EP0912914A1 - Liquid crystal device comprising anchoring means on at least one confinement plate providing a degenerated orientation - Google Patents

Abstract

The invention concerns a liquid crystal display device comprising a liquid crystal material sandwiched between two confinement plates (1, 2), characterised in that at least one of the plates (2) is provided with a degenerated or almost degenerated treatment defining an azimuthal anchorage without orientation memory.

Description

LIQUID CRYSTAL DEVICE COMPRISING MEANS

ANCHORING ON AT LEAST ONE PLATE

CONTAINMENT GIVING A DEGENERATED ORIENTATION

The present invention relates to the field of liquid crystal display devices.

The present invention is the result of a collaboration between the CNRS, the University of Paris-Sud and the Institut Curie.

STATE OF THE ART It is well known to those skilled in the art that the molecules of nematics and liquid crystals in general in contact with a bordering surface orient in one or more directions, thanks to the interaction with the substrate. .

For example, on a solid surface rubbed in one direction, the mesogenic molecules generally orient in a direction close to the direction of friction. This orientation, called monostable, is characterized by a single "easy" direction for the nematic, usually defined by the angles θo and φ ₀ (see Figure 1): the zenith angle θo, between the easy axis f and the normal z to the substrate; and the azimuth angle φo between the easy axis f and a fixed direction chosen in the plane of the substrate.

The easy axis f corresponds to a minimum of the interaction energy of the nematic with the bordering phase.

By applying an external field we can redirect the nematic on the surface. The surface energy increases and this additional energy, due to the deviation of the surface director n _s with respect to the easy axis f, is called anchoring energy W and it is a function of the angles θ and φ which define ns :

W = W (θ, φ)

In the case of monostable anchoring, the anchoring energy W has a single minimum, which corresponds to a single easy axis (generally, we do not differentiate between the direction n _s and the opposite direction -n _s , because that the nematic phase is non-polar). A large number of treatments (see [1]) give this simple anchor, which is widely used in liquid crystal display devices.

There are, however, anchors with several W energy minima and therefore with several easy directions. For example, on SiO layers evaporated under vacuum under certain conditions, the orientation of the nematics is bistable, with two easy directions f1 and f2 (see Figure 2) defined by Θ02 = Θ01 and φ ₀ 2 = - φoi- Several devices were proposed and produced, using the bistable nematic anchors. See documents [2-5], Another well-known class of anchors are degenerate anchors. In this case, there is a whole continuum of easy directions, which correspond to the same zenith angle θo and an arbitrary azimuth angle. The anchoring energy in this case is a function only of θ and does not depend on φ: W = W (Θ)

We can say in this case that the azimuth anchor does not exist or that it is infinitely soft.

Depending on the value of θo we can distinguish the planar degenerate anchor (θo "= 90 °), with n _s parallel to the surface, or the degenerate conical anchor (0 <θ ₀ <90 °), with n _s which can turn on an opening cone 2Θ ₀ (see figure 3). Another special case corresponds to θo = 0 (homeotropic anchoring), which in fact gives a monostable anchoring, with the molecules perpendicular to the surface (φ is not defined when θ = 0).

Degenerate anchors are typical on a flat interface between the nematic and an isotropic phase. Indeed, nothing imposes an azimuthal direction in this case and by symmetry the minimum of anchoring energy is achieved for all possible angles φ (0 <φ <360 °).

Experimentally, degenerate anchoring has often been observed on the free surfaces of nematic drops or on the nematic - liquid interfaces. This type of anchoring has been studied from an academic point of view, but has so far found no application, due to its unstable nature: the liquid-liquid or liquid-gas interface is very easy to deform, it create easily faults and it is difficult to apply an electric field through this interface.

One will find presentations on the state of the art relating to degenerate anchors in document [6]. BASIS OF THE INVENTION

It results from the studies carried out by the inventors that in principle, a degenerate anchoring can be obtained on all isotropic solid surfaces, for example mineral or organic glasses, but that in reality, this anchoring is rarely observed, because two main phenomena intervene.

The first of these phenomena identified by the inventors corresponds to an adsorption of the mesogenic molecules on the surface.

At the first contact between the nematic and the substrate, for example during the filling of the cell, no azimuthal direction is imposed and the orientation of the molecules is defined by chance or by flow. The initial orientation is therefore usually very inhomogeneous, with θ = θo and φ arbitrary, but with φ depending on the position on the substrate. Very quickly, the nematic molecules in direct contact with the surface are adsorbed on the substrate. Thus their order and their orientations are memorized on the surface and imposed on the nematic molecules which remain in the volume near the substrate. Although theoretically adsorption is a reversible process, in practice the characteristic times for desorption at room temperature are very long (days or even years). The orientation of the sample therefore remains ill-defined, inhomogeneous and strongly anchored on the substrate.

The second of these phenomena identified by the inventors corresponds to an orientation of the substrate by the nematic.

In fact, in the case of relatively soft substrates, for example polymer layers, a second phenomenon can occur. Even if the adsorption is weak, the interaction of the substrate with the nematic can make it anisotropic, for example by local orientation of the polymer chains. Again we obtain a memorization on the substrate of the poorly defined and inhomogeneous initial state, also accompanied by an azimuthal anchoring energy which destroys the degenerate anchoring.

Following these two phenomena of orientational memory, degenerate anchoring on solid surfaces seems difficult to produce and use. For this reason, the degenerate anchoring has not found any application for the moment.

The present invention now aims to improve the liquid crystal devices to allow the operation of a degenerate or almost degenerate anchor. More specifically, the present invention aims to provide new means for obtaining degenerate anchors, or almost degenerate, and without memory of liquid crystals on solid substrates and allowing the use of these anchors in display devices. These objects are achieved in the context of the present invention by means of a liquid crystal display device comprising a liquid crystal material sandwiched between two confinement plates, characterized in that at least one of the plates is provided with '' a treatment which defines a degenerate azimuthal anchoring, without azimuthal orientation memory. According to another characteristic of the present invention, the treatment is a passivation treatment of the surface of at least one of the plates by inhibiting the adsorption sites on this surface.

It may be a treatment operating by saturation of the adsorption sites. According to another advantageous characteristic of the present invention, the treatment comprises a coating comprising a polymer comprising fluid or very mobile chains, or even self-lubricating, that is to say without site capable of adsorbing the liquid crystal.

DESCRIPTION OF THE FIGURES Other characteristics, objects and advantages of the invention will appear on reading the detailed description which follows, and with reference to the appended drawings, given by way of nonlimiting example and in which: FIG. 1 represents the azimuthal orientation of "easy" direction of the nematic director with respect to a confinement plate and the zenith orientation of the easy direction with respect to normal to the latter,

FIG. 2 represents the orientation of two easy directions in the case of a bistable anchoring,

- Figure 3 shows schematically the case of a conical anchor,

FIG. 4 represents the energy of the zenith anchor for a monostable planar alignment,

FIG. 5 represents the energy of the zenith anchor for a symmetrical bistable anchor or for a degenerate conical anchor,

FIG. 6 represents the texture of the liquid crystal during six successive stages of a switching process,

FIG. 7 represents the texture of the liquid crystal during four successive stages of a switching process in accordance with another variant of the present invention,

- Figure 8 shows four successive stages of another process according to the present invention without breaking the anchor;

FIG. 9 represents the optical response between crossed polarizers of a cell according to the present invention, FIG. 10 represents two flat textures capable of being obtained with a planar anchoring on a counter blade,

FIG. 11 represents the registration thresholds as a function of the duration of the control pulses, measured on a cell comprising a polyisoprene treatment, - FIG. 12 represents the registration thresholds as a function of the duration of the control pulses, measured on a cell comprising a polystyrene treatment,

- Figure 13 shows the spontaneous erasure time of a cell as a function of the square of the thickness of the cell concerned, - Figure 14 shows different curves illustrating the optical response of a cell according to the present invention for electrical control pulses of a fixed duration and of different amplitudes, FIG. 15 represents the optical signal as a function of time for refresh pulses of fixed repetition frequency and for different amplitudes of these pulses,

FIG. 16 represents the average light intensity as a function of the voltage rms,

FIG. 17 illustrates a variant in which the two electrodes are arranged on the same plate,

FIG. 18 represents the angle φ (z) for two displays, one in accordance with the invention, the other conventional, - FIG. 19 illustrates the twist in a chiralised cell,

FIG. 20 represents the optical response of a horizontal field cell,

FIG. 21 represents a device with a chiralised or cholesteric nematic, FIG. 22 represents the optical response of this device,

FIG. 23 represents the optical response for a similar device having a different thickness and pitch, and

- Figure 24 shows the optical response of a long-pitch cell.

DETAILED DESCRIPTION OF THE INVENTION I - Characteristics of the invention relating to the passivation of the substrate by saturation of the adsorption sites with surfactants.

As indicated previously, according to a first approach of the present invention, the undesirable memory of the surface of a containment plate is eliminated by virtue of a thin layer of an appropriate surfactant on the substrate. The role of the surfactant is to occupy the adsorption sites available on the surface and in this way to "passivate" the substrate, making it impossible to adsorb the liquid crystal itself.

This passivation material can be formed of any surfactant which reacts strongly with the surface of the containment plate and which is easily adsorbed on it, preferably in a definitive manner. Preferably, this surfactant is further adapted so as not to interact strongly with the liquid crystal, for example not to orient itself easily by the anisotropic interactions with the mesogenic molecules. Once covered by the molecules of the surfactant, the surface of the confinement plate becomes inert: it no longer adsorbs the molecules of the liquid crystal, and moreover it does not become anisotropic under their action.

For a rigid and flat substrate, for example mineral glass, a monomolecular layer of the surfactant is sufficient to saturate all the available adsorption sites and to passivate the surface. For a highly rough and porous substrate, for example a layer of evaporated SiO, the surfactant film is preferably thicker to ensure passivation. For soft substrates, for example polymers, the passivation material is advantageously formed by a thicker layer to strongly screen the substrate-liquid crystal interaction and to thereby avoid the orientation of the polymer and the hysteresis which results therefrom. results.

In order to passivate different substrates by saturating the adsorption sites, the inventors tested in particular several surfactants belonging to the family of organofunctional silanes, comprising chlorosilane (Si-Ci), silanol (Si-OH), or alkoxysilane ( Si-C _n H _2n OH). To cling to the surface, these products have the property of being easily adsorbed on glass or other substrates and, after heat treatment, react chemically with the surface and with each other. The surfactant layer obtained in this way is very solid and impenetrable for the mesogenic molecules.

The organofunctional group is chosen so as to minimize the interaction between the surfactant and the mesogenic molecule, to avoid the phenomena of memory and hysteresis. The invention is however not limited to the family of organofunctional silanes.

It extends to any equivalent compound, that is to say any compound capable of fulfilling the two aforementioned functions: a) to be anchored on the substrate, b) but without interacting with the liquid crystal material, for example with chromium complexes . Examples of chromium complexes capable of defining a homeotropic anchoring are found in document [7]. Experimental results have shown that most of the silanes tested reduce the memory of the substrates on which they are deposited.

Very interesting and reproducible results have notably been obtained with the product 3-Glyceryloxypropyl Trimethoxysilane (GLYMO). This product was deposited in thin layers (from 20A to 1000A) on the substrates from an isopropanol solution (from 0.01% to 0.5%). The layers were baked for one hour at 120 ° C or 200 ° C to make them insoluble in liquid crystal. To test the anchoring, the inventors used the pentylcyanobiphenyl (5CB) nematic at room temperature. The test cells were constructed using two different slides: the plate tested, treated with surfactant, and a "standard" slide with strong and monostable anchoring (evaporated SiO).

On all the isotropic substrates used (glass, float glass, double oxide of Indium and Tin), the inventors observed that the deposition of a layer of GLYMO thicker than about 100A leads to planar anchoring (θo ≈ 90 °) degenerate without any orientation memory: the orientation of the nematic in the test cell is uniform, defined by the standard slide, and under the action of an external electric or magnetic field this orientation changes in a uniform manner and without memory . The azimuthal anchoring energy on substrates provided with a passivation layer thicker than 100A is strictly equal to zero. Layers thinner than 100A give very low azimuth anchoring energy, less uniform anchoring and less reproducible results. The inventors also tested the action of the passivation layers of the GLYMO product on anisotropic substrates. The substrates tested were glass slides with transparent ITO electrodes and a thin layer (5 to 1000 Å) of SiO, evaporated in vacuum at a grazing angle (75 °). Without passivation layer the orientation of the 5CB nematic on these substrates is according to the conditions of evaporation, planar monostable, bistable inclined and monostable inclined. In all cases, the substrates show a very strong memory of the initial orientation of the nematic, due to the strong adsorption of mesogenic molecules on the porous and polar layer of SiO.

After applying a thin passivation layer (> 20A), the orientational memory disappears almost completely. On all substrates, the alignment of 5CB on the passivation layers is planar (without pretilt) monostable, in a direction perpendicular to the evaporation plane. The energy of the azimuth anchoring is very strong for the thinnest layers, with an extrapolation length L <4θA. This anchoring energy gradually weakens when the thickness d of the GLYMO film increases and for d> 200A, L diverges and the anchoring becomes planar degenerate.

According to an advantageous characteristic of the present invention, the thickness of the passivation layer is preferably between 20 A and 500 A. Similar results have been obtained by passivating with a layer of GLYMO glass or ITO substrates, made anisotropic by friction. Without passivation, these substrates align the 5CB in a monostable planar fashion, with a strong azimuth anchoring and a very large surface memory. By depositing a layer of GLYMO on the substrates, the inventors observed a complete disappearance of the memory and a weakening of the anchoring force. Again, the anchoring energy gradually decreases when d increases and at d> 200A it cancels out completely: the anchoring becomes planar degenerate, with negligible memory. Similar results have also been obtained with a unidirectional friction of the layer of GLYMO previously deposited on an isotropic substrate.

These results confirm that the orientational memory of a containment plate can be eliminated by an appropriate surfactant treatment, which saturates the adsorption sites available on the substrate. Two different objectives are achieved by this treatment: a) strongly reduce or completely eliminate the surface memory on isotropic substrates. The resulting degenerate planar or conical anchors are very homogeneous, easy to use and without orientation hysteresis. b) on anisotropic substrates this treatment also makes it possible to modify the azimuthal anchoring force. Thus the strength of monostable or bistable anchors can be reduced, making them almost degenerate, with a very low break point of the azimuthal anchor.

According to another variant in accordance with the present invention, the passivation treatment can be formed by depositing a coating thick enough to prevent access to the adsorption sites formed on the substrate, by the liquid crystal molecules, without for the coating does not saturate each of these sites. II - Characteristics of the invention relating to the "lubrication" of the azimuth anchoring on surfaces by polymer layers with fluid or very mobile chains

The present invention also proposes, in order to avoid surface hysteresis, to apply to the substrate an isotropic liquid or vitreous layer, the molecules of which can not only inhibit the adsorption sites on the substrate as indicated above, but also can easily reorient or deform so that the confinement plate thus treated loses all the memory of the previous orientation. This layer plays the role of an orientational lubricant: it allows the orientation of the nematic director n _s on the surface to slide on the substrate without any friction or hysteresis. It should be noted that in this case it is not annoying that the mesogenic molecules adsorb on the lubricating layer: when a torque is applied to n _s , the adsorbed molecules will easily reorient without desorption, thanks to the deformation and the reorientation of the "soft" molecules of the lubricating layer. Such a "anchoring lubricant" product can be formed from polymers which are in the liquid phase at room temperature (or more precisely, throughout the thermal stability range of the mesogenic phase used). In this case, it is necessary to graft the lubricating layer on the substrate to strongly attach its molecules and avoid their solution or diffusion in the liquid crystal. The layers thus prepared are both very stable macroscopically and very mobile at the molecular level. Alternatively, it is also possible to use as anchoring lubricants solid polymers, in their vitreous state, provided that their chains are fairly mobile and their viscosity is moderate. This condition is easy to meet if the polymer is close to its melting point at room temperature.

The inventors have in particular tested several liquid polymers from the family of siloxanes and glassy polymers, with a melting temperature close to ambient (for example polyisoprene and polybutadiene). These products have great mobility in their chains. Two nematics at room temperature were used in this study: 5CB and the nematic mixture MLC 6012 (Merck). All the products, deposited in thin layers on the substrates, gave degenerate anchors of the two nematics studied and without any orientation memory.

We will describe a typical result, obtained with the polyisoprene polymer which is glassy to the ambient. A thin, uniform layer (<300A) is deposited with the spinner from a solution (0.3 to 3%) of cyclohexane. The layer is used without any heat treatment to prevent the polymer from passing into the liquid phase, which would destroy the uniformity of the film. In the long term (days), the inventors observed phenomena of transport of the polyisoprene from the slide studied to the standard slide, used in the test cells. For this reason, the inventors used standard blades, the anchoring of which is not sensitive to the slow migration of polyisoprene molecules: layers of rubbed polyimide, which give a strong monostable, planar or inclined anchoring.

The polyisoprene layers deposited on isotropic substrates (ordinary glass, float glass, transparent ITO electrode on glass) give an excellent degenerate orientation of the nematics studied. No anchoring memory was detected in the test cells: the orientation of the easy axis on the polyisoprene always remains in the same plane, only on the standard blade. Under the action of the external fields, the easy axis on the studied slide is reoriented in a uniform way without hysteresis and without any azimuthal anchoring energy.

When the polyisoprene layers are deposited on anisotropic substrates (glass or rubbed ITO, evaporated SiO), which usually give a strong monostable anchoring, the azimuthal anchoring energy decreases. By changing the thickness of the deposited layer, the inventors observed a gradual transition between the highly monostable anchoring of the substrate and the degenerate anchoring typical for the polyisoprene layer. This is also accompanied by a complete disappearance of the orientation memory.

Finally, the zenith angle θ ₀ of the degenerate anchoring on the polyisoprene depends on the nature of the substrate and the nematic. For the nematic mixture MLC 6012 (Merck), the anchoring is always planar degenerate (θo = 90 °). For the 5CB, the anchor is planar degenerate for polyisoprene deposited on evaporated SiO, while it is degenerate conical (θo close to 55 °, the "magic" angle) if the polyisoprene film is deposited directly on the glass.

Very similar results were obtained by grafting onto the substrates of polystyrene terminated with a chlorosilane group, which was synthesized specially for this study. The polystyrene used to modify the surfaces (hereinafter called functional PS) was synthesized anionically according to a conventional method, so as to obtain polymer chains terminated at one end with SiCl3. The molecular mass of the functional PS is typically 40,000 g / mol but can be varied from 1,000 to 10 ⁶ g / mol.

To graft the polymer, a solution of functional PS is first prepared in anhydrous toluene, at a volume fraction of the order of 5%. The surfaces to be grafted are cleaned in a stream of oxygen, under UV. The functional PS solution is deposited on these surfaces suitable for the spinner. After the toluene has evaporated, the surfaces covered with functional PS are placed in the vacuum oven, typically for 24 hours, at a temperature of 160 ° C. The excess functional polymer is then removed by cold dissolution in toluene, possibly with the help of ultrasound. The amount of grafted PS is measured by ellipsometry.

The grafting of polystyrene on isotropic substrates (glass or ITO) results in a degenerate conical anchoring of the 5CB nematic, without any memory, very homogeneous and reproducible. In addition, the treatment with grafted polystyrene is very stable over time: the grafted molecules do not dissolve in the liquid crystal and remain attached to the surface even after mechanical treatments, for example after unidirectional friction on tissue.

The grafting of polystyrene on anisotropic substrates (glass or rubbed ITO, evaporated SiO) reduces the azimuthal anchoring energy and removes the anchoring memory. By varying the initial anisotropy of the substrate or the density of the grafting, the inventors observed a continuous variation of the azimuthal anchoring energy between the usual high value for the untreated substrates and the typical degenerate anchoring of the grafted layer. In this way the inventors have succeeded in obtaining almost reproducible, almost degenerate anchors. Similar results have also been obtained by mechanical friction of the already grafted polystyrene layer. According to an alternative embodiment, it is possible to use a self-lubricating material, that is to say without a site capable of adsorbing the liquid crystal, without however comprising fluid or mobile chains for producing the coating of the confinement plate or producing this plate itself directly without any coating. This arrangement is particularly suitable when one of the plates does not have an electrode. An example of such an arrangement will be described later.

IMPLEMENTATION OF THE INVENTION Most nematic display devices use only volume effects. In these displays the texture changes are carried out by continuous deformations in the volume and without any reorientation on the surfaces. By their very nature, these devices require strong monostable anchors for their operation. Recently, nematic displays using a break in the anchoring on the surfaces have been proposed and produced. See documents [2- 5], [8], [9]. In these devices, the orientation of the liquid crystal on the surface changes suddenly during the operation of the display, thus making it possible to transform the volume texture and to switch between two bistable textures (ie textures which remain stable for long times without the application of external fields). The main advantages of these devices are their bistability and their switching speed. However, nematic displays with surface bistability also have certain drawbacks: their anchors are more difficult to produce and to control.

The display proposed in documents [2-5] requires fairly complex anchors: the two states used are distinguished by both their azimuth and zenith angle (pretilt angle). For the moment these anchors remain difficult to achieve. The device proposed in documents [8] and [9] uses simpler anchors (monostable). But to decrease the duration and the voltage of the control pulses, it requires moderate or weak overhead anchor energies, the production technologies of which are not yet well mastered. The inventors are now proposing new means for producing degenerate (i.e. without azimuthal anchoring energy or surface memory) or almost degenerate (i.e. with low azimuth anchoring energy and without surface memory) anchors. These anchors are easy to produce and control with external electric fields. Their zenith anchoring force is moderate or even weak, and for this reason they can be used in display devices.

The switching command for the degenerate anchor displays can be done by a break in the degenerate anchor and then a command for the broken anchor. Another solution is the application of an azimuth torque which rotates the degenerate anchor.

I.- Devices with degenerate or almost degenerate anchorage switching. 1-1) Breakage of the degenerate anchor.

To understand the surface break in the case of a degenerate anchor, we will first recall the simplest case of the break of a monostable anchor. In Figure 4 is presented the energy of the zenith anchor, which corresponds to a monostable planar alignment. In the absence of an external field, the director n _s on the surface is oriented according to the minimum of W (θ), which corresponds to θ = + 90 ° (these two directions are physically equivalent, for this reason we consider anchoring as monostable). Under electric field E, the molecules in the volume orient themselves along the field (assuming that the dielectric anisotropy of the liquid crystal Δε is positive). A couple is exerted on n _s : the nematic is redirected on the surface under the action of the field. In general, the stronger the external torque, the more the new orientation of n _s approaches the direction of the field, without ever reaching it (because the return torque due to the surface anchoring is opposed to it). An important exception to this rule is the case where the field E is oriented in the direction which corresponds to a maximum of the surface energy (the direction θ = 0 in FIG. 4). In this direction, the restoring torque, due to the anchoring, is canceled out and cannot oppose the field E: π _s therefore becomes parallel to E when the field is strong enough, greater than a threshold value E _c .

If we now cut the field, the surface is at θ = 0 in an unstable equilibrium and can return to one or the other of the positions of stable equilibrium (θ = ± 90 °). The choice of this position is made by chance, due to fluctuations, or can be induced by a weak control effect exerted on the cell.

In practice, the only direction on which it is easy to apply a strong E field is normal to the plates. The anchoring break already described is therefore possible, by symmetry, only for planar anchors, or for other symmetrical anchors with respect to θ = 0.

The zenithal energy of such an anchor, the symmetrical bistable anchor, is shown in Figure 5. This anchor can be broken by a field normal to the cell in two different directions: θ = 0, if Δε>0; θ = ± 90 °, if Δε <0. The same figure 5 can also be interpreted as the zenith part of a degenerate anchor, which by definition has zero azimuthal energy (the two branches θ> 0 and θ <0 correspond in this case to the same zenith angle θ and to two angles azimuths which differ by 180 °). For this degenerate conical anchoring, the same conclusions remain valid as for the bistable conical anchoring: it breaks at θ = 0 or at θ = 90 ° (and φ arbitrary).

Within the framework of the invention, the bistable symmetrical anchors can be considered as anchors very close to the degenerate anchor: the energy of the degenerate anchor W (θ) has a cylindrical symmetry and it is a function of θ only; if one superimposes on this energy a weak azimuthal energy Wφ (φ), symmetrical compared to φ = 0, one obtains the symmetrical bistable anchoring. The anchoring obtained by this superposition is called "almost degenerate anchoring" in the context of the present patent application.

A special case of almost degenerate anchoring is obtained if we start from a degenerate planar anchor (θo = 90 °): it is a planar anchor with a very low azimuthal energy.

In the case of degenerate (or almost degenerate) anchors, the azimuthal energy of the anchor is zero (or very weak), but the zenithal energy is arbitrary and can be strong. In practice, these anchors are less anisotropic than monostable anchors and their zenith anchor is usually moderate or weak. I-2) Control of the broken anchor. Different means can be used to control the broken anchor.

According to a preferred embodiment, this control is ensured by a hydrodynamic surface flow. Such a control by hydrodynamic surface effect can be in accordance with the provisions described in documents [8] and [9] to which reference will usefully be made for a good understanding of the present invention. We will first consider the cell shown in Figure 6a. The plate 1 has a conventional monostable and strong anchoring, preferably with a pretilt (θoι <90 °). The blade 2 has a degenerate conical or planar anchoring (θo ₂ ≠ 0, φo ₂ arbitrary) in accordance with the present invention. The elastic energy of volume is minimized for the plane texture with φ ₂ = 180 °, presented on figure 6a.

If an electric field is applied E _c ι>E> E _C 2 perpendicular to the plates 1 and 2, the anchoring on the blade 2 with degenerate anchoring breaks and the practically homeotropic texture of FIG. 6b is obtained. E _c ι and E _c2 correspond to the breaking thresholds respectively on the two plates.

If the field E is suddenly cut, the blade 2 is in an unstable equilibrium, with no torque applied to it. The blade 1, on the other hand, is in imbalance and a strong restoring torque, due to the anchoring, acts on the molecules in its vicinity, forcing them to return to their initial orientation. This return creates a mass flow, which diffuses rapidly through the cell to the plate 2 (Figure 6c). By the interaction with this flow, the molecules on slide 2 leave in the direction opposite to their initial orientation (φ2 = 0, Figure 6c).

As soon as θ ≠ O, the restoring torque due to the zenith anchoring of the blade 2 reappears and accelerates the relaxation of the blade 2 towards the state θ ₂ = θ ₂ o, 9 ₂ = 0 (Figure 6d). This texture (FIG. 6d), produced quickly (a few tens of microseconds) after the end of the control pulse, is different from the initial texture - the display is registered.

The texture registered in accordance with FIG. 6d, however, is not stable if Θ ₁ and θ ₂ are sufficiently large, because it has significant elastic bending energy. Spontaneously after a few milliseconds, it relaxes towards a volume conical helical texture without changing the surface anchors (fig 6e). This torsion in volume applies an azimuth torque on the degenerate anchoring of the plate 2 which rotates the director on the anchoring cone (fig 6f) and finally causes it to return to the equilibrium position φ = 180 ° by unrolling l conical propeller (fig 6a). The movement on the cone is slowed down by the surface viscosity and lasts a few tens of milliseconds.

On the other hand, the texture of figure 6d can be stable if θ-i and Θ ₂ are weak. The resulting device thus has bistable properties. A monostable display is therefore obtained, which can be written with very short pulses (τi≤I Oμs) and which spontaneously disappears in a much longer time (τ _e > 10ms). In addition, depending on the application, τ _e can be adjusted within wide limits by modifying the anchors and the thickness of the cell. Of course, other processes can be used to control the broken anchor.

If necessary, the display in Figure 6 can be transformed into a bistable display. On the one hand, if the mean angle of the inclination of the molecules (Θ1 + Θ2V2 is less than a critical value θ _c , the bent state (Figure 6d) is stable. Indeed, to pass from the bent state in the inclined uniform state (figure 6a) of minimum energy, the texture must pass through the twisted state of 180 ° (figure 6e). This intermediate state forms a barrier which stabilizes the bent state as long as (Θι + θ2 ) / 2 <θ _c . The critical angle Θ _c varies from about 45 ° for compounds - whose torsional energy is very low, to 90 ° (flexed state always stable) for compounds whose elastic energies of bending and twisting are equal.

On the other hand, it suffices to replace the anchoring on the blade 2 with an almost degenerate anchoring. In this case only the positions φ ₂ = 0 and φ ₂ = 180 ° on the cone 02 = 020 are stable and a weak barrier due to the azimuth anchoring W _φ separates them. The texture 6d is then transformed into a twisted conical texture, which remains stable if the anchoring barrier W _φ is greater than the volume energy of torsion. We therefore obtain a bistable display. To erase the twisted texture, just apply a new impulse and this time gradually cut the field. The hydrodynamic interaction is now absent and a weak elastic volume interaction favors the return to the "uniform" state of FIG. 6a: the display is erased. Of course, the present invention is not limited to the particular embodiment which has just been described, but extends to any variant in accordance with its spirit.

In particular, in the context of the present invention, it is possible to envisage:

- control of the anchor broken by a hydrodynamic volume effect,

- a control of the broken anchor by the superposition of two hydrodynamic effects - of volume and surface, - a switching without complete break of the surface by rotation of the director on the cone of the degenerate anchor.

According to yet another variant, it is possible to consider breaking the anchoring on the two plates (In the case of a liquid crystal of positive dielectric anisotropy, one then obtains under field a uniform homeotropic state, respectively planar for a negative anisotropy) . Without additional control after deletion of the field, the final state can be random. But if desired, the final state of the structure can be checked with an additional command, such as an electric field or a flow of adequate orientation, for example horizontal. I.2 a) Control of the broken anchor by a hydrodynamic volume effect.

The hydrodynamic effect previously described (fig 6c) is created by the rapid return of the molecules on the blade 1 to their initial orientation, under the action of the anchoring torque on this surface. If the anchoring on the blade 1 is very strong, during the command pulse, the molecules near this surface do not change their orientation and we obtain the texture of figure 7b: the texture is homeotropic everywhere, except in a thin volume layer in the vicinity of the plate 1. After the field is cut, the molecules on the surface 1 do not reorient themselves and therefore, there is no hydrodynamic surface effect. However, the relaxation of the strongly deformed volume layer towards a more homogeneous texture, creates a hydrodynamic volume flow (Figure 7c), known as the "blackfiow" effect (see document [10]). This diffuse flow towards the blade 2 and controls the broken anchor on this blade, in exactly the same way as in the case of a hydrodynamic surface effect.

We then obtain a state equivalent to that illustrated in FIGS. 6d and 7d, ie either a stable state if θ-i and Θ ₂ are weak, or a state which evolves towards the states illustrated in FIGS. 7e and 7f .

In FIG. 7 is illustrated a planar anchoring on the blade 1. However as a variant, this anchoring on the blade 1 could be oblique. I.2 b) Control of the broken anchor by the superimposition of them hydrodynamic effects - of volume and surface. Finally, the flows created by the two hydrodynamic effects, surface and volume, are in the same direction and therefore they are added: in practice, the broken surface is controlled by the superposition of the two effects. I.2 c) Switching without complete break of the surface by rotation of the director on the cone of the degenerate anchor.

The switching described in FIG. 6 requires a complete break of the zenith anchor: under electric field E> E _C2 , the molecules in the vicinity of the blade 2 are aligned exactly parallel to its normal. However, degenerate anchors are very anisotropic: the break point E _c2 of the zenith anchor is finished, but the azimuth anchor is infinitely soft and the corresponding breakpoint is strictly zero (or very low for almost degenerate anchors) . This anisotropy makes it possible to easily switch from the texture illustrated in FIG. 6a to the texture illustrated in FIG. 6d without going through the normal to the plate, but by turning around it on the cone of the degenerate anchoring, or on a cone closer to normal (Figure 8). Under an electric field lower than the break point (E <E _c2 ) the molecules on slide 2 remain at a finite angle compared to normal (Figure 8b). At the cut of the field, the hydrodynamic flow created by the other blade and the deformed volume layer, lead the director towards φ = 0. The molecules now rotate (figure 8c) around the normal (by chance left or right) ), staying almost at the same zenith angle θ ₂ . Under the action superimposed on the flow and the zenith anchor (which pulls them towards θ ₂ = Θ ₂₀ ) the system relaxes towards the texture (8d), identical to that of Figures 6d and 7d. For degenerate anchoring, only a small elastic torque prevents the switching. For an almost degenerate anchoring, a weak azimuthal anchoring couple is added. In both cases, however, the switching is much more effective than in the case of complete breakage of the zenith anchor. 1.3 - EXPERIMENTAL RESULTS

To study the operation of the proposed device, the inventors produced several cells with degenerate or almost degenerate anchors.

A first type of cell uses a slide covered with a thin layer of GLYMO (with a thickness of the order of 100A) on a transparent ITO electrode, which gives a degenerate planar anchoring of the 5CB nematic. GLYMO is chemically grafted onto the surface and is crosslinked to form a stable and insoluble layer in the liquid crystal. The other plate in the cell is always treated by evaporation of SiO (82 °, 105 nm, strong anchoring, monostable, inclined at Θ ₁₀ of the order of 55 °). The thickness of the cells was between 1 μm and 4 μm. After filling, the cells still show a flat texture, without any twisting. After the application of the short electrical pulses, the inventors observed a switch to a twisted texture of 180 °. This texture is transient and spontaneously transforms again into the initial texture, after an erasure time τ _e . This time is of the order of 10 to 100 ms and it is proportional to the square of the thickness of the cell.

The zenithal anchoring energy for these cells is relatively strong, comparable to that obtained with monostable evaporated SiO. Ec is of the order of 10V / μm for τ = 1 ms. FIG. 9 is the recording of the optical transmission of the cell between crossed polarizers following the application of a control pulse 30. We have added in FIG. 9 the correspondences with the states explained with reference to FIG. 6. We note that the maximum transmission is obtained for state 6e. On this cell, whose optical response is not optimized, the inventors have measured a contrast better than 100: 1.

Another series of typical cells was carried out with blades 2 covered with polyisoprene which give for the 5CB a degenerate conical anchoring. As counter-blades 1, the inventors used glass plates with transparent electrodes, covered either by rubbed polyimide (strong planar anchoring, monostable), or by evaporated SiO (82 °, 105 nm, strong anchoring, monostable, inclined at around 55 °). When filling, all the samples show flat textures, with the director in the plane defined by the monostable anchoring on the counter blade 1. In the case of counter blade 1 treated with SiO (inclined anchoring), a single inclined texture , almost uniform, is observed, similar to the texture of Figure 6a. In the case of a planar counter blade 1 (rubbed polyimide), the two flat textures shown diagrammatically in FIG. 10 coexist, separated by defects.

By applying short electrical pulses, the inventors observed a transition of the cells in the "registered" state, twisted at 180 °. The inscription thresholds E _c (τ) measured for these cells are presented in FIG. 11. The static surface fracture threshold is very low for polyisoprene (E _c of the order of 1.5 V / μm, one order smaller than polyimide). From the practical point of view this property is very important: the device can be controlled with low voltages, even for short pulses (U = 26V for τ of the order of 10μs and a cell of 2μm). Despite its low value, the anchoring energy is very uniform over the surface of the cell, this results in practice in a very well defined and constant registration threshold over large surfaces.

In FIG. 12 is presented the breaking threshold of the anchoring of the 5CB as a function of the duration of the pulse for a treatment of polystyrene grafted on the ITO. For this coating also the threshold is reasonably low (E≈3volt / μm for τ = 1 ms) and very well reproducible. The same threshold was also obtained for the almost degenerate anchors obtained by mechanical friction of the grafted polystyrene layer.

The time τ _e is an important characteristic of the monostable display (which disappears spontaneously) and it must be adjusted according to the refresh rate. This time depends on several parameters: cell thickness, geometry of the anchors, duration and voltage of the control pulse.

In Figure 13 τ _e is presented as a function of the square of the thickness of cell "d" for different samples, containing the nematic 5CB. The degenerate anchor blade 2 is always covered by polyisoprene (degenerate conical anchor without any memory). Two types of counter-blades 1 were used: monostable planar anchoring on rubbed polyimide (curve a) and inclined monostable anchoring on evaporated SiO (curve b). The dependence of τ _e on d ² is well known to those skilled in the art: it can be deduced from the condition of equilibrium between the elastic torque of volume torsion which forces the director to turn on the anchoring cone , and the viscous couple that opposes it. We also observe that τ _e is longer for an oblique anchoring on the counter blade: these cells have a smaller mean angle θ and therefore the energies of their bent (Figure 6d) and twisted (Figure 6e) texture are very close. The elastic torque becomes small and the relaxation time τ _e becomes longer.

In Figure 14 is presented the optical response of a cell (5CB, polyisoprene plate 2, 1 oblique SiO counter blade, thickness 2.3 μm) for control pulses of a fixed duration (τ = 50 μs) and different tensions. More precisely in FIG. 14, 4 curves a, b, c and d have been represented which correspond respectively to four control voltages of 13.4 V, 13.5 V, 14 V and 15 V. It can be seen in FIG. 14 that, below a well defined threshold U _c of the order of 13.4 volts, no transmission is observed. AU> U _c the transmission increases with the voltage applied during the control pulse. At greater than or equal to US = 15 V, the intensity of the transmission saturates at its maximum value and does not change any more. This dependence, discovered by the inventors, can be explained as follows: at high tension (U> US) the anchoring on the polyisoprene blade is completely broken and, at the end of the impulse, the hydrodynamic control is very effective, requiring a rapid return of the director to the position θ = θo, φ = 0 ° (figure 6d) helped by the zenith anchoring. The bent texture thus produced is flat and the optical transmission is weak. For the moment, no azimuth couple is applied to the texture and the anchoring does not slip. As already explained, the texture (Figure 6d) is unstable and it turns into the twisted texture of Figure 6e, which pulls on the surface 2 and forces the anchor to slide, leading through the texture of Figure 6f to the final texture of figure 6a. During the consecutive transformations of the texture of Figure 6d to the texture of Figure 6a, the texture is twisted and the transmitted intensity is strong.

When the control voltage is low (U of the order of U _c ), the zenith anchoring of the blade 2 is not completely broken and, at the end of the pulse, the director turns around normal on a cone with small aperture Θ ₂ <θ ₂ o- This time, the hydrodynamic effect and the zenith anchor pull the molecules in two different directions: respectively towards the half-plane φ = 0 ° and towards the anchor cone Θ ₂ = θ ₂ o * At the end of the flow, we don't get the plane texture of figure 6d, but a twisted texture, according to figure 6f. This texture quickly untwists and gives the initial texture of Figure 6a. In addition, all the intermediate textures are close to the homeotrope and therefore the corresponding transmission between crossed polarizers is weak.

This model explains the behavior of the curves l (t) in Figure 14, in particular the increase in relaxation time and maximum intensity with tension. Similar curves have been obtained and explained by the inventors, with a fixed control voltage (U = 25 volts) and variable duration τ: for τ = 14 μs the optical response is zero; at 14 μs <τ <16 μs the transmitted intensity increases progressively, at τ greater than or equal to 16 μs the optical signal saturates and no longer depends on τ. The dependence of the intensity transmitted after the pulse of τ and U is very important from a practical point of view: it makes it possible to use the display offered in a "shades of gray" operating mode, very important for color display. This is the ability to change the brightness of the pixel by changing the voltage or the duration of the short control pulse. Thus to define shades of gray, it is possible to apply pulses close to the break point, for example in the form of pulses of fixed duration and of amplitude between U _c and U _c -20% or of pulses of amplitude fixed and of duration between τ _c and τ _c -20% (τ _c representing the pulse duration required to break the anchor for a given amplitude, see for example Figures 11 and 12).

The high fidelity of the thresholds U _c and τ _c and the rapid rise in brightness for a small change in the parameters (U or τ) make possible a high rate of multiplexing of the display, retaining the shades of gray. For example, you can enter the display line by line with a voltage Uligne = (U _c + Us) / 2 on the active line and (U _c - Us) / 2 <Ucol <(U _s - U _c ) / 2 on the columns. In this way, the resulting voltage in the pixels on the active line varies between U _c and Us- The intensity of the transmitted light can therefore vary between zero and the maximum value. The tension on the unaddressed lines remains lower than the tension of the instability of Fredericks [10].

To avoid flickering of the image, it is possible to use an image refresh period less than the erasure time τ _e .

FIG. 15 gives the optical signal as a function of time for refresh pulses of fixed repetition frequency and different amplitudes of these pulses. In this case, the inventors have shown that the device operates in the rms mode: the average optical signal is a function of the average of the square of the applied voltage.

Figure 16 shows the average light intensity as a function of the voltage rms (square root of the mean of the square of the voltage). We can see that the voltage threshold is steep. The ratio M between the voltage giving a light intensity of 90% and that giving 10% is close of 1.025. This makes it possible to produce a screen whose number of lines is given by the formula of Alt and Pleshko [11]:

a screen of 1600 lines. Finally, the inventors also studied the almost degenerate anchors, obtained by superposition of a conical or planar degenerate anchor (polyisoprene, polystyrene or GLYMO layer) with a weak monostable alignment (friction of the blade before or after deposition of the layer degenerate anchoring, deposit of this layer on evaporated SiO or another equivalent anisotropic surface). As already explained, these anchors are symmetrical, bistable anchors, with very low azimuthal anchoring energy.

According to yet another variant, the anisotropy can be induced by irradiation using a beam of polarized ultraviolet or visible light.

After the application of the control signals, the almost degenerate anchors break and the cell registers, giving the texture illustrated in FIG. 6e. The breakpoints are the same as for the corresponding degenerate anchors. The inscribed texture does not, however, spontaneously disappear by sliding the anchor, and remains stable over the seconds scale. At longer times, the cell is erased by movement of the faults.

If necessary, this spontaneous erasure can be eliminated by adding a chiral dopant to the liquid crystal material, as proposed in document [9], in another context for monostable anchoring.

In short, the display proposed in the context of the present invention has the following characteristics:

- anchors simple to produce,

- very fast registration (τ <10μs for U of the order of 26 V), - spontaneous erasure after an adjustable time τ _e , of the order of 20 to 100 ms, ie comparable to the refresh rate of the video image , - shades of gray (approximately 7 to 8 binary shades of gray for the non-optimized cell),

- possibility of obtaining bistability.

We can also consider the case where the zenithal breakpoint Ec ₂ of the degenerate anchor is greater than the zenithal breakpoint Eci of the conventional zenithal anchor. For a field E such as Ec ₂ >E> Ecι, the degenerate anchoring plate becomes the master plate, that is to say that it will create the hydrodynamic flow of volume and surface which gives the state bent in the case of the inscription or the elastic interaction which creates the uniform state of FIG. 6a in the case of the erasure.

We can also consider the case of a treatment deleting any azimuthal orientation memory on the two plates.

In addition, it may be envisaged in the context of the present invention to use either a liquid crystal of positive dielectric anisotropy, or a liquid crystal of negative dielectric anisotropy.

II.- Devices using an azimuth couple to turn the degenerate anchor.

In the devices that we have described so far, the applied field is normal to the blades and the high electric torque breaks the __ zenithal anchoring of the degenerate azimuthal anchoring blade. After the control pulse a new volume texture is produced, which in the case of the monostable returns to the initial texture by azimuthal rotation of the molecules on the degenerate anchor plate. We will now describe other devices, where an azimuth torque applied in the cell produces an azimuthal rotation. No anchoring break occurs in this case: the degenerate azimuth anchoring freely reorients under such a torque and the texture of the cell changes in a progressive and reversible manner since by definition the breaking threshold of the azimuth anchoring degenerate is zero. However, these new devices differ from traditional displays by the fact that, during their switching, the director's orientation turns on the degenerate anchor blade: instead of the traditional strong anchor they use an "infinitely weak" azimuth anchor ". 11.1 - Display with degenerate anchorage in horizontal field. a) Geometry of the device

In this case, the two electrodes are arranged on the same blade, for example the blade 1 in FIG. 17; they are parallel to the y axis and distant from L. The electric field is in the plane of the cell parallel to the x axis. On the blade 2 there are no electrodes and its anchoring is conical or degenerate planar, without orientation memory. On blade 1, the anchoring is traditional, monostable, planar or inclined with an azimuth angle φ1 relative to the x axis (fig. 17). The distance between the blades is d. Without a field, the plane texture of Figure 17a is achieved: everywhere in the cell, the molecules are parallel to a vertical plane, defined by φ (z) - = φ1. This texture is imposed only by the anchoring on the blade 1 and by the elasticity of distortion of the nematic, because there is no surface azimuth couple on the blade 2.

When a voltage U is applied between the two electrodes, the electric field (E = U / L) orients the director n (z) parallel (if Δε> 0), or perpendicular (if Δε <0) to the x axis. On blade 1 the azimuth anchoring is strong and the director does not rotate (φ (0) = φ1). On plate 2 the azimuth anchoring is free and the edge condition is given by dφ / dz / _{z = d} = 0. Eh general we obtain under field a twisted texture, presented on fig. 17b, with an orientation φ2 of the molecules on the degenerate plate, depending on the field. This variation in the orientation of the nematic on blade 2 with the field distinguishes the device proposed from other already known displays, which also used horizontal control fields but, the anchoring of which on blade 2 is strong and monostable (homeotropic [ documents 12 and 13], planar or inclined (see references cited in document 14]). Those skilled in the art know that horizontal field displays are much more difficult to produce and therefore more expensive than vertical field devices (normal to the blades); they require control voltages L / d "1 times higher, the arrangement of the electrodes in the pixel is complicated, an active matrix is compulsory for multiplexing. However, horizontal field displays also have a great advantage: their angle of view is very large, thanks to the fact that in all their states the director remains parallel to the blades. b) Comparison between the proposed system and displays in horizontal field and traditional anchors. To understand the advantages of the proposed device, it is necessary to compare the variation of the angle φ (z) through the cell (fig. 18) for two very similar displays: the device proposed in fig. 17a and the same type of display, but with a strong monostable anchoring on the blade 2, which requires φ (d) = φ ₂ = φ-i. Without a field (U = 0), the two displays have a uniform texture with φ (z) = φ1 (fig. 18, curve a). To simplify, we assume here φ1 = 90 ° and Δε> 0, without of course limiting ourselves to this case. Up to a certain threshold value of the voltage U _s the surface torque, due to the anchoring, dominates the electrical volume torque and the texture remains uniform. For the traditional monostable anchoring the threshold is defined by U's = LU ₂ / d, where U2 is the Frederiksz threshold in torsion geometry [document 10], around 0.5V for the 5CB. For our device with conical anchoring on the blade 2, the threshold is twice lower U "s = LU ₂ 2d, thanks to the free anchoring condition on the blade 2. Above the threshold the electric torque becomes sufficiently strong to distort the texture. In the case of traditional anchoring on the two blades, φ (z) varies in volume (fig. 18, curve b), but remains unchanged on both surfaces. In the case of azimuthal anchoring degenerate on slide 2 the molecules also rotate on this surface (fig. 18, curve c). The texture twists in this case is much stronger. To observe the texture transformations we place the cell between crossed polarizers, one of which is parallel to the director on the slide 1. In the case of the two displays, if U <U _S , their cell is a birefringent plate whose slow axis is parallel to the polarization of the incident light, the transmission of these displays (cell plus polarizers) is zero: the display is in its state black.

For the known display, with strong anchoring on the two blades, if U> U _S , but close to U _s the torsion in the cell corresponds to FIG. 18 curve b, the Mauguin condition is satisfied (dφ / dz "Δn / λ). Then the polarization of the light follows the direction of the molecules, the twists up and down cancel their effects, the display remains black. For higher voltages the torsion of the cell becomes strong, the Mauguin condition is no longer satisfied near the surfaces; the light at the exit of the cell is strongly elliptical. The transmission increases quickly with tension but in a different way for the different colors.

In the case of degenerate anchoring, if U> U _S but close to U _s , the cell remains in the waveguide regime and plays the role of a polarization rotator: after having crossed the cell, the polarization of the light rotated by an angle φ_-φ., the same (or almost) for all wavelengths. The transmission therefore increases very regularly with the voltage as soon as the threshold is exceeded and the transmitted light remains white.

All this remains true also in the general case φ1 <90 ° for degenerate anchoring, but in this case there is no clear threshold U _s and the twist of the texture gradually increases with U. Thus, the shades of gray are extended over a larger range of voltages and are therefore easier to control but the contrast decreases slightly, because the total torsion in the cell φ1 - φ2 becomes smaller than 90 °. To avoid this drawback it suffices to chirize the nematic by transforming it into a cholesteric with a long pitch of 2 μm <P <10 μm. The torsion in a chiralised cell is shown schematically in FIG. 19, curve a, without field, curves b, c and d in increasing field, c) Advantages of the proposed display Some of the advantages of the device of FIG. 17 compared to other horizontal field displays become evident from the previous discussion. The degenerate anchoring on the blade 2 makes it possible to lower the control voltage U by a factor of two, or to use L / d ratios twice as large at fixed U and therefore to double the size of the pixels. The transmitted light is white, without any color dispersion. Of course, the angle of view is large, as in all other displays in the horizontal field. Degenerate anchoring also gives great freedom of choosing the geometric parameters of the cell to optimize its behavior. For example, we can optimize the optics of the cell by chiralising the nematic and independently choosing the orientations φ1 and φ2 on the two plates relative to the field E. It is also possible to control the device by the polarity of the field: two surfaces being different, one can create a density of the volume or surface dipole in the cell (for example flexoelectric [document 13], ordoelectric [document 15]). This polarization couples with the polarity of the field and according to its sign the texture is twisted to the left or to the right, minimizing the energy due to this linear coupling in E. This effect can be important when E 1 n or when the field is weak , because in these two cases the linear effect is stronger than the dielectric effect (quadratic in E). d) Experimental results

The inventors have produced several cells with a horizontal field between a blade 1 of strong planar anchoring (oblique evaporation of SiO) and a blade 2 of degenerate conical anchoring (grafted polystyrene). The two transparent ITO electrodes were placed on slide 2 and separated by 100 μm. The duration of the control pulses was chosen to be 40 ms, compatible with the duration of the video image. In fig. 20a is presented the optical response of such a cell (d

= 2.0 μm, φi = 70 °, nematic 5CB pure) at the variable control field E = U / L. For comparison, in fig. 20b are presented the results obtained with a similar cell, but with monostable planar anchoring on the slide 2. In both cases the contrast is very good (-200) and the angle of view is wide. In the case of monostable anchoring (fig. 20b) the transmitted light is more colorful and the control fields are approximately twice as high, as expected. II.2 - Display with degenerate azimuth anchoring in "vertical" field.

As we have already explained, the advantage of devices using horizontal fields is their wide angle of view. On the other hand, the advantage of "vertical" field displays (perpendicular to the blades, obtained between two transparent electrodes deposited on the two blades) is their simplicity and low control voltage. We will now demonstrate that by using conical anchors it is possible to combine the advantages of the two types of displays, a) Geometry of the device In this case (fig. 21), the two electrodes are deposited respectively on the two blades 1 and 2. The electric field E = U / d is parallel to the normal of the blades. The anchoring on the blade 1 is monostable (planar or oblique). On the blade 2 the anchoring is planar or degenerate conical, without memory. The liquid crystal between the plates is a chiralise or cholesteric nematic with a pitch P relatively large compared to the wavelength of visible light. The degenerate conical or planar anchoring on the blade 2 distinguishes the proposed device from the monostable homeotropic anchoring devices already proposed in documents [16] and [17]. Those skilled in the art know that without external constraint, imposed by a field or by the edges, the cholesteric rolls up in a helix. If the axis of the helix is parallel to z, the molecules are perpendicular to this axis (θ = 90 °) and φ = φ (z) = qoz, where q ₀ = 2π / P is the wave vector of spontaneous cholesteric torsion. The anchoring conditions of the cell in Figure 21 require a helical texture, the axis of the helix is perpendicular to the blades. However, the alignments on the blades and the applied electric field, impose a zenith angle θ = θ (z), in general different from 90 °. A coupling now appears between θ (z) and dφ (z) / dz due to the anisotropy of the elastic constants of the nematic (KιΞK ₃ > K ₂ ). Assuming that Kι = K ₃ = K> K ₂ , and in the case of free azimuthal anchoring on one of the blades we have the relation:

relationship in which

Ki = elastic constant of the fan deformation of the liquid crystal, K = elastic constant of the torsion, K ₃ = elastic constant of bending.

There is then a coupling between the zenithal and azimuthal distortions. If the texture is close to the planar (θ ≡ 90 °, fig. 21a) we return to dφ / dz = qo. If the texture is close to the homeotropic (θ "90 °) we obtain a decrease in the twist dφ / dz = qoK ₂ / K.

In the device proposed by the inventors, the coupling between φ and θ plays the role of a torque "converter". It transforms the overhead couples, created directly by the vertical field, into azimuthal couples. Thus, thanks to the free azimuth anchoring on the plate 2, the angle φ ₂ changes under the action of the field (fig. 21 b, we assume in this figure Δε> 0). Between crossed polarizers a change in the transmitted intensity is obtained. If P and Δπ are large, the texture is almost planar, we work in the waveguide regime and at the cell output the polarization is more or less rotated, depending on the field applied. This regime is easily achievable with oblique anchors on the two blades and Δε <0 (θ <90 ° without field, θ≡90 ° under field). If P is small and / or the texture is close to the homeotrope (θ "90 °), the waveguide regime is disturbed and at the exit of the cell the light is more or less elliptical, depending on the field. This regime is easily achieved with planar anchors (or close to the planar) on the two blades and Δε> 0 (almost homeotropic texture under field), b) Advantages of the proposed device

The proposed device has many advantages compared to devices based on traditional anchors. Compared to horizontal field displays, these advantages are:

• simplicity of implementation,

• possibility of making pixels of arbitrary size and shape,

• low control voltages, possibility of operating in passive mode (without active matrix), • good colorimetry in the case of waveguide operation.

Compared to vertical field displays and traditional anchoring, these advantages are: • wide angle of view,

• good colorimetry in case of waveguide

• possibility of strong changes in texture and therefore of φ, without appearance of defect (these variations can thus be very rapid and reversible). b) Experimental results

To demonstrate the performance of the device proposed in the vertical field, the inventors produced several cells between two glass slides, with transparent ITO electrodes on the internal surfaces of the slides. In all cases, the anchoring is traditional and monostable on the blade 1 and conical or planar degenerate on the blade 2. The nematic 5CB is chiralized to obtain a spontaneous torsion, with a pitch P much greater than the wavelength.

In FIG. 22 is presented the optical response of such a cell to an electrical pulse of duration τ = 40 ms (compatible with the video frequency) and of voltage U between 1.0 and 3.0 V. The anchoring is oblique monostable (SiO) on slide 1 and degenerate conical (grafted polystyrene) on slide 2. The cell thickness is d = 4.0 μm and the cholesteric pitch is P ≡ 5.0 μm. The torsion in the cell varies between -270 ° without field and -160 ° under strong field. This strong twist ensures a wide angle of view, comparable to supertwisted displays. The response times are compatible with the frequency of the video image and the contrast (not optimized) is approximately 20.

In Figure 23 is presented the optical response for another cell, with the same anchors, but with d = 2.9 μm and P ≡ 2.6 μm. Thanks to the thinner thickness, the relaxation times are faster on this cell, allowing more frequent image refreshes (for example 16.6 ms for the NTSC standard). The contrast of this cell is very good, around 140. In these two cells, the pitch of the helix is not large enough to satisfy the Mauguin condition ΔnP »λ. The waveguide regime is therefore disturbed and the color dispersion is not completely avoided. Also, the inventors observed in these cells with high total torsion of the modulations of the texture, already observed for hybrid cells [document 17]. These continuous modulations, without fault line, are due to the conical anchoring on the blade 2 and for small thicknesses (d ~ P) they replace the real faults ("fingerprints") observed in cholesteric patients with monostable anchors. The response times in our case are not influenced by the texture modulations: there is no hysteresis, because there is no creation or transport of defects. The modulations are observed in the fieldless state, which is the clear state for the proposed display, and they do not degrade the contrast.

In Figure 24 we present the optical response of a long-pitch cell (P ~ 13 μm), which works in the waveguide regime and which has better colorimetry. This cell has a thickness of ~ 3.7 μm, monostable planar anchoring (SiO) on blade 1 and degenerate planar anchoring (treatment by Glymo) on blade 2. The total torsion of this cell is small and there is no texture modulation. BIBLIOGRAPHY

1. J. Cognard, "Alignment of Nematic Liquid Crystals and their mixtures", Mol. Cryst. Liq. Cryst. Suppl. 1, pp. 1-77 (1982);

2. Durand et al. US Patent 5,357,358;

3. R. Barberi, G. Durand, Appl. Phys. Lett. 58, 2907 (1991);

4. R. Barberi, M. Giocondo, Ph. Martinot-Lagarde, G. Durand, J. Appl. Phys. 62, 3270 (1993); 5. R. Barberi, M. Giocondo, G. Durand, Appl. Phys. Lett. 60, 1085 (1992);

6. B. Jérôme, "Surface effects and anchoring in liquid crystals", Rep. Prog. Phys. 54, 391 (1991);

7. S. Matsumoto, M. Kawamoto, N. Kaneko, "Surface-Induced Molecular Orientation of Liquid Crystals by Carboxylatochromium Complexes", Appl. Phys. Lett. 27, 268 (1975);

8. Patent application PCT / FR 96/01771 of 8 Nov. 1996;

9. I. Dozov, M. Nobili, G. Durand, "Fast bistable nematic display using monostable surface switching", Applied Physics Letters 70, p. 1179 (1997). 10. PG de Gennes, J. Prost, "The Physics of Liquid Crystals", Clarendon Press, Oxford, 1993.

11. Alt. P.M. and Pleshko P .; 1974 IEEE Trans. Elec. Dev. ED-21: 146

12. G. Durand, Ph. Martinot-Lagarde, I. Dozov FR 8207309 "Improvement in optical cells using liquid crystals"

13. I. Dozov, Ph. Martinot-Lagarde, G. Durand "Flexoelectrically controlled twist of texture in a nematic liquid crystal" J. Phys. Lett. 43, 365-369 (1982) 14. MASAHITO OH-E et al. "Dependence of viewing angle characteristics on pretilt angle in the in-plane switching mode Liquid Crystals, 1997, vol. 22, n ° 4.

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Claims

1. Liquid crystal display device comprising a liquid crystal material sandwiched between two confinement plates (1, 2), characterized in that at least one of the plates (2) is provided with a treatment which defines a degenerate azimuth anchor, without azimuthal orientation memory.

2. Device according to claim 1, characterized in that the treatment is a passivation treatment of the surface of at least one of the plates (1, 2) by inhibiting the adsorption sites on this surface.

3. Device according to one of claims 1 or 2, characterized in that the treatment is obtained by the choice of a material constituting the containment plates or a coating thereon.

4. Device according to claim 3, characterized in that the material comprises a polymer comprising fluid or very mobile chains or having no adsorption site of the liquid crystal.

5. Device according to one of claims 1 to 4, characterized in that the treatment includes the grafting of a coating.

6. Device according to one of claims 1 to 5, characterized in that the treatment operates by saturation of the adsorption sites.

7. Device according to one of claims 1 to 6, characterized in that the treatment is adapted so as not to strongly interact with the liquid crystal, in particular not to be easily oriented by the anisotropic interactions with the mesogenic molecules of liquid crystal .

8. Device according to one of claims 1 to 7, characterized in that the treatment comprises a compound chosen from the family of organofunctional silanes.

9. Device according to one of claims 1 to 8, characterized in that the treatment comprises a compound chosen from the family of organofunctional silanes comprising chlorosilane (Si-Ci), silanol (Si-OH), or alkoxysilane ( Si-C _n H _2n OH).

10. Device according to one of claims 1 to 9, characterized in that the treatment comprises chromium complexes.

11. Device according to one of claims 1 to 9, characterized in that the treatment is formed of 3-Glyceryloxypropyl Trimethoxysilane (GLYMO).

12. Device according to claim 4, characterized in that the material comprises polymers in liquid phase throughout the thermal stability range of the mesogenic phase used and that said polymers are grafted on the surface.

13. Device according to claim 4, characterized in that the material comprises polymers insoluble in the liquid crystal, in their vitreous state.

14. Device according to one of claims 4 or 12 and 13, characterized in that the material comprises liquid polymers of the family of siloxanes or glassy polymers, at room temperature.

15. Device according to claim 14, characterized in that the material comprises polyisoprene, polybutadiene or polystyrene.

16. Device according to one of claims 1 to 15, characterized in that the treatment is adapted to define a degenerate planar anchoring.

17. Device according to one of claims 1 to 15, characterized in that the treatment is adapted to define a degenerate conical anchoring.

18. Device according to one of claims 1 to 17, characterized in that it comprises means capable of controlling a change of state, by hydrodynamic surface effect.

19. Device according to one of claims 1 to 18, characterized in that it comprises means capable of controlling a change of state, by hydrodynamic effect of volume.

20. Device according to one of claims 1 to 19, characterized in that it comprises control means applying to the liquid crystal a pulse less than the breaking threshold on the plate provided with the treatment.

21. Device according to claim 20, characterized in that it comprises control means creating a hydrodynamic effect of volume or surface inducing an azimuthal rotation of a degenerate or almost degenerate anchoring without breaking of the zenithal anchoring.

22. Device according to one of claims 1 to 19, characterized in that it comprises control means applying to the liquid crystal a pulse greater than the breaking threshold on the plate provided with the treatment.

23. Device according to one of claims 1 to 22, characterized in that the treatment is deposited on an isotropic substrate to define a degenerate planar or conical anchoring and without orientation hysteresis.

24. Device according to one of claims 1 to 22, characterized in that the treatment is deposited on an anisotropic substrate.

25. Device according to claim 24, characterized in that the treatment is deposited on a layer of SiO placed on a containment plate.

26. Device according to one of claims 1 to 25, characterized in that it comprises means superimposing an anisotropy on a degenerate anchor conical or planar to produce an almost degenerate anchor of non-zero azimuthal energy but without memory or hysteresis .

27. Device according to claim 26, characterized in that the means superimposing an anisotropy on a degenerate conical or planar anchoring are chosen from the group comprising: friction of the confinement plate (2) before or after the coating is deposited, depositing the coating on an anisotropic surface or inducing anisotropy by irradiating the passivation coating using a beam of polarized ultraviolet or visible light.

28. Device according to one of claims 1 to 27, characterized in that the display device is bistable and that in the absence of a field at least two textures are stable among the three possible textures: plane / inclined ( fig. 6a, 7a), flat / bent (fig. 6d, 7d), and the texture twisted (fig- 6f).

29. Device according to claim 28, characterized in that the inclinations (θ) of the molecules at rest relative to a normal to the confinement plates are small enough to stabilize the flat / bent texture.

30. Device according to one of claims 28 or 29, characterized in that it comprises a chiral dopant to stabilize the twisted texture.

31. Device according to one of claims 1 to 23, characterized in that the display device is monostable, that the state induced by the control means is unstable and that the device returns spontaneously to the initial stable state by a continuous path.

32. Device according to claim 31, characterized in that the inclinations (θ) of the molecules at rest relative to a normal to the confinement plates are large.

33. Device according to claim 20, characterized in that the control means are adapted to selectively apply to the liquid crystal a pulse chosen from several control pulses close to the break point to define the selected shades of gray.

34. Device according to claim 33, characterized in that the control impulses have a fixed duration and a variable amplitude close to the break point.

35. Device according to claim 33, characterized in that the control pulses have a fixed amplitude and a variable duration close to the breaking threshold.

36. Device according to one of claims 33 and 34, characterized in that it is organized in a matrix and that the control means are adapted to apply a voltage Uligne = (U _c + Us) / 2 on the active line and (U _c - U _s ) / 2 <Ucol <(U _s - U _c ) / 2 on the columns, U _c and U _s respectively representing the thresholds for appearance and saturation of change of state of the device.

37. Device according to claim 28, characterized in that to erase an inscribed texture, the control means apply a impulse that promote the elastic interaction between the two surfaces over their hydrodynamic interaction.

38. Device according to one of claims 33 to 35, characterized in that the repetition period of the control pulses which refresh the image is less than the erasure time of the device and that thus the selection of the gray levels is given by the value of the mean square voltage applied to the pixel.

39. Device according to one of claims 1 to 32, 37 and 38, characterized in that there is provided control means capable of breaking the anchor on the two plates.

40. Device according to claim 39, characterized in that there are provided additional control means capable of creating a horizontal field or flow of orientation.

41. Device according to one of claims 1 to 21 and 23 to 40, characterized in that the zenith breakpoint of the degenerate anchoring provided on a plate is greater than the zenith breakpoint of a conventional anchorage provided the other plate, and means are provided capable of applying a field lying between the two anchoring thresholds thus defined.

42. Device according to one of claims 1 to 41, characterized in that the two plates (1, 2) have a treatment defining an degenerate azimuth anchoring without azimuthal orientation memory.

43. Device according to claims 1 to 17, characterized in that one of the plates induces a strong azimuth anchoring and the other a planar or conical degenerate azimuth anchoring.

44. Device according to claim 43, characterized in that it comprises means capable of applying an electric field perpendicular to the plates and that the liquid crystal is cholesterized to form in the cell a regular helical texture, the pitch of which changes under the field action thanks to the degenerate azimuth anchoring of one of the blades.

45. Device according to claim 44, characterized in that the liquid crystal is of positive anisotropy and that under field the regular helical texture is preserved but its pitch varies.

46. Device according to claim 44, characterized in that the liquid crystal is of negative anisotropy and that at least one of the anchors on the plates is inclined; thus the applied field increases the zenith angle and changes the pitch of the helix.

47. Device according to claim 43 and characterized in that it comprises means capable of applying an electric field parallel to the plates which controls the torsion of the molecules in the vicinity of the degenerate azimuthal anchoring plate.

48. Device according to claim 47, characterized in that the liquid crystal is nematic with positive dielectric anisotropy and that the field E is applied perpendicular to the uniform texture of the nematic without field.

49. Device according to claim 47, characterized in that the liquid crystal is nematic with negative anisotropy and that the field E is applied parallel to the uniform texture of the nematic without field.

50. Device according to claim 47, characterized in that the liquid crystal is nematic and that the field is applied in a direction different from 0 and 90d relative to the uniform texture.

51. Device according to claim 47, characterized in that the liquid crystal is cholesterized to obtain a helical texture in the absence of the field.