FR3063443A1 - NANOSTRUCTURE POLYMERIC FILMS AND PROCESS FOR THEIR PREPARATION - Google Patents

Abstract

The present invention relates to a process for preparing a film of a polymer M1 containing spherical particles of a material M2, said method comprising a step of impregnating a substrate with a solution, said substrate consisting of a support material M3 comprising at least one layer of spherical particles, said substrate being obtained by the deposition of said particles on said support by the Langmuir-Blodgett technique, and said solution comprising at least one oligomer and optionally at least one crosslinking agent.

Description

Holder (s): UNIVERSITY OF BORDEAUX Public establishment, POLYTECHNICAL INSTITUTE OF BORDEAUX Public establishment, NATIONAL CENTER OF SCIENTIFIC RESEARCH Public establishment.

Extension request (s)

Agent (s): LAVOIX.

NANOSTRUCTURED POLYMERIC FILMS AND THEIR PREPARATION METHOD.

FR 3,063,443 - A1 ^The present invention relates to a method for preparing a film of a polymer M1 containing spherical particles of a material M2, said method comprising a step of impregnating a substrate with a solution, said substrate consisting of a support made of M3 material comprising at least one layer of spherical particles, said substrate being obtained by depositing said particles on said support by the Langmuir-Blodgett technique, and said solution comprising at least one oligomer and optionally at minus a crosslinking agent.

NANOSTRUCTURED POLYMERIC FILMS AND THEIR PREPARATION METHOD

The present invention relates to nanostructured polymer films and their preparation process. It also relates to their use in particular in the field of biology, surface optics, mechanical sensors and microfluidics.

Techniques for assembling particles on surfaces are particularly targeted to date because of potential applications, particularly in the fields of electronics and optics.

Among these techniques, the following techniques can for example be cited: dip coating (dip coating in English), spin coating (spin coating in English) or even controlled evaporation. However, these techniques have the disadvantage of limiting the size of the beads that can be used as well as the surface area of the sample and its geometry. In addition, the thickness obtained on the treated surface is not homogeneous and we often see the presence of cracks (especially during drying).

Among the conventional nanostructuring techniques, we can also cite electronic lithography. However, such a technique has many drawbacks, particularly in terms of cost.

The methods existing to date therefore have many drawbacks such as the high cost, the treatment of small areas but also a only two-dimensional structuring as well as a too high implementation time.

To date, there is therefore a need for a low-cost and simple process making it possible to obtain nano- or microstructured films which can be applied to large areas.

The present invention therefore aims to provide new nanostructured polymeric films according to a simple process to implement and inexpensive.

It also aims to provide micro- or nanostructured polymeric films making it possible to increase the active surface and therefore the sensitivity.

The present invention also aims to provide flexible micro- or nanostructured polymeric films, which can be fine (that is to say a thickness of a few tens of microns).

The present invention also aims to provide transparent and biocompatible micro- or nanostructured polymeric films, which can therefore be used, for example, in the field of biology.

Thus, the present invention relates to a method of preparing a film of a polymer M1 containing spherical particles of a material M2, said method comprising a step of impregnating a substrate with a solution, said substrate consisting of '' a support made of M3 material comprising at least one layer of spherical particles on its surface, said substrate being obtained by depositing said particles on said support by the Langmuir-Blodgett technique, and said solution comprising at least one oligomer and optionally at least a crosslinking agent.

The invention therefore relates to a new process for manufacturing films of micro- and nanostructured polymers. These polymeric films (in particular of the polydimethylsiloxane (PDMS) type) are transparent, flexible, and of controllable thickness (ranging from a few tens of microns to a few centimeters). They contain a 2D or 3D structure, formed by one or more layers of monodisperse particles, preferably inorganic, self-assembled in a compact hexagonal network.

The monolayers or multilayers of particles trapped in the polymer are produced by the Langmuir-Blodgett technique which makes it possible to compactly organize particles of sizes ranging from 10 nm to 20 microns, preferably from 100 nm to 2 microns, and then deposit on a large surface support, very precisely controlling the number of layers.

The present invention therefore consists in particular in preparing a polymeric film containing spherical particles, commonly called "beads". The film thus obtained is based on the abovementioned M1 polymer in which spherical particles are trapped.

The process of the invention can be considered as comprising a step of molding the spherical particles and this via their assembly with the film.

The film obtained according to the process of the invention therefore comprises spherical particles, in particular at least one layer of spherical particles, covered with a film of polymer M1.

The film obtained according to the process of the invention can be considered as a composite product comprising on the one hand a polymeric film (based on polymer M1) and spherical particles.

The film obtained according to the process of the invention can be considered as a composite product comprising on the one hand a polymeric film (based on polymer M1) and spherical particles, the whole being deposited on a support.

The process of the invention makes it possible, at low cost, to produce micro- and nanostructured polymer films suitable for large areas (tens of cm ² ) and for different geometries (plane, cylindrical, etc.).

According to one embodiment, the polymer M1 is obtained by polymerization from at least one oligomer chosen from the group consisting of styrene, siloxanes, and their mixtures. Mention may also be made of resins activated by UV or by heat, epoxy resins or alternatively methyl methacrylate.

Preferably, the polymer M1 is polystyrene or PDMS, optionally loaded with nanoparticles or nanowires. The polymer M1 can also be a polymerized resin or poly (methyl methacrylate) (PMMA).

According to one embodiment, the thickness of the film of the polymer M1 is greater than or equal to 10 μm, preferably greater than or equal to 40 μm.

According to one embodiment, the thickness of the film of the polymer M1 is less than or equal to 10 cm, preferably less than or equal to 5 cm.

As indicated above, the method of the invention comprises the implementation of spherical particles. These particles can also be designated by the term "beads".

In the context of the present invention, the term size designates the diameter, in particular the average diameter, of the particles.

Preferably, the diameter of the above-mentioned spherical particles (or beads) is between 10 nm and 20 μm, preferably between 100 nm and 2 μm.

The spherical particles according to the invention consist of a material M2. Preferably, the material M2 is silicon dioxide (SiO ₂ ).

Among the M2 materials, mention may also be made of organic polymers, preferably crosslinked, such as for example polystyrene-co-divinylbenzene.

According to the invention, the spherical particles can be functionalized or modified at the surface. For example, they can be modified by grafting chemical molecules and / or nanoparticles.

The method of the invention comprises a step of impregnating a substrate as defined in more detail below with a solution comprising at least one oligomer.

This impregnation step can consist in running said solution on the substrate or in immersing the substrate in said solution.

According to one embodiment, said solution further comprises at least one crosslinking agent. This crosslinking agent is chosen from crosslinking agents well known to those skilled in the art, depending on the oligomer used, and more particularly divinylbenzene (in the case of the styrene oligomer) or else a branched siloxane chain (such as component B presented in Example 1 below for oligomers of the siloxane type).

According to one embodiment, when the polymer M1 is obtained from an oligomer chosen from resins activated by UV radiation or by heat, the method of the invention comprises a step of UV radiation or of heating in order to obtain a resin polymerized from the oligomer. According to this embodiment, the above-mentioned solution does not include a crosslinking agent but it is UV radiation or heat which plays this role and makes it possible to obtain a crosslinked resin.

The substrate used in the process of the invention consists of a support made of a material M3 covered with at least one layer of spherical particles of M2. The substrate therefore comprises at least one layer of spherical particles of M2 on the surface. In other words, the substrate comprises a layer of material M3 covered, on its surface, with a deposit comprising at least one layer of spherical particles.

According to one embodiment, the material M3 is chosen from materials which are naturally hydrophilic, or whose surface can be made hydrophilic by suitable physical or chemical treatment.

Preferably, the material M3 is chosen from glass, plastic materials and metallic or semiconductor materials. More particularly, the material M3 can be silicon, gold, copper, platinum, nickel or zinc.

According to one embodiment, the support has a plane or cylindrical, hollow or solid geometry. In other words, the support used for the process of the invention can have a flat surface or be a cylinder, hollow or solid.

According to one embodiment, the support used for the process of the invention is a capillary, the section of which can be, for example circular, square or rectangular.

As indicated above, the substrate according to the invention is obtained by the Langmuir-Blodgett technique, well known to those skilled in the art.

Historically, the Langmuir-Blodgett technique is a process which has been used to deposit on a substrate multilayers of surfactant molecules. The general principle is as follows:

- spreading at a plane interface (gas-liquid or liquid-liquid) of a solution containing "amphiphilic" molecules having affinities for the two phases in contact (in general, air and water);

- after evaporation of the dispersion solvent, compression of the molecules using a mobile barrier in order to organize them into a more or less compact two-dimensional film (called "Langmuir film");

- immersion of a substrate through the interface, generally perpendicularly, and transfer onto its surface of the film previously formed (by adjusting parameters such as immersion speed, emersion speed, number of cycles).

This process can however be implemented using larger amphiphilic objects, in particular nanoparticles or organic or inorganic particles of submicron or micron size.

The Langmuir-Blodgett method makes it possible to obtain homogeneous deposits on a large scale, without cracking, with object sizes greater than one micron, and the thickness of which can be controlled very precisely - to the nearest layer - including on large substrates with flat or cylindrical geometry. The silica particles necessary for manufacturing have also been synthesized and functionalized in situ.

According to one embodiment, the substrate used for the process of the invention comprises from 1 to 100, preferably from 1 to 20, layers of spherical beads.

Indeed, the process of the invention is particularly advantageous in that it allows the number and nature of the layers to be controlled.

According to a preferred embodiment, the method of the invention comprises the following steps:

a step of preparation by the Langmuir-Blodgett technique of a substrate consisting of a support of material M3 covered by at least one layer of spherical particles of a material M2;

- a step of preparing a solution comprising at least one oligomer and optionally at least one crosslinking agent; and

- a step of impregnating the substrate with said solution.

The above-mentioned impregnation step therefore corresponds to a step of impregnating the deposit covering the support with said solution, and this as detailed above.

According to a specific embodiment, the step of preparing the above-mentioned solution comprises a step of mixing at least one oligomer and at least one crosslinking agent, followed if necessary by a step of degassing the mixture thus obtained . This degassing step can for example be carried out by placing said mixture in a desiccator.

According to one embodiment, the method of the invention comprises the use of a mold to place the substrate there before adding the solution. According to this embodiment, the substrate, as obtained by the Langmuir-Blodgett technique, is in a suitable mold and then the solution is poured onto the substrate placed in the mold. The use of the mold then makes it possible to obtain a defined and chosen shape for the polymeric film obtained at the end of the process.

This step can also be followed by an additional degassing step, in particular by placing the assembly in a desiccator.

The process of the invention therefore makes it possible in particular to obtain a support covered with a polymer film M1 containing spherical particles as defined above.

It also makes it possible to obtain a film of polymer M1 containing spherical particles as defined above. To obtain the polymer film M1 containing spherical particles as defined above, the method of the invention then further comprises a step of recovering the film of polymer M1 containing the spherical particles, in particular by separating it from the support. Such a step therefore consists in detaching said film from the support.

To detach or separate the film from the support, it can be envisaged to lift the film by a corner. Another possibility is to place the support covered with said film in deionized water or in a suitable solvent such as acetone or hexane, and to subject the assembly to an ultrasonic treatment, for example for 30 minutes. Then the film can be cut around the edges and then lift a corner of the polymeric film.

According to one embodiment, the method of the invention further comprises a step of dissolving at least part of the spherical particles, preferably all of the spherical particles.

This embodiment then makes it possible to obtain the polymeric film without part of the spherical particles or even the polymeric film without the spherical particles.

The polymeric film without the spherical particles can then be assimilated to a porous film whose pores correspond to the imprint of the spherical particles.

The process of the invention can also include a post-functionalization step of the polymer M1, for example by surface functionalization or by impregnation.

According to one embodiment, the surface properties of the polymer M1 can be modified by a physical or chemical treatment, in particular by modifying its surface tension, for example by UV / ozone treatment, Piranha treatment, or by grafting of new chemical functions like for example -NH ₂ .

The present invention also relates to a film of a polymer M1 as defined above containing spherical particles of a material M2 as defined above, capable of being obtained according to the above-mentioned process. The process of the invention can also include a step of post-functionalization of the surface of the particles of M2 material.

According to one embodiment, in the above-mentioned film, the spherical particles form a compact stack, each of the particles being in contact with at least one of the other particles of said stack.

The present invention also relates to a micro- or nanostructured film comprising spherical pores, said pores being organized in compact form and at least one of the pores intercepting or flush with at least one surface of the film.

The term “micro- or nanostructured film” designates a polymer including, in all or part of its volume or surface, particles of nanometric or micrometric size, or a combination of the two, these particles possibly being able to be dissolved in a selective manner to finally obtain a porous material having a greater specific surface.

The main characteristic of this film is that the spherical particles form a compact stack at least locally or by domains (because of the Langmuir-Blodgett deposition technique). Each of the particles is in contact with at least one other particle in the film. The particles intercept or surface on at least one of the surfaces of the polymer film. The particles are limited to a given thickness in the film (they are not everywhere since they exclusively form layers).

According to one embodiment, the aforementioned film further comprises a metallic film deposited on its surface, for example a gold film. This metallic film is notably deposited on the surface of the aforementioned pores.

The present invention also relates to a process for the preparation of a film of a polymer M1, comprising a step of thermal or chemical treatment, of a film of a polymer M1 containing spherical particles of a material M2, capable of be obtained according to the above-mentioned process, whereby the spherical particles are dissolved.

This heat or chemical treatment step therefore consists in dissolving the spherical particles and therefore makes it possible to obtain the polymeric film alone. Such a step may for example consist in treating the film of a polymer M1 containing spherical particles of a material M2 with an organic solvent or with a strong acid. Preferably, this step consists of treatment with hydrofluoric acid when M2 is SiO ₂ .

The present invention also relates to a polymeric film, capable of being obtained according to the method as described above.

The films obtained according to the invention can be used in the field of biology, surface optics (photonic, plasmonic crystal effect), mechanical sensors and microfluidics.

The process of the invention makes it possible to obtain micro- and / or nanostructured films and this structuring makes it possible to impart an optical effect to the films.

DESCRIPTION OF THE FIGURES

Figures 1,2, 3 and 4 are SEM pictures of microstructured PDMS.

Figure 1 shows a PDMS film with 10 layers of 1.2 µm diameter silica beads trapped inside. Figure 2 shows a film of

PDMS with 10 layers of "pores" (after treatment with HF). Figure 3 shows a channel structured in PDMS with 5 layers of beads. Figure 4 shows a channel structured in PDMS with 5 layers of "pores" (after treatment with HF).

FIG. 5 represents an embodiment of the process of the invention in which silica beads and PDMS are used.

Figures 6 and 7 show different embodiments of the method of the invention depending on the geometry of the support used (respectively flat surface and cylinder).

EXAMPLES

Example 1: Manufacturing of PDMS films

1. Obtaining a Langmuir film of silica particles:

A Langmuir film of silica particles (SiO ₂ ) is prepared as follows:

The silica particles used are synthesized according to the procedures described in the references Stober et al. J. Colloid Interface Sci. 1968, 26, 62-69; Reculusa et al. Chem. Mater., 2002, 14 (5), pp 2354-2359 or Reculusa et al. Chem. Mater., 2003, 15 (2), pp 598-605, from TetraEthoxysilane in a highly basic hydroalcoholic medium. These are then functionalized on the surface with an agent of the aminopropyltriethoxysilane type which gives the particles an amphiphilic character suitable for the formation of the Langmuir film. These particles can be stored in the form of an aqueous suspension.

In the following, objects of 1.2 micron in diameter were used obtained by a two-step process such as that described in Reculusa et al. Chem. Mater., 2002, 14 (5), pp 2354-2359.

The mass of beads necessary to form a compact film of desired area is removed, washed with ethanol several times by centrifugation cycles and then redispersed in fine in an ethanol / chloroform mixture (20/80 v / v). Thus, for beads of 1.2 μm in diameter, 80 mg of particles represent an area of approximately 500 cm ² . The volume of suspension, approximately 5 mL, is then spread (deposited dropwise) on the surface of a Langmuir tank (“Alternate layer Langmuir-Blodgett trough” by NIMA, model 622) filled with distilled water, including maximum working area is 1200 cm ² .

After evaporation of the solvent, the particles present on the surface are compressed using mobile barriers until they form a compact monolayer, generally obtained when a modification of the imposed surface pressure no longer induces movement of the barriers mobile. Usually, this value of the surface pressure is between 6 and 10 mN.m ' ¹ .

2. Deposition of the film on a substrate by Langmuir-Blodgett technique:

The support on which it is desired to deposit the particles is then treated differently depending on its nature.

In the case of the flat or cylindrical glass supports described here, they were used without prewash because they were kept away from dust and organic contaminants. As a precaution, exposure to an oxygen plasma, soaking in an acid solution (Piranha type) or degreasing with a solvent (ethanol, acetone, chloroform) can be used to ensure the cleanliness of the surface.

The support is then positioned on the articulated arm or the soaking system before being quickly and vertically immersed through the Langmuir film (generally 50mm.min-1). The support is then reassembled slowly (in general, 1 to 2 mm per minute) while maintaining a constant surface pressure to allow the transfer of the monolayer. By repeating the process n times, it is also possible to deposit n layers of particles on the support considered, for example with n = 5 or n = 10 on a cylindrical capillary (cf. Figures 1 and 3).

3. PDMS preparation:

In the examples presented below, the PDMS which was used is Sylgard 184 (Dow Corning) prepared from two components, an oligomer A and a crosslinking agent B, in a mass ratio A: B of 10: 1. It is however possible to incorporate more crosslinking agent (10: 2 or 10: 3 ratios, for example) to obtain a harder final material.

Oisigomers of Sîloxane Crosslinking agent of Sîloxane

A, ^c -P ⁸ ,, ^r

R is generally>: '· i' CH ₃ , sometimes H

In a first stage of preparation, the two components were added in the desired proportions (10g of A and 1g of B) in a container of suitable size. The mixture of precursors A / B was then vigorously stirred for 5 min and then degassed in a desiccator by drawing under vacuum until the bubbles have completely disappeared so that it becomes perfectly translucent. It is then ready to be used.

4. Formatting of the PDMS:

Two methods were used depending on the geometry and the size of the substrate:

• in the case of 7cm * 5cm glass slides, the slide is placed in a larger plastic container such as a square plastic petri dish. The substrate is laid flat so that the layer of beads is on the upper face. The appropriate volume (equal to L x I xe with L the width of the container, I the length of the container and e the thickness of the polymer film) of A / B mixture is then poured into the container so as to completely cover the surface of the substrate. It is also possible to use conventional coating techniques such as centrifugal coating (spin-coating) to form the finest films (tens of microns) (see Figure 5);

• in the case of cylindrical glass capillaries 5cm long and 0.5 to mm in diameter, the substrate, the surface of which is covered with particles, is placed in a parallelepipedal plastic tank of dimensions 10mm * 10mm * 50mm and kept as vertically as possible. The tank is then filled with the A / B mixture to the appropriate height (for this example, 4 mL of mixture have been added).

Once the substrate has come into contact with the A / B mixture, the assembly is returned to degassing in order to remove the air which would have possibly been incorporated and to allow the mixture to well infiltrate the deposit of particles. A heat treatment is then carried out to promote the crosslinking of the monomer and to form the PDMS proper. The duration of this heat treatment is directly linked to its temperature and corresponds to the following values:

- 48 to 72 hours at room temperature (20-25 ° C)

- 4h at 60 ° C

- 2h at 80 ° C

- 1 hour at 100 ° C

5. Release of the PDMS:

For flat structures, demolding is simple. First, we detached the substrate from the container (following the contours of the substrate using a scalpel) and then we lifted a corner of the polymer film to be able to slowly peel it off its support. If the desired PDMS thickness is 1 to 2 mm, the film is easily deformable.

For capillaries, we first remove the PDMS block by breaking the plastic tank. Then, the block is immersed in a solvent such as hexane or THF. After a residence time of approximately 1 h, the PDMS matrix is sufficiently swollen with solvent so that it suffices to gently pull on the capillary to detach it from the block. So that it returns to its initial dimension, the evaporation of the solvent can be accelerated by placing everything in an oven under vacuum for 1 hour at 100 ° C.

Example 2: Subsequent treatments of materials in PDMS

According to the protocols detailed above, the silica particles which were initially present on the surface of the substrates are trapped in the film or the PDMS block. However, it is possible to selectively dissolve the silica particles by briefly exposing them to dilute hydrofluoric acid. For this, a few drops of an aqueous solution of HF at 5% by volume are deposited on the surface of the film (in plane geometry) or the acid solution is brought up in the capillary channel (cylindrical geometry) and the acid is left at contact of the particles for a few minutes. The time varies depending on the thickness of the silica deposit, but just a few minutes are enough.

Once the treatment is complete, the PDMS is rinsed in water and then in ethanol in order to dry the material well. Whatever the geometry of the sample, Scanning Electron Microscopy makes it possible to ensure that the silica particles which were present before the treatment have indeed disappeared after it (cf. Figures 2 and 4).

It is possible to form gold nanoparticles on the surface of the microstructured PDMS by bringing the latter into contact with a 10 mg / ml solution of HAuCI ₄ 3H ₂ O in ethanol (or H ₂ O or THF). Thus, by circulating, by means of a peristaltic pump, in the channel in PDMS - or, in the case of planar geometries, by covering the surface with a 10 mg / mL solution of HAuCI ₄ 3H ₂ O in ethanol (or H ₂ O or THF) -, a violet color is obtained after a few minutes which reflects the formation of these nanoparticles which can then be used to form a thicker layer of gold. For this, it is sufficient to circulate continuously, always by means of a peristaltic pump - or to cover the surface with - a solution called electroless plating prepared from the following constituents:

o gold salt (HAuCI ₄ , 3H ₂ O) at a rate of 5mg / mL o potassium hydrogen carbonate KHCO ₃ at a rate of 50mg / mL o glucose at a rate of 5mg / mL o potassium hydroxide KOH (5.6.10 ³ mg / ml_) in order to adjust the pH towards 10

Claims (10)

1. A method of preparing a film of a polymer M1 containing spherical particles of a material M2, said method comprising a step of impregnating a substrate with a solution, said substrate consisting of a support of material M3 comprising at least one layer of spherical particles on its surface, said substrate being obtained by depositing said particles on said support by the Langmuir-Blodgett technique, and said solution comprising at least one oligomer and optionally at least one crosslinking agent. 2. Method according to claim 1, in which the material M3 is chosen from glass, plastic materials and metallic or semiconductor materials such as silicon, gold, copper, platinum, nickel or zinc. 3. Method according to any one of claims 1 or 2, wherein the polymer M1 is obtained by polymerization from at least one oligomer selected from the group consisting of styrene, siloxanes, and their mixtures, and is by for example polystyrene or PDMS, possibly loaded with nanoparticles or nanowires. 4. Method according to any one of claims 1 to 3, in which the thickness of the film of the polymer M1 is greater than or equal to 10 μm, preferably greater than or equal to 40 μm. 5. Method according to any one of claims 1 to 4, in which the material M2 is silicon dioxide (SiO ₂ ) or an organic polymer, the particles being optionally functionalized or surface modified. 6. Method according to any one of claims 1 to 5, comprising the following steps: a step of preparation by the Langmuir-Blodgett technique of a substrate consisting of a support of material M3 covered by at least one layer of spherical particles of a material M2; - a step of preparing a solution comprising at least one oligomer and optionally at least one crosslinking agent; and - A step of impregnating the deposit covering the substrate with said solution. 7. Method according to any one of claims 1 to 6, comprising 5 also a step of recovering the film of polymer M1 containing the spherical particles, in particular by separating it from the support. 8. Film of a polymer M1 containing spherical particles of a material M2, capable of being obtained according to the method according to any one of claims 1 to 7. 9. Micro- or nanostructured film comprising spherical pores, said pores being organized in compact form and at least one of the pores intercepting or flush with at least one surface of the film. 10. A method of preparing a film of a polymer M1, comprising a step of thermal or chemical treatment, of a film of a polymer M1 containing spherical particles of a material M2, capable of being obtained according to the process according to any one of claims 1 to 7, whereby the particles 20 sphericals are dissolved. 1/5 2/5 ZZ Ο Ο LL E ZL O O 3 < m ID 3/5 ω -π ΜΜΙ (Ν Ο vj LD ο LL. 4/5 Layer of KD O 5/5 "M" Λ dd □ u Έ ro Q, ® TS ® £ w □ δ î: o Q. tt. □ “Λ e Φ E ® 2 " "M ’Wî 3 *. □ · <Υ " o VS Ut <0 LJ 'VS ® G » E □ Sm Φ- * Qj O ® & 0 o υ irt Φ Qj U o VS fs 3 O XJ CU E E '® V ut vs o at o. TS E œ u -® 3 us U ί- Φ α. 3 > S < "VS œ > / ·. QJ T3 <U ~ G d) ® o Φ * e ® a, Ü -® a Z ® M ni Top view Top view Top view r ** O