EP3769302A1

EP3769302A1 - Porous acoustic phase mask

Info

Publication number: EP3769302A1
Application number: EP19712210.4A
Authority: EP
Inventors: Olivier Mondain-Monval; Thomas BRUNET; Olivier Poncelet; Yabin JIN; Raj Kumar; Samuel Marre; Artem KOVALENKO
Original assignee: Centre National de la Recherche Scientifique CNRS; Ecole National Superieure dArts et Metiers ENSAM; Universite de Bordeaux; Institut Polytechnique de Bordeaux
Current assignee: Centre National de la Recherche Scientifique CNRS; Ecole National Superieure dArts et Metiers ENSAM; Universite de Bordeaux; Institut Polytechnique de Bordeaux
Priority date: 2018-03-23
Filing date: 2019-03-25
Publication date: 2021-01-27
Also published as: FR3079340A1; US20210142778A1; FR3079340B1; WO2019180270A1

Abstract

The invention relates to an acoustic phase mask, the phase mask having a variation of the acoustic index n, characterized in that the phase mask comprises a body comprising: - at least one matrix formed by a deformable solid material, having a shear modulus of less than 10 MPa, and - pores formed in the matrix, the pores being predominantly filled with gas, the deformable solid material extending between the pores, the body having a porosity φ of less than or equal to 50%, and a controlled porosity φ gradient leading to a variation of the acoustic index n spatially in the body.

Description

POROUS ACOUSTIC PHASE MASK

FIELD OF THE INVENTION

The invention relates to an acoustic phase mask for the spatial manipulation of acoustic wave fronts, for example an acoustic lens for focusing an acoustic wave.

STATE OF THE ART

The spatial manipulation of acoustic wave fronts, and particularly the focusing of acoustic waves, is typically performed using metamaterial devices, especially in a frequency range corresponding to the audible and / or ultrasonic frequencies.

Zhu et al. (Zhu, H., & Semperlotti, F. (2015), Improving the performance of structured-embedded acoustic lenses via gradient-local index inhomogeneities, International Journal of Smart and Nano Materials, 6 (1), 1 - 13.) for example a device for focusing an ultrasonic wave propagating in an aluminum plate. Inhermalities or inclusions are formed by openings in the aluminum plate so as to form a waveguide, for focusing an initially radial ultrasonic wave. This method is difficult to transpose industrially to the manipulation of acoustic waves in three dimensions, and in other acoustic wave propagation media in which it is not possible to create openings.

Martin et al. (Martin, TP, Naify, CJ, Skerritt, EA, Layman, CN, Nicholas, M., Calvo, DC, ... & Sanchez-Dehesa, J. (2015), Transparent gradient-index lens for underwater sound based on phase advance, Physical Review Applied, 4 (3), 034003.) discloses a device comprising an anisotropic array of aluminum hollow cylinders arranged to form an acoustic index gradient n in the device. An incident sound acoustic wave, passing through the array of cylinders, is focused in predefined regions of space. The manufacture of such a device requires a precise and expensive mechanical assembly, limiting its industrial application. SUMMARY OF THE INVENTION

An object of the invention is to provide an acoustic phase mask that is easier to implement or to manufacture than the devices of the prior art.

These objects are achieved in the context of the present invention by means of an acoustic phase mask, the mask having a variation of the acoustic index n, characterized in that the phase mask comprises a body comprising:

at least one matrix formed of a deformable solid material having a shear modulus of less than 10 MPa, and

pores formed in the matrix, the pores being predominantly filled with gas, the deformable solid material extending between the pores,

the body having a porosity of less than or equal to 50%, and a controlled porosity gradient leading to a variation of the acoustic index n spatially in the body.

The invention may be advantageously completed by the following characteristics, taken individually or in any of their technically possible combinations:

the phase mask is an acoustic lens, the porosity gradient being such that the lens is able to focus an incident plane acoustic wave transmitted by the phase mask into at least one point of the space,

the phase mask has two opposite planar faces extending parallel to a main plane and having at least one porosity gradient f oriented in a direction parallel to the main plane,

the porosity is distributed in the body so as to correspond to an index n evolving linearly in the direction, in at least part of the phase mask,

the porosity is distributed in the body so as to correspond to an index n evolving hyperbolic manner in the direction, in at least a part of the phase mask,

the phase mask comprises a juxtaposition of layers comprising a matrix and pores, each layer having a constant porosity, the porosity of a layer being different from the porosity of a directly adjacent layer, the phase mask comprises a support having cavities, each cell containing a matrix, at least two matrices having different porosities,

the two opposite planar faces are separated by a thickness d, the thickness d being between 100 μm and 10 mm. Indeed, this range of thicknesses d is suitable for the manipulation of acoustic waves having a wavelength of between 100 kHz and 10 MHz.

Another aspect of the invention relates to a method for handling acoustic wave fronts, comprising a step of installing an acoustic phase mask previously described in the propagation space of an incident plane acoustic wave having a length wave l.

The phase mask may advantageously have a thickness d according to a direction of propagation of the incident acoustic wave, the thickness d being strictly less than the wavelength λ.

Another aspect of the invention relates to a method of manufacturing a phase mask described above, the method comprising steps of:

forming a plurality of emulsions, each emulsion having on the one hand a first liquid phase and, on the other hand, a second phase comprising monomers and at least one type of surfactant, so as to form drops of the first liquid phase in the second phase, at least two emulsions having respective fractions in first phase different,

crosslinking the monomers of the emulsions so as to form a deformable solid material defining the matrix or matrices and the pores comprising the first liquid phase,

drying to remove the first liquid phase so as to fill the majority of the gas pores.

the drying step is a supercritical drying step of the first liquid phase, the first liquid phase comprises, during the step of successively supercritical drying of water, a liquid chosen from ethanol and acetone, and carbon dioxide,

the first liquid phase comprises a liquid compound adapted to decompose spontaneously at room temperature into a gas and a liquid, and in which, during the drying step, the decomposition of the liquid compound is expected so as to form a phase gas in the pores,

the compound is hydrogen peroxide,

the crosslinking of the monomers is carried out by exposing the emulsions to ultraviolet radiation.

DESCRIPTION OF THE DRAWINGS

Other features and advantages will emerge from the description which follows, which is purely illustrative and nonlimiting, and should be read in conjunction with the appended figures, among which:

FIG. 1 illustrates a phase mask according to one embodiment of the invention,

FIGS. 2a, 2b and 2c illustrate a method of manufacturing a porous material,

FIG. 3 illustrates a supercritical drying step,

FIGS. 4a, 4b and 4c are photomicrographs of porous materials,

FIG. 5 is a diagram illustrating the evolution of the porosity of a porous material as a function of the volume fraction in first dispersed phase after drying for different drying methods,

FIG. 6 illustrates the manipulation of a plane wave incident by a phase mask according to one embodiment of the invention,

FIG. 7 illustrates the evolution of the longitudinal velocity of an acoustic wave in a porous elastomeric material according to one embodiment of the invention as a function of the porosity of the porous elastomeric material,

FIGS. 8a, 8b, 8c, 8d, 8e and 8f illustrate a method of manufacturing a phase mask according to one embodiment of the invention,

FIGS. 9a, 9b, 9c and 9d illustrate a method of manufacturing a phase mask according to one embodiment of the invention, FIGS. 10a and 10b illustrate the deflection of a plane wave incident at a predetermined angle, as well as the phase mask adapted for this deflection,

FIGS. 11a and 11b illustrate the focusing of an incident plane wave towards a predetermined focal point, as well as the phase mask adapted for this focusing,

FIGS. 12a, 12b and 12c illustrate experimental measurements enabling the deflection and focusing of acoustic waves produced by phase masks according to embodiments of the invention.

DEFINITIONS

The porosity of a porous material will be defined as the ratio between the pore volume of the porous material and the total volume of the porous material (i.e. sum of the pore volume of the porous material and the volume of solid material extending between the pores).

"Manipulation" of an acoustic wave (or acoustic wavefront) refers to any deliberate and controlled modification of said front (local phase, amplitude and / or polarization), for example the deflection or focusing of an acoustic wave. .

The term "phase mask" denotes any device that locally modifies the phase of an incident wave passing through it. Preferably, the phase mask is a planar device.

The term "emulsion" denotes a mixture of two immiscible liquid substances, one of which is dispersed in the form of drops in the other in a homogeneous manner.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

General architecture of an acoustic phase mask

Figure 1 illustrates a section of an acoustic phase 1 mask. The phase mask 1 comprises a body 2. The body 2 allows a simplified use of the phase mask 1, with regard to other known devices, such as obstacle networks or transducer networks. The body 2 comprises at least one matrix 3, formed in a deformable solid material, and pores 4 formed in the matrix 3. The majority of the pores 4 are filled with gas, which allows the body 2 to be compressible (compressibility is between 10 ⁹ Pa ¹ for the body 2 non-porous to 10 ⁶ Pa ¹ for the body 2 having 40% pores filled with gas). The deformable solid material extends between the pores 4. This material has a shear modulus preferably less than 10 MPa.

The pores 4 give the body 2 a local porosity. The body 2 has a porosity of less than 50%, that is to say that the local porosity at any point of the body 2 is less than 50%. In other words, the maximum porosity of the body 2 is less than 50%.

The body 2 has a porosity gradient f. This porosity gradient in the body 2 is controlled and causes a spatial variation of the acoustic index n in the body 2. The variation of the acoustic index n, or the acoustic index gradient n, makes it possible to cause a modulation incident wavefront during its passage through the phase mask 1, allowing for example to focus an incident wave, particularly an incident plane wave.

FIG. 1 illustrates a phase-1 mask in section: the phase mask 1 has two opposite planar faces extending parallel to a main plane 7 and the body 2 of the phase-1 mask has a porosity gradient oriented in a parallel direction 7. With reference to FIG. 1, the main plane 7 is parallel to the plane defined by the x and y axes, and the gradient is oriented along the x axis. More specifically, the body 2 has a porosity f decreasing along the x axis.

Manufacture of a porous material

The body 2 is at least partly made of porous materials, that is to say a material comprising the matrix 3 of deformable solid material, and the pores 4, the deformable solid material of the matrix 3 extending between the pores 4 The porous material is produced by polymerization of an aqueous emulsion in a polymerizable solvent, for example thermally or by irradiation under UV and then by drying.

The deformable solid material comprises elastomeric polymers. These polymers have transition temperatures vitreous less than room temperature. In particular, the polymers of the deformable solid material have a glass transition temperature below -50 ° C., preferably below -80 ° C., and preferably below -100 ° C.

Due to the high porosity of the porous material, the filling of the pores 4 mainly with gas, and the deformability of the solid material, the body 2 may have a higher compressibility than the known porous materials.

The acoustic index n of a material can be defined by the formula (1): n - ^c ref / ^c mat 0) where C _re f and c _mat are respectively the speeds of propagation (or celerities) of the longitudinal acoustic waves in a reference material, i.e. water in the invention, for which _{cre /} = 1500 ms ¹ , and in the material under consideration. As the velocity of propagation of an acoustic wave in a material depends on the porosity of the material, the acoustic index n depends on the porosity. A porosity gradient of the porous material thus leads to an acoustic index gradient n in the body 2 of the phase 1 mask.

The porous material may have a wide range of propagation velocity. Indeed, the speed of propagation of sound in a material can be written c _mat = (M / p) ^1/2 , where p is the density of the material and M the elastic modulus of compression of the material. The velocity of an acoustic wave in the porous material decreases when both the elastic modulus of compression of the porous material decreases and the density of the porous material increases.

Thus, the porosity of the porous material can be adjusted to have propagation speeds of typically between 10 ms ^-1 and 1000 ms ¹ . The known devices do not make it possible to achieve variations in propagation speeds with such a high amplitude.

With reference to FIGS. 2a, 2b and 2c, the manufacture of a phase 1 mask, and in particular the porous material of one or more matrices 3 of the phase 1 mask, comprises the steps of: a) forming a plurality of emulsions 12, each emulsion 12 having, on the one hand, a first liquid phase 13 and, on the other hand, a second phase 14 comprising monomers and at least one type of surfactant, in order to form drops of the first liquid phase in the second phase 14.

At least two emulsions 12 have different first-phase respective fractions. The step of forming an emulsion is illustrated in FIG. 2a.

b) crosslinking the monomers of the emulsions 12 so as to form a deformable solid material 3 defining the matrix or matrices 3 and the pores 4 containing the first liquid phase 13.

The crosslinking step is illustrated in FIG. 2b.

c) drying the porous material obtained in step b) to remove the first liquid phase so as to fill most of the pores 4 gas.

The drying step is illustrated by Fig. 2c.

In step a), each inverse emulsion 12 is made between, on the one hand, a second phase 14 comprising monomers and a suitable surfactant, and on the other hand a first aqueous phase. The emulsion

12 can be achieved using a shearing device (eg Rayneri, Ultraturrax, or any mechanical device allowing sufficient shear of the two phases). The emulsion can also be formed by exposing the first phase 13 and the second phase 14 to ultrasonic waves. The mother emulsion is emulsion 12 thus obtained. The volume fraction of the mother emulsion 12 in the first phase 13 can be between 0% and 90%. The choice of the surfactant is adapted to the monomers chosen in the second phase 14. In general, the surfactant has an HLB number of less than or equal to about 8. Thus, an inverse emulsion 12, that is, to say including drops of first phase

13 in a second lipid phase is favored. The diameter of the first phase drops 13 formed in the second phase 14 is typically between 0.1 and 100 μm.

In step b), the emulsions 12 are deposited in one or more containers. At least two emulsions 12 have different first-phase respective fractions. The fraction in first phase 13 can also vary in a controlled manner during the deposition of an emulsion 12 in a container, forming a plurality of emulsions 12 in a continuous manner. The container may be for example a mold or a cell. A wall of the container may be formed by an emulsion 12 already crosslinked. Once deposited, the monomers of the second phase 14 of the emulsion 12 are crosslinked so as to form a deformable solid material. Crosslinking of the monomers may be preferably carried out by exposure of the monomers to ultraviolet radiation or heating. At the end of step b), one or more matrices 3 are obtained in deformable solid material. Pores 4 are formed by the matrix or matrices 3. These pores 4 are filled with first phase 1 3.

During step c), the matrix or matrices 3 are dried. This step makes it possible to replace, at least in majority and preferably completely, the first liquid phase 13 contained in the pores 14 by gas. Typically, the drying of the first phase 13 is carried out by pervaporation of the first liquid phase 13 through the polymer matrix 3.

In the known drying methods, a drying front propagates in the matrix 3, and more particularly at the interface between the matrix 3 and the pores 4. The matrix 3 is then subjected to a drying pressure P _drying equal to two the surface tension g of the first liquid phase with the air, divided by the radius r of the micropores by which the first phase 13 is evaded during the _evaporation , or P _drying = 2 g / r. If it is difficult to know the exact value of r, we can estimate that r is typically less than or equal to 1 nm. Thus, for a first phase 13 of water (g = 72 mN / m), P _drying = 144 MPa. This indicative value is greater than the typical shear moduli of the deformable solid material, elastomeric polymers. As a result, pore collapse is observed during the implementation of known drying methods, and the porosity of a porous material is thus limited because no gas replaces the disappearance of the first phase 1 3 in the pores 4.

According to one aspect of the invention, the drying step is a supercritical drying step of the porous material for removing the first liquid phase. The supercritical drying method is known for drying porous brittle materials, such as aerogels. She is by example described by Marre et al. (Marre, S., made Aymonier, C. (2016), Preparation of Nanomaterials in Flow and Supercritical Conditions from Coordination Complexes in Organometallic Flow Chemistry (pp. 177-211), Springer, Cham.). During supercritical drying, a liquid phase contained by the pores 4 is transformed into a gaseous phase, without phase transition, by imposing temperature and pressure conditions making it possible to bypass the critical point of the compound (s) contained in the pores. The absence of passage through a phase transition line makes it possible to avoid a drying front between a liquid phase and a gaseous phase. Thus, the drying pressure is decreased or equal to zero, and it is possible to avoid crushing the porous material on itself during drying.

The drying fluid used for supercritical drying can be C0 ₂ . The C0 ₂ has a critical point corresponding to a pressure Pc = 73.9 atm and a temperature of Te = 31 ° C. These conditions of temperature and pressure are easy and economical to implement.

Figure 3 illustrates the supercritical drying of the porous material using CO2. Following step b), the pores 4 contain a first aqueous phase. The first phase 13 is first exchanged with liquid ethanol. Thus, the liquid contained in the pores 4 is miscible with the CO2. The exchange of the first phase 1 3 with a liquid phase of ethanol is carried out by immersing the porous material in an aqueous solution bath which is progressively enriched in ethanol by a system of pumps at ambient temperature and pressure. The exchange occurs gradually, on a time scale adapted to avoid imposing excessive mechanical stresses on the solid deformable material.

The ethanol is then extracted with CO2. The extraction is carried out by placing the porous material bathed in pure ethanol in a high pressure reactor, in which the pressure and temperature conditions can be adjusted by means of injection pumps and a pressure regulator. output. The temperature of the reactor is first adjusted beyond the theoretical critical temperature of the CO 2 / ethanol mixture (ie between 45 and 50 ° C. for a 90/10 molar composition) while the reactor is slowly pressurized with CO2. up to a value greater than the critical pressure of the CO 2 / ethanol mixture (ie 1 10 bar). These variations in pressure and temperature correspond to the trajectory shown in dotted lines starting from point A and going to point B in FIG.

The C0 ₂ is then pumped continuously through the porous material at a constant flow rate (11 g / min), while the operating conditions are kept constant (the pressure is controlled by means of an outlet pressure controller). C0 ₂ mixes with ethanol and forms a supercritical monophasic mixture. During this mixing phase, the ethanol contained in the pores 4 is gradually replaced by a C 2 / ethanol supercritical mixture which progressively enriches with CO2. At the same time, the fluid ethanol / CO ₂ mixture is extracted from the reactor in order to maintain a constant internal volume of fluid. Once all the ethanol is replaced by CO2, the pressure of the system is slowly reduced to 1 bar, for example in one hour, so as to bring the CO2 gas phase without ironing the liquid state. This pressure variation corresponds to the trajectory shown in dotted lines starting from point B and going to point I in FIG.

Finally, the temperature is decreased to room temperature. This variation in temperature corresponds to the trajectory shown in dashed lines starting from point I and going to point A in FIG. 3. Thus, the fluid contained in the pores 4 is continuously replaced by a gas, avoiding the appearance of a triple solid / liquid / gas interface (non-zero surface tension) in the pores 4.

According to another aspect of the invention, the first liquid phase comprises a liquid compound adapted to decompose spontaneously at ambient temperature into a gas and a liquid. The kinetics of decomposition of the liquid into gas and liquid product can be fixed by the proportion of liquid capable of decomposing in the dispersed phase. This kinetics can be adjusted so that the characteristic gas bubble onset time is typically slower (i.e. typically greater than 30 minutes) than the time required for emulsification. During the drying step, the decomposition of the liquid compound is expected so as to form a gas phase in the pores 4. The compound 1 may be hydrogen peroxide H2O2. Hydrogen peroxide decomposes at constant ambient temperature and pressure into water (liquid) and gaseous oxygen. The proportion of H 2 O 2 in the first liquid phase may preferably be 1/3 in total mass of the first phase 13. For a proportion of H2O2 in water of 1/3, the characteristic time of appearance of the gas bubbles is of the order of 30 minutes. This kinetics can be slowed down or accelerated by varying this proportion or by adding a controlled concentration catalyst in the dispersed phase (for example iodide ions I which are known to catalyze the reaction of decomposition of H2O2 into oxygen and water). This method makes it possible to compensate for the pressure potentially exerted during drying by contact lines in the pores 4 by an increase in the gas pressure caused by the decomposition of the compound. It is thus possible to avoid the collapse of the pores 4 on themselves during the drying of the porous material. This method does not require external control of the pressure and / or temperature imposed on the porous material. Thus, the use of the compound makes it possible to dry the porous material by using a simpler and less expensive material than during a supercritical drying. The drying by introduction of the compound is for example carried out by placing the porous material in an oven, in which the temperature is controlled at 40 ° C. under ambient atmosphere.

The two drying processes described above (supercritical drying and introduction of a compound in the first liquid phase 13) make it possible to obtain a porosity of the porous material substantially equal to the volume fraction in the first phase 13 obtained during step a) of the process.

Figures 4a, 4b and 4c are photomicrographs obtained by scanning electron microscopy, illustrating porous materials of the body 2, having different porosities. With reference to FIG. 4a, the porosity f of the porous material of the body 2 is substantially equal to 5%. With reference to FIG. 4b, the porosity f of the porous material of the body 2 is substantially equal to 10%. With reference to FIG. 4b, the porosity f of the porous material of the body 2 is substantially equal to 15%.

FIG. 5 illustrates the evolution of the porosity of a porous material after a drying step carried out according to a known method (illustrated by curve (a)), a supercritical drying step (illustrated by curve (b)) and by a drying step by introducing a compound into the first phase 13 (illustrated by the curve (c)). The drying methods illustrated by curves (b) and (c) make it possible to obtain a porous material having a porosity f substantially equal to the volume fraction of the first phase 13 in the emulsion 12. In contrast, when using a drying by a known method (for example a simple drying in an oven), the collapse of the matrix 3 on itself prevents an increase of the porosity above a threshold value (substantially 10%) and thus limits a possible acoustic index gradient n of the body 2.

Examples of manufacture of the porous material

Example 1

The second phase 14 comprises silicone oil Silcolease UV poly 200 from Bluestar Silicones, 4% by weight of catalyst Silcolease UV cata 21 1 from Bluestar Silicones, 0.4% by weight of surfactant (2-octyl-1-dodecanol) and 200 ppm of Rahn Genocure ITX. The first phase 13 comprises 1.5% by weight of sodium chloride. The amount of aqueous phase incorporated in the organic phase is dependent on the desired porosity for the porous material. The formation of an emulsion is carried out in a mortar by adding the first phase 13 dropwise during shear, and then it is refined, either using blade tools (Rayneri or Ultraturrax type), either by ultrasound. The crosslinking step is carried out by exposing the emulsion to ultraviolet radiation using the Dymax BlueWave 200 lamp. The porous material is then dried by supercritical drying or by introducing a compound as described above.

Example 2

The second phase 14 comprises 64% by weight of ethylhexyl acrylate, 5.5% by weight of styrene, 10.5% by weight of divinylbenzene and 20% by weight of SPAN 80 surfactant. sodium chloride 25.10 ³ mol / L and potassium peroxodisulfate 5.10 ³ mol / L. The amount of first phase 13 incorporated into the organic phase is dependent on the desired porosity for the final material. The formation of an emulsion is carried out using a Rayneri type blade tool by adding the first phase 13 dropwise during shearing. The crosslinking of the monomers is carried out by heating at a temperature of 60 ° C. The porous material is then dried by supercritical drying or by introduction of a compound, as described above. Example 3

A deformable silicone-based solid material (denoted SiVi / SiH) can be obtained by thermal polymerization of the PDMS via a hydrosilylation reaction. The second phase 14 comprises 8.8 g of PDMS-vinyl (BLUESIL FLD 621 V1500), 1.8 g of PDMS-silane (BLUESIL FLD 626V30H2.5) and 0.352 g of platinum catalyst (SCLS CATA1 1091 M), (BlueStar Silicones Company). In order to be able to prepare the emulsions 12 before the crosslinking of the monomers, 4.4 mg of polymerization retarder (1-ethynyl-1-cyclohexanol, ECH of Sigma Aldrich) are added. To stabilize the emulsion, 2-octyl-1-dodecanol or Silube J208-812 can be used. The emulsions 12 are prepared by introducing a first aqueous phase comprising 1.5% by weight of NaCl with stirring. The emulsion is then poured into a Teflon mold and then heated at 60 ° C for 24 hours. The porous material is then dried by supercritical drying or by introducing a compound as described above.

Fabrication of an acoustic phase 1 mask

Due to the presence of a porosity gradient leading to a spatial variation of the acoustic index n in the body 2, it is possible to locally control the celerity of the acoustic waves and thus to bend the acoustic rays by mirage effect (version 3D of the gradient medium) or to control delays / phase advances at wave fronts (2D version of these media, for example a phase 1 mask). Thus, it is for example possible to concentrate the acoustic beams, that is to say, focus them, to deflect the acoustic beams and / or to separate the acoustic beams. The term "manipulation" of the acoustic wavefront is understood to mean at least one of the effects previously described on a plane incident acoustic wave.

Referring to Figure 6 and according to one aspect of the invention, the phase mask 1 (shown in section in Figure 6) may have a subwavelength thickness. The phase mask 1 has two opposite planar faces extending parallel to the main plane 7 and has at least one porosity gradient f oriented in the direction 8 parallel to the plane principal 7. The thickness d denotes the distance between the two opposite planar faces. The manipulation of an incident plane acoustic wave front plane 6, of length l can be implemented with a phase mask 1 of a thickness d strictly less than the wavelength 1. Preferably, the incident wavelengths 6 are between 100 kHz and 10 MHz, in particular for water as a surrounding medium. Thus, the thickness d of the phase mask is preferably between 100 μm and 10 mm.

FIG. 6 illustrates an incident plane acoustic wave 6 presenting a simple physical wavefront (uniform / flat for example) at the input of the phase 1 mask. The transmitted wave 19 or target wave 19 has a wavefront different from that of the incident plane wave 6 at the output of the phase mask 1. The output wavefront results in a "target" volume acoustic field (a converging field, for example, for focusing).

FIG. 6 also illustrates a method using a phase 1 mask for generating a sound pressure target field p _c (at non-planar phase fronts) from an incident plane wave 6 or an excitation plane front ( by an ultrasonic transmitter for example).

In general, so that the output pressure field is as close as possible to the target field (chosen by a user), the phase mask 1 is manufactured so that the transmission of a plane wave incident 6 by the mask phase 1 reproduces exactly at the output of the phase mask 1 the selected target field, that is to say in z = 0, and preferably on a surface of the plane defined by the axes x and z greater.

The target pressure field may, for example, be a harmonic field of pulsation w. The target pressure can be written is the pressure amplitude, O _c the target phase, and t the time. The acoustic field P _m immediately at the output of the phase mask 1 is equal to phase mask 1 makes it possible to impose the phase of the wave transmitted directly at the output of the phase 1 mask, which makes it possible to establish the relationship _C = F (x, y, z = oj. U is possible to consider that the phase mask 1 presents an acoustic index n variable in the xy plane and constant in its thickness d, supposed small compared to 1, that is to say to consider that n depends on x and y. The phase mask locally shifts the field m (x, y) k _Q d

incident of an amount 6 at each point of the output of the phase mask 1 (x, y, z = d) such that

. The incident plane wave 6 may correspond to a uniform incident phase front in the xy plane i.e. _inc x V ^z = = F ₀ (which can be set arbitrarily equal to 0). Thus, the spatial distribution of the acoustic index n of the phase mask 1 must verify the formula: to be adapted to generate the target field P _c . The distribution of the porosity in the phase mask 1 is thus chosen so as to produce a phase 1 mask having an acoustic index n satisfying the formula (2).

FIG. 7 illustrates the evolution of the celerity of an acoustic wave in the phase 1 mask with the porosity of the body 2. The measured celerities correspond to an acoustic index n of between approximately 1.5 and 40. The phase mask 1 is, in general, adapted to have an acoustic index between 1, 5 and 40.

With reference to FIGS. 8a, 8b, 8c, 8d, 8e and 8f, the body 2 of the phase mask 1 can be manufactured by stacking layers 9, each layer 9 comprising a matrix 3 and pores 4 and having a constant porosity f , the porosity of a layer 9 being different from the porosity of a layer 9 directly adjacent.

With reference to FIG. 8a, a first emulsion 12 is deposited in a mold 19 comprising a support of polytetrafluoroethylene (PTFE) and two transparent side walls.

With reference to FIG. 8b, the monomers of emulsion 12 are crosslinked by exposing emulsion 12 to UV radiation. This exposure is possible thanks to the transparent walls 20. Thermal crosslinking of the emulsion 12 is also possible. The thickness d of the mold (distance between the two walls 20) can be between 0.5 mm and 5 mm when the crosslinking is carried out by UV. The thickness may be greater when the crosslinking is carried out by heating.

With reference to FIG. 8c, a first phase volume fraction emulsion 12 different from that of the emulsion 12 described in FIG. 8a, for example greater, is deposited in the mold 19 on the crosslinked layer 9 described in FIG. 8b.

With reference to FIG. 8d, the monomers of the emulsion 12 described in FIG. 8c are crosslinked so as to form two juxtaposed layers 9.

The different layers 9 can be dried before each deposit of a new emulsion.

With reference to FIG. 8e, the various layers 9 can be extracted from the mold 19. The different juxtaposed layers 9 thus form the body 2 of a phase 1 mask.

FIG. 8f is a front view photograph of a phase 1 mask manufactured according to the method described in FIGS. 8a to 8e. The phase mask 1 thus has a porosity gradient, the porosity being maximum in the middle of the phase 1 and minimum mask at the low and high ends of the phase 1 mask. The dotted lines correspond to the boundaries between the different layers 9.

The layers 9 deposited have a height h smaller than the wavelength 1 of the incident acoustic wave 6. For example, for a frequency of 100 kHz, the wavelength 1 of an incident plane acoustic wave 6 is 15 mm. The layers 9 have a height equal to 8 mm (ie about 1/2), which is sufficient for the acoustic index gradient n is actually perceived as continuous for the incident plane wave 6. It is obviously possible to reduce the width of the strips, for example up to about 1 mm.

Referring to Figures 9a, 9b, 9c and 9d, the body 2 may comprise a support 10 having cells 1 1, each cell 1 1 containing a matrix 3, at least two of the matrices 3 having different porosities.

With reference to FIGS. 9a and 9b, the support 10 may for example be manufactured by 3D printing. The parameters of the 3D printing are chosen so as to manufacture a support 10 in which cells 1 1 are delimited by thin thicknesses (typically about 100 micrometers) of polylactic acid (PLA), polyamide (PA) or any other printable polymer. The emulsions 12 of different volume fractions in the first phase 1 3 are introduced into the cells 1 1.

With reference to FIG. 9c, the monomers of all the emulsions 12 may be crosslinked simultaneously, for example by exposure to UV through a wall 20. The phase mask 1 may comprise the support 10. In fact, the thickness of the support 10 may be typically 0.1 mm, and considered negligible compared to the acoustic wavelengths. The acoustic experiments carried out show that the presence of such a support 10, not filled with material, introduces no change in the acoustic field.

With reference to FIG. 9d, the phase mask 1 comprises the body 2, the body 2 comprising a succession of strips or layers 9 and the support 10. The body 2 has a porosity gradient due to the formation of the different matrices 3 of different porosities.

Manipulation of acoustic wave fronts

The phase mask 1 may comprise a succession of strips or layers 9, as described above.

With reference to FIGS. 10a and 10b, the phase mask 1 may make it possible to modify the propagation angle θ between the propagation direction of an incident plane acoustic wave 6 and that of a transmitted wave 19, ie an angle of deflection Q with respect to the main plane 7.

The incident incident plane wave 6 of incident field P _in is ^{nsi have} transformed into plane wave transmitted or deflected 19 of target field

_ ik _Q sin qc ik _Q cos q _z - jui

P _c - P ₀ ^eee The spatial distribution of n satisfies being the index at the center of the mask of

Phase 1 . The gradient is preferably constant, which corresponds to a linear evolution of the porosity of the body 2 in space. In this case, it is equal to sin 9 / d and oriented along the x axis.

With reference to FIGS. 11a and 11b, the phase mask 1 can be used to focus an incident plane acoustic wave 6. The plane wave incident 6, in normal incidence with the phase mask 1, to manipulate

_ ik ₀ z -iut

the field P _inc - P _Q e is thus transformed into a convergent cylindrical wave at the focal point of coordinate z = F with respect to the phase mask 1 whose output face corresponds to the coordinate z = 0. Using a field approximation far, the target field is expressed in the form

spatial distribution of the index n in the phase mask 1 is thus given with reference to FIG. 1 1 b, the index

acoustic n evolves hyperbolic way in a part of the body 2.

In all of the embodiments, it is possible to match the porosity of the body 2 with a given index n, as described above, by using the measured relation between the speed of the acoustic wave and the porosity of the material traversed. by the acoustic wave, illustrated in Figure 7.

Referring to Figures 12a, 12b and 12c, the deflection, and in particular the focusing of an incident plane wave 6 by a phase mask 1 are tested. Phase 1 masks with a thickness equal to 2 mm are deposited on the surface of an ultrasonic transducer (supplied by the company Imasonic) emitting at a center frequency of 1 50 kHz and having side dimensions of 150 mm × 40 mm in the plane defined by the x and y axes. The whole is immersed in a tank filled with water allowing measurements in immersion, as performed in underwater acoustics. The ultrasonic transducer is positioned in the upper part of the tank and its active face is oriented towards the bottom so as to generate an incident plane wave propagating from top to bottom along the z axis. The ultrasonic transducer is powered via a function generator (provided by Agilent) to generate in the water an ultrasonic wave train (30 cycles) centered at 150 kHz. The sound pressure emitted is then mapped in the central zone of the field near this transducer by means of a needle hydrophone having a diameter of 1 mm (supplied by Accuracy Acoustics) in the XZ plane (60 mm x 100 mm). The pitch in x and z between each measurement is 2 mm, that is to say 5 times smaller than the wavelength of the ultrasound used. The time signals were recorded by means of an acquisition card (provided by Alazartech) with a sampling frequency of 1 MHz over a duration of 300 ps for each measurement position.

With reference to FIG. 12a, the transducer is not covered by a phase 1 mask. The wave fronts are planar, parallel and horizontal, and are characteristic of a plane wave propagating vertically from top to bottom of the tank as expected along the z axis.

With reference to FIG. 12b, the transducer is covered with a phase 1 mask. The phase mask 1 comprises a body having a constant gradient of acoustic index (ie a variation of n linear acoustic index). The plane, parallel and inclined wavefronts show a deflection of the ultrasonic waves related to the presence of the phase mask 1 at the surface of the ultrasonic transducer. As theoretically anticipated, the deflection angle θ of the ultrasonic beam is connected to the index gradient and the thickness of the porous material by 2 mm, and is substantially equal to 5 °.

With reference to FIG. 12c, the transducer is covered with a phase 1 mask. The phase mask 1 comprises a body having a variation of n hyperbolic acoustic index. The mapping of the diffracted acoustic field in FIG. 12c illustrates the existence of a small central zone whose width is slightly smaller than the wavelength (ie approximately 10 mm) in which the energy of the acoustic beam is concentrated. Moreover, the convergent (and divergent) curved wave fronts respectively visible above and below this focal task underline the focusing effect of the phase 1 mask.

Claims

Acoustic phase mask (1), the phase mask (1) having a variation of the acoustic index n, characterized in that the phase mask (1) comprises a body (2) comprising:

at least one matrix (3) formed of a deformable solid material having a shear modulus of less than 10 MPa, and

pores (4) formed in the matrix (3), the pores (4) being filled with gas, the deformable solid material extending between the pores (4),

the body (2) having a porosity of less than or equal to 50%, and a controlled porosity gradient leading to a spatial variation of the acoustic index n in the body (2).

2. Phase mask (1) acoustic according to claim 1, the phase mask (1) being an acoustic lens (5), the porosity gradient being such that the lens is able to focus an incident plane acoustic wave (6) transmitted by the phase mask (1) in at least one point of space.

3. phase mask (1) acoustic according to claim 1 or 2, the phase mask (1) having two opposite planar faces extending parallel to a main plane (7) and having at least one gradient of porosity f oriented according to a direction (8) parallel to the main plane (7).

4. Acoustic phase mask (1) according to claim 3, in which the porosity is distributed in the body (2) so as to correspond to an index n evolving linearly in the direction (8), in at least a part phase mask (1).

5. Acoustic phase mask (1) according to claim 3 or 4, wherein the porosity is distributed in the body so as to correspond to an index n evolving hyperbolic manner in the direction (8), in at least a part of the phase mask (1).

6. acoustic phase mask (1) according to one of claims 1 to 5, comprising a juxtaposition of layers (9) comprising a matrix (3) and pores (4), each layer (9) having a constant porosity f , the porosity of a layer (9) being different from the porosity of a layer (9) directly adjacent.

7. Acoustic phase mask (1) according to one of claims 1 to 6, comprising a support (10) having cells (11), each cell (11) containing a matrix (3), at least two matrices (3). ) with different porosities.

8. phase mask according to one of claims 3 to 7, wherein the two opposite planar faces are separated by a thickness d, d being between 100pm and 10mm.

9. A method of manufacturing an acoustic phase mask (1) according to one of claims 1 to 8, the method comprising steps of:

- forming a plurality of emulsions (12), each emulsion (12) having on the one hand a first phase (13) liquid, and secondly, a second phase (14) comprising monomers and at least one type of surfactant, so as to form drops of the first phase (13) liquid in the second phase (14), at least two emulsions (12) having respective fractions in first phase (13) different,

crosslinking the monomers of the emulsions (12) so as to form a deformable solid material (3) defining the matrix or matrices and the pores (4) comprising the first liquid phase (13),

- Drying to remove the first phase (13) liquid so as to fill the majority of the pores (4) of gas.

10. A method of manufacturing an acoustic phase mask (1) according to claim 9, wherein the drying step is a supercritical drying step of the first phase (13) liquid.

1. A method of manufacturing an acoustic phase mask (1) according to claim 10, wherein the first liquid phase (1 3) comprises, during the supercritical drying stage successively water, a selected liquid. among ethanol and acetone, and carbon dioxide.

12. A method of manufacturing an acoustic phase mask (1) according to claim 10, wherein the first liquid phase (13) comprises a liquid compound adapted to decompose spontaneously at room temperature into a gas and a liquid, and in which, during the drying step, decomposition of the liquid compound is expected to form a gaseous phase in the pores (4).

A method of manufacturing an acoustic phase mask (1) according to claim 12, wherein the compound is oxygenated water.

14. A method of manufacturing an acoustic phase mask (1) according to one of claims 9 to 13, wherein the crosslinking of the monomers is carried out by exposing the emulsions to ultraviolet radiation.