### Results (2008-2013)

*1) Multistable nanostructures; Ultra-low power NVRAM; Towards new paradigms for information processing*

*1) Multistable nanostructures; Ultra-low power NVRAM; Towards new paradigms for information processing*

1.A – New concept of Nonvolatile MagnetoELectric Random Access Memory (MELRAM)

*2) Critical State (CS) materials, bosonic micro-nano-structures & composites; Towards functional electronics and theragnostics*

*2) Critical State (CS) materials, bosonic micro-nano-structures & composites; Towards functional electronics and theragnostics*

2.A – New phenomena of multi-phonon coupling and supercritical explosive instability of ultrasound in solids

2.B – Tunable and reconfigurable Quasi-Phononic crystals

2.C – Liquid flow velocimetry using Wave phase conjugation

2.D – Nonlinear imaging of defects and contact problems

2.E – Nonlinear behavior of polymeric chains

2.F – Composite materials and structures

*3) Flexible electronics*

*3) Flexible electronics*

3.A – Agile millimeter wave antennas & phase shifters

*4) Functional micro-fluidics, interface dynamics & MEMS for active flow control*

*4) Functional micro-fluidics, interface dynamics & MEMS for active flow control*

4.A – Microscale acoustofluidics

4.B – Faraday instabilities

4.C – Micro-jet actuators and micro-sensors for active flow control

#### 1.A – New concept of Nonvolatile MagnetoELectric Random Access Memory (MELRAM)

We proposed and patented a new concept of magneto-electric memory that aims at reducing the energy cost by two orders of magnitude relatively to the state of the arts. This work was done in the frame of the ANR PNANO project NAMAMIS and the CPER-CIA North Pas de Calais.

In this cell, the magnetization of a memory layer can be unequivocally switched between two orthogonal stable positions by an electric field in an artificial multiferroic structure. Such kind of film-film structures was elaborated by the group for the first time and demonstrated the record value of magnetoelectric coefficient at the date. In the memory cell, the electric field applied to a piezoelectric or ferroelectric relaxor induces a stress that is transmitted to a magnetostrictive material and affects its magnetization thanks to the magnetoelastic coupling. The electric writing ensures a high energy efficiency. This original approach has been acknowledged by the granting of a patent with a worldwide extension [1]. Theoretical study (based on the Eshelby theory) and finite element micromagnetic simulations were led to assess the performances of the system: for a typical magnetoelastic nanostructure subjected to realistic stress pulses, writing time is less than 1 nanosecond, and the required energy is less than 2000 k_{B}T, i.e. about a hundred-fold less than state of the art in magnetic memories. The energy barrier between the two states is sufficient for a 10-year retention time of the information.

An original theoretical analysis of destructive thermal effects [2], based on Langevin and Fokker-Planck techniques, has shown that the maximum ratio of thermal energy to the volume of a memory cell (k_{B}T/v)_{max} = 10^{3} J/m^{3} for an error probability equal to 10^{-8}. This result, confirms the operability of the nanoscale MELRAM at room temperature. Experimental validation has been shown at the macroscopic scale (Fig 1a,b) using off-the-shelf piezo-actuators [3,4]. Reading can be done with magnetoresistive structures such as magnetic tunnel junctions (MTJ) or Giant Magneto-Resistive (GMR) structures.

Efforts are now focused on the downscaling of the devices to show the integration possibilities. Multiphysics simulations show that patterned pillars of PMN-PT can transmit sufficient levels of stress into a magnetoelastic-based MTJ structure. Technological development of such a prototype is on-going in the clean room facilities of IEMN (Fig 1b).

Fig. 1: (a) MELRAM cell; (b) experimental evidence of electric field magnetisation switching in magnetostrictive nanostructure deposited on single crystal PMN-PT substrate; (c) sub-micrometre MTJ structures embedding magnetoelastic layers (deposition by RF-Sputtering and patterning by Ion Beam Etching).

[1] Patent WO2011158208 (A1) 2011-12-22

[2] S. Giordano, Y. Dusch, N. Tiercelin, P. Pernod, V. Preobrazhensky, EPJB, 86: 249 (2013)

[3] N. Tiercelin, Y. Dusch, A. Klimov, S. Giordano, V. Preobrazhensky, P. Pernod, *Appl. Phys. Lett., *99*, 192507 (2011)
[4] Y. Dusch, *N. Tiercelin, A. Klimov, S. Giordano, V. Preobra-zhensky, P. Pernod J. Appl. Phys., 113, 17C719 (2013)

#### 2.A – New phenomena of multi-phonon coupling and supercritical explosive instability of ultrasound in solids

Multi-boson interactions is a hot topic in the modern fundamental physics. **Pioneering theoretical and experimental works** on multi-phonon coupling in solids were carried out these last years by the group [1-3]. **Multi-phonon bound excitations were demonstrated on an example of three-phonon parametric coupling**. Modulation of nonlinear acoustic parameters of a medium by electromagnetic field **was proposed first** as a physical mechanism of phonon coupling. The development of the phonon triad dynamics in the form of **explosive instability was predicted theoretically** [1]. The threshold of instability depends on initial number of phonons. Over the threshold the number of coupled phonons increases drastically up to arising of singularity at finite time. **For travelling acoustic** waves the explosive instability is accompanied by the **special localization of phonon concentration**. The theoretical predictions are confirmed experimentally on an example of three-phonon excitations in antiferromagnetic α-Fe_{2}O_{3} and FeBO_{3} crystalline resonators under RF magnetic field [2,3]. The mechanisms of phonon intensity limitation near the point of explosion were defined and methods of their compensation in part were elaborated. Fig. 2 illustrates explosive dynamics of phonon triads observed in α-Fe_{2}O_{3 }single crystal. The results on multi-boson coupling have a general physical meaning and can be applied for wide range of dynamic systems with controllable nonlinearity from condensed matter physics to MEMS/NEMS.

Fig. 2: Explosive supercritical dynamics of three-phonon excitation in antiferromagnetic resonator [2]: (a)-FM pumping; (b)-pumping without modulation; (c)-FM pumping without initial excitation.

[1] V. Preobrazhensky, O. Bou Matar, P. Pernod. Phys. Rev. E., 78, 046603 (2008)

[2] V. Preobrazhensky, O. Yevstafiev, P. Pernod, V. Berzhansky. ** **JMMM, 322,

**585-588 (2010)**

[3] O. Yevstafiev, V. Preobrazhensky, P. Pernod, V. Berzhansky. JMMM, 323, 1568-1573 (2011)

#### 2.B – Tunable and reconfigurable Quasi-Phononic crystals

We have for the first time proposed and demonstrated feasibility of tuning the band structure of phononic crystals by employing magnetostrictive materials and applying an external magnetic field [1,2]. Taking into account hybridisation of phonons with magnetic excitations in magnetostrictive substances the structures under consideration were named as quasi-phononic crystals. The introduction of a magnetoelastic constituent opens the possibility of easy controllability of the properties of a phononic crystal without any contact. We shown a contactless tunability of more than 20% of the absolute band gaps of a two-dimensional phononic crystal composed of an epoxy matrix and Terfenol-D inclusions. Moreover, the tunable quasi-phononic crystal behaves like a transmission switch for elastic waves when the magnitude of an applied magnetic field crosses a threshold. In this case, the large variations of elastic properties are linked to giant magnetostriction of Terfenol-D.

We have explored a second original way to obtain huge tuning of the band structures of a magneto-elastic phononic crystal: the use of spin-reorientation phase transitions. In this case, the influence of the amplitude and orientation of applied magnetic field on wave propagation have been studied in great details [3]. Applications to tunable selective frequency filters with switching functionnality and to reconfigurable waveguides (see Fig. 3) and demultiplexing devices have been demonstrated [4]. The opportunity to design and create quasi-phononic crystal devices with new or enhanced functionalities, opened by this tuning capability, is now explored in the ANR project MIRAGES.

Fig. 3: Band structure of a square-lattice 2D phononic crystal with cylindrical Terfenol-D rods of 0.5 mm radius embedded in an epoxy matrix with a filling factor *f*=0.6. The applied magnetic field is (a) 20 kOe and (b) 1 kOe along the Z axis. (c) Three components of the particle displacement of a plane wave with a frequency of 970 kHz impinging on a square-lattice phononic crystal containing a linear waveguide. Only the out of plane transversely polarized mode is transmitted through the waveguide.

[1] O. Bou Matar, J. Vasseur, P.A. Deymier, Chapter 8 of the book Phononic Crystals and Acoustic Metamaterials, Springer Series in Solid-State Sciences, vol. 173 (2013)

[2] J.F. Robillard, O. Bou Matar, J. Vasseur, P.A. Deymier, A.C. Hladky-Hennion, M. Stippinger, Y. Pennec, B. Djafari-Rouhani, Appl. Phys. Lett., 95, 124104 (2009)

[3] O. Bou Matar, J.F. Robillard, J. Vasseur, A.-C. Hladky-Hennion, P.A. Deymier, P. Pernod, V. Preobrazhensky, J. Appl. Phys., 111(5), 020205 (2012)

[4] J. Vasseur, O. Bou Matar, J.F. Robillard, A.C. Hladky-Henion,** **P.A. Deymier,** **AIP Advances, 1, 041904 (2011)

#### 2.C – Liquid flow velocimetry using Wave phase conjugation

We have **demonstrated for the first time** that **break of time reversal invariance due to the presence of flows** in a medium **is detectable** **by means** of parametric wave phase conjugation **(WPC) technique**. The theoretical approach for linear and nonlinear propagation of phase conjugate waves in heterogeneously moving media was developed [1]. **The new concept of WPC Harmonic Imaging was proposed** and applied successfully for reliable imaging of inhomogeneous spatial distributions of liquid flow velocities (Fig. 4). **The method has been extended to tomography** for the direct reconstruction of complex, in particular vortex, flows [2]. The filtered Back Projection (FBP) algorithm was developed to obtain the magnitude of velocity vector as well as velocity direction in each point of the reconstructed area. Last but not least, **a new method allowing ****simultaneous**** measurements** of spatial distribution of velocity and concentration of binary liquid flows was developed and **patented** [3]. This technique is now under adaptation for the characterization of hydrodynamic instabilities (Faraday instabilities) in the frame of an ESA Topical team and the FP7 Marie-Curie IRSES project *PAS *(see 4B).

On the same basis, **a new concept of quantitative diagnostics of air micro-bubbles in decompressed media was demonstrated** experimentally and based theoretically in the framework of the ANR project *SMARTUS* [4]. We have shown that operation by coherent back scattering of phase conjugate waves in multi-scattering media makes available authentic quantitative information on bubbles concentration and noticeably accelerates the testing procedure in real time.

Fig. 4: WPC harmonic imaging of inhomogeneous spatial distributions of liquid flow velocities.

[1] V. Preobrazhensky, P. Pernod, Yu. Pyl’nov, L.M. Krutyansky, N. Smagin, S. Preobrazhensky Acta Acustica united with Acustica, 95(1), 36-42 (2009)

[2] Yu. Pyl’nov, S. Koshelyuk, P. Pernod, Yu. Kutlubaeva, Physics of Wave Phenomena, 20(3), 231–234 (2012)

[3] P. Pernod, V. Preobrazhensky, Yu. Pyl’nov, N. Smagin, Patent FR2010/000713 (2009)

[4] P. Shirkovskiy, V. Preobrazhensky, P. Pernod S. Kosheluk, Proc. of the ICU 2013, 452-457.

#### 2.D – Nonlinear imaging of defects and contact problems

Nonlinear acoustic techniques are extremely sensitive to the presence of damage and defects. An important challenge is to pass from the nonlinear characterization of materials to imaging. The original approach that we have developed consists in the use of the combination of Time Reversed Acoustics (TRA) with Nonlinear Elastic Wave Spectroscopy (NEWS) for the detection and localization of micro-damage in solids. We demonstrated the advantages of “chaotic cavity transducer focusing” to enhance the localization of microdamage in conjunction with NEWS methods. Chaotic cavity transducer focusing is defined as the hardware-software combination of a piezoelectric ceramic glued on a cavity of chaotic shape on the one hand with the reciprocal time reversal or inverse filter technique on the other hand. Additional optimization through the use of chirps and inverse filtering techniques have been explored, and applied for the first time to image a crack in a steel sample [1]. Moreover, experimental data for reverberant and nonreverberant composite plates shown that the use of a chaotic cavity transducer significantly enhances the focusing process, and enables focusing in a nonreverberant medium using only one transducer. The application of the chaotic cavity transducer concept for synthetic imaging has been demonstrated in a composite plate, revealing properties similar to phased arrays [2]. This technique is now explored in the framework of the FP7 European project ALAMSA.

For numerical modeling of waves in complex structures we have developed a high performance numerical tool based on the nodal Discontinuous Galerkin Finite Element Method. We have applied the method to solve the linear and nonlinear elastic wave equation on unstructured meshes, for heterogeneous media, with an arbitrary order of accuracy in space (ANR project ANL-MEMS) [3].

In parallel we have **established theoretically the physics-based boundary conditions** corresponding to internal contacts or cracks by solving the problem of frictional contact between solids in quite a general case (profiles including random and, in particular, fractal shapes and taking into account friction which makes the system hysteretic and memory-dependent). An **original Method of Memory Diagrams (MMD)** [4-6] that maps complex memory-dependent stress distributions onto a simple graphical object has been developed. The MMD algorithm is fully equivalent to the complete mechanical analysis of stresses and strains in the contact system and automatically accounts for virtually any loading protocol. This research presents a major advance in contact mechanics in general. We are now working on its use for prediction of nonlinear acoustic signals in solids with cracks and to estimate physical and geometric parameters of defects.

[1] O. Bou Matar, Y.F. Li, K. Van Den Abeele, Appl. Phys. Lett., 95, 141913 (2009)

[2] B. Van Damme, K. Van Den Abeele, Y.F. Li, O. Bou Matar, J. Appl. Phys., 109, 104910 (2011)

[3] O. Bou Matar, P.Y. Guerder, Y.F. Li, B. Vandewoestyne, K. Van Den Abeele, J. Acoust. Soc. Am., 131(5), 3650-3663 (2012)

[4] V. Aleshin, K. Van Den Abeele, J. Mech. and Phys. Solids, 57, 657–672 (2009)

[5] V. Aleshin, K. Van Den Abeele, J. Mech. and Phys. Solids, 60, 14-36 (2012)

[6] V. Aleshin, K. Van Den Abeele, Int. J. Non-Linear Mech, 49, 15–30 (2013)

**2.E – Mechanical behavior of polymeric chains**

The recent development of mechanical experiments on single molecules provided a deeper understanding of intermolecular and intramolecular forces, thereby introducing crucial additional information about the thermodynamics and kinetics of several biomolecular processes. Typically, mechanical methods allow the manipulation of a polymer molecule in two ways: the stretching of the chain by the direct action of an external force or by the application of an external field.

Single-molecule experimental methods to apply a force can be typically based on laser optical tweezers, magnetic tweezers, or atomic force microscope. These experimental techniques have been extensively applied to nucleic acids (DNA, RNA, and DNA condensation), proteins (protein-protein interaction and protein folding), molecular motors, and other long-chain biopolymers. Some investigations performed on doublestranded DNA determined the extension of the polymer as a function of the applied force, providing results in very good agreement with the worm-like chain (WLC) model and the freely-jointed chain (FJC) model. In Ref. [1] and [2] we formulate analytical expressions and develop Monte Carlo simulations to quantitatively evaluate the difference between the Helmholtz (isometric conditions) and the Gibbs ensembles (isotensional conditions) for a wide range of polymer models of biological relevance. We consider generalizations of the FJC models and of the WLC models with extensible bonds. In all cases we show that the convergence to the thermodynamic limit upon increasing contour length is described by a suitable power law and a specific scaling exponent, characteristic of each model.

Alternatively, it is possible to manipulate single molecules by an external field. In this case, the external field acts on the molecules from a distance or, in other words, without a defined contact point for applying the traction. A nonuniform stretching performed by an external field can be induced either via a hydrodynamic (or electrohydrodynamic) flow field, or via an electric (or magnetic) field. In Ref.[3], we describe the non-uniform stretching of a single, non-branched polymer molecule by an external field (e.g., fluid in uniform motion, or uniform electric field) by a universal physical framework, which leads to general conclusions on different types of polymers. We derive analytical results both for the freely-jointed chain and the worm-like chain models based on classical statistical mechanics. Moreover, we provide a Monte Carlo numerical analysis of the mechanical properties of flexible and semiflexible polymers, which accurately confirms the analytical achievements.

Finally, we presented in Ref. [4] a statistical mechanics analysis of the finite-size elasticity of model polymers, consisting of domains that can exhibit transitions between more than one stable state at large applied force (Fig.1a). The constant-force (Gibbs, Fig1b) and constant-displacement (Helmholtz, Fig1c) formulations of single-molecule stretching experiments are shown to converge in the thermodynamic limit. Moreover, Monte Carlo simulations of continuous three-dimensional polymers of variable length are carried out, based on this formulation. We demonstrate that the experimental force-extension curves for short and long polymers are described by a unique universal model, despite the differences in chemistry and rate-dependence of transition forces.

[1] F. Manca, S. Giordano, P. L. Palla, R. Zucca, F. Cleri and L. Colombo, Elasticity of flexible and semiflexible polymers with extensible bonds in the Gibbs and Helmholtz ensembles, Journal of Chemical Physics 136, 154906 (2012).

[2] F. Manca, S. Giordano, P. L. Palla, F. Cleri and L. Colombo, Monte Carlo simulations of single polymer force-extension relations, Journal of Physics: Conference Series 383, 012016 (2012).

[3] F. Manca, S. Giordano, P. L. Palla, F. Cleri, and L. Colombo, Theory and Monte Carlo simulations for the stretching of flexible and semiflexible single polymer chains under external fields, Journal of Chemical Physics 137, 244907 (2012).

[4] F. Manca, S. Giordano, P. L. Palla, F. Cleri, and L. Colombo, Two-state theory of single-molecule stretching experiments, Physical Review E 87, 032705 (2013).

(a) (b) (c)** **

Fig.1. (a) Potential energy function with an energy barrier. Folded and unfolded configurations of the domains are schematically represented. (b) Force-extension curves for the Gibbs ensemble. (c) Force-extension curves for the Helmholtz ensemble.

#### 2.F – Composite materials and structures

The central problem in predicting the physical (dielectric, magnetic, elastic and coupled) behavior of heterogeneous materials (like, e.g., composite or nanostructured systems, powders or mixtures) consists in the evaluation of their effective macroscopic properties, still taking into account the actual microscale material features. This leads to the concept of homogenization, a coarse graining approach addressed to determine the relationship between the microstructure and the effective behavior: the prediction of the effective properties of a composite material from those of its constituent material phases is the major objective of various homogenization models. The resulting effective properties can be observed at the macroscale, where the refined effects of the morphology cannot be directly measured. Moreover, all results are valid in dynamic regime if the wavelength of the propagating wave is much larger than the typical size of the microstructure. Many factors strongly influence the effective or macroscopic response of composite materials or structures: imperfect interfaces among constituents, strong nonlinearity and/or anisotropy of some components, presence of local defects as cracks, and so forth. This line of research is devoted to the analysis of these topics, in order to develop efficient homogenization scheme for complex structures.

In Ref.[1] we elaborated a blended continuum/atomistic theoretical picture of the nonlinear elastic properties of nanostructured materials, looking at diverse aspects such as dispersions of inhomogeneities within a matrix, random or graded nanograined materials, two-dimensional atomic sheets (graphene and graphane). In particular, we discuss the possible onset of length-scale effects and we establish the limits and merits of continuum versus atomistics.

Probably the most important result in the extensive literature on elastic composites is the Eshelby theorem on the response of a single ellipsoidal elastic particle in an elastic space subjected to a remote strain. Eshelby in 1957 proved that an applied uniform strain results in a uniform strain within the ellipsoidal inhomogeneity. In Ref.[2] we considered a generalization of the Eshelby theory concerning the elastic behavior of prestrained or prestressed inhomogeneities. The theory, in its original version, deals with a configuration where both the ellipsoidal particle and the surrounding matrix are in elastostatic equilibrium if no external loads are applied to the system. In our work, we considered slightly different shapes and sizes for the particle and the hosting cavity (whose surfaces are firmly bonded together) and, therefore, we observed a given state of strain (or stress) even without externally applied loads. We developed a complete procedure able to determine the uniform elastic field induced in an arbitrarily prestrained particle subjected to arbitrary remote loadings.

Also, the electrical and thermal conduction properties of disordered solids and the possible degradation processes induced by the generation of cracks are central issues in the field of the heterogeneous materials. In Ref.[3] we obtained an exact result for the fields induced within an elliptic anisotropic inhomogeneity embedded in a different anisotropic (two-dimensional) conductor. Then, we applied it to show that the degradation process strongly depends on the statistical orientational distribution of defects: in particular we theoretically prove that parallel cracks lead to the power law decay log *σ *~ *− *log*N *while random oriented cracks lead to the exponential law decay log *σ *~ *−**N *(where *σ *is the effective conductivity of a region with a large number *N *of defects), as recently predicted by numerical findings.

Nanomaterials composed of a population of particles dispersed in a matrix represent the building block for the next generation of several technologies: energy storage and conversion, thermal management, electronics, and photovoltaics. When interfaces between particles and matrix are imperfect (see Fig.2), the size of the particles may strongly influence the effective linear and nonlinear response of the whole system. In our research, we study these scale effects focusing both on the linear [4] and on the nonlinear [5] transport behavior of composite structures.

[1] L. Colombo and S. Giordano, Nonlinear elasticity in nanostructured materials, Report on Progress in Physics 74, 116501 (2011).

[2] S. Giordano, P. L. Palla, E. Cadelano and M. Brun, Elastic behaviour of inhomogeneities with size and shape different from their hosting cavities, Mechanics of Materials 44, 4 (2012).

[3] S. Giordano and P. L. Palla, Conduction degradation in anisotropic multi-cracked materials, The European Physics Journal B 85, 59 (2012).

[4] F. Pavanello, F. Manca, P. L. Palla, and S. Giordano, Generalized interface models for transport phenomena: Unusual scale effects in composite nanomaterials, Journal of Applied Physics 112, 084306 (2012).

[5] F. Pavanello and S. Giordano, How imperfect interfaces affect the nonlinear transport properties in composite nanomaterials, Journal of Applied Physics 113, 154310 (2013).

Fig. 2. Scheme of a nanostructure composed of nonlinear particles embedded in a linear matrix: the imperfect interfaces are modeled either through the T-model or the Π-model (introduced in Ref.[4]). The multiscale approach leads to evaluate the nonlinear effective behavior of the overall system (see Ref.[5] for details).

#### 3.A – Agile millimeter wave antennas & phase shifters

On the basis of the technological developments of the group on ultra-soft metal/polymer actives structures, remarkable results were obtained in the field of agile RF devices in the 60GHz band for short distance high-speed communications or anti-collision radar systems (CPER-CIA project and collaboration with the MiRCTECH team of the Georgia Institute of Technology). This research was started at LIMMS (CNRS-IIS UMI 2820) by IEMN researchers in 2006 who were the first to propose such an approach and transferred in the group back to France and developed in collaboration with the BIOMEMS Group. The concepts of agile devices using a moveable polymer membrane developed recently in the group led to a granted patent [1]. Figure 5 illustrates the brightest results:

- § A suspended patch antenna with pneumatic actuation showed 8% tuning capability around 53GHz [2].
- § A phase shifter with moving ground plane showed a state-of-the-art figure of merit of 118°/dB
_{(insertion loss)}[3]. - § A directive antenna capable of a +/- 90° mechanical beam steering with a constant gain at 57GHz was realized.

Fig. 5: mm-wave devices on soft polymer membrane, from left to right: pneumatic tunable patch antenna, phase shifter, beam steering antenna.

[1] Patent WO2011086298 (A1) 2011-07-21

[2] S. Hage-Ali, N. Tiercelin, P. Coquet, R. Sauleau, V. Preobra-zhensky, P. Pernod, IEEE Antennas and Wireless Propagation Letters, 9,, 1131-1134 (2010) [3] S. Hage-Ali, Y. Orlic, N. Tiercelin, R. Sauleau, P. Pernod, V. Preobrazhenski, P. Coquet, IEEE MTT-S International Microwave Symposium, IMS 2012

#### 4.A – Microscale acoustofluidics

Microscale acoustofluidics opens new perspectives for the conception of original labs-on-chips and the characterization of microflows and suspensions. This field of research focuses on the study of linear and nonlinear interactions between acoustic fields and fluids at micrometric scales. Acoustic fields can be used to actuate flows through nonlinear effects [1,2] and conversely, microflows can produce sound [3] or modify the propagation of acoustic waves [4,5].

An extensive literature is dedicated to the manipulation of micrometric fluid samples with surface acoustic waves. However little work deals with the use of the complex nonlinear coupling between acoustic fields and fluid dynamics to obtain the desired fluid response with moderate excitation power. **We showed that** by modulating the acoustic signal and adapting it to droplet eigenmodes, **parametric and harmonic excitation of droplets can be achieved**, leading to their oscillations [1,2]. These oscillations (Fig. 6) **improve the depining of the contact line** and lead to **drastic reduction of the acoustic power required** to move them, and consequently the temperature increase in the drop. This is essential in the perspective of using SAW actuators for biofluids manipulation* *(ANR project* AWESOM*).

Another original aspect of microscale acoustofluidics developed in the team is the study of **sound produced by two-phase microflows**: brutal ruptures of gas-liquid interfaces result in the generation of pressure waves. A good understanding of the relation between the flow features and the resulting sound is essential (i) for the development of sensors in microsystems able to characterize microflows from the acoustic signature and (ii) in the medical field since such flows occur for example in the lungs in pathological situations; the resulting sound is used as a diagnostic tools by physicians. To improve our knowledge of these flows, we studied the dynamics of liquid plugs in synthetic networks and showed that airways reopening occurs through cascade of plugs rupture with specific spatio-temporal distribution [3]. We are now working on the relation between these specific flow structures and the resulting acoustic signature.

Fig. 6: Dipolar oscillations of a drop of 7.5 microlitres subjected to a SAW of carrier frequency of 19.5 MHz, and modulation frequency of 52.5 Hz.

[1] P. Brunet, M. Baudoin, O. Bou Matar and F. Zoueshtiagh, Phys. Rev. E, 81, 036315 (2010)

[2] M. Baudoin, P. Brunet, O. Bou Matar, E. Herth, Appl. Phys. Lett., 100, 154102 (2012)

[3] M. Baudoin, Y. Song, P. Manneville, C.N. Baroud, Proc. Natl. Acad. Sci., 110, 859-864 (2013)

[4] M. Baudoin, J.L. Thomas, F. Coulouvrat, J. Acoust. Soc. Am., 123, 4127-4139 (2008)

[5] M. Baudoin, J.L. Thomas, F. Coulouvrat, C. Chanéac, J. Acoust. Soc. Am., 129, 1209-1220 (2011)

#### 4.B – Faraday instabilities

The scientific aim here is the study of complex flow instabilities at fluid interfaces, the development of a new non-invasive ultrasonic characterization method providing simultaneously velocities and density gradient in such flows, and the development of the resulting innovations for spatial and medical applications (e.g. fuel mixers & BioMEMS microfluidic mixers, both with real time monitoring).

These activities are taking place in the Framework of the European Spatial Agency (ESA) Topical Team «Liquid interfaces subjected to oscillations» (involving the Japanese Spatial Agency JAXA, Florida University, and other international partners), and within the FP7 European Marie-Curie IRSES project PAS (10 international partners). The french space agency CNES is the main sponsor.

In the case of hydrodynamic instabilities at fluid interfaces, **we have been the first to experimentally succeed in generating Faraday instability between two miscible liquids**. The results show that it is possible to exploit the phenomenon for the development of a new, alternative industrial technologies for mixing liquid using hydrodynamic instabilities (Fig. 7) [1-2]. This mixing method is of special interest in absence of the force of gravity (since gravity has in general a stabilizing role). Experiments at zero gravity will be made in the near future within the project. In presence of gravity, we have also **demonstrated the important role of the gravity force on the threshold of instability appearance and the resulting mixing** [2]. Furthermore, we successfully generated experimentally Faraday waves in confined geometry which closely approximates stress-free behavior along the sidewalls. This is achieved with an original immiscible, two-liquid system which forms a thin film of the more wetting fluid along the glass sidewall. Further adjustment of the film fluid viscosity renders the sidewall dissipation negligible in comparison to bulk viscous effects. The system is **the first in over 60 years of experiments to show agreement with linear stability theory, without the incorporation of phenomenological parameters**.

In parallel, our new method of simultaneous measurements of fluid velocity and concentration based on ultrasonic WPC (see 2D) is under adaptation for the characterization of Faraday or Marangoni instabilities. This method, without tracers and applicable for non-transparent media, is a major innovation with a high potential in various applications such as space field and terrestrial field.

Fig. 7: Faraday instabilty occuring between two miscible liquids.

[1] F. Zoueshtiagh, S. Amiroudine, R. Narayanan, J. Fluid Mech., 628, 43-55 (2009)

[2] S. Amiroudine, F. Zoueshtiagh, R. Narayanan, Phys. Rev. E, 85, 016326 (2012)

#### 4.C – Micro-jet actuators and micro-sensors for active flow control

Innovative state of the arts integrated microjets, based on ultra-soft active polymers and Micro-Magneto-Mechanical Systems (MMMS) have been developed for aerodynamic active flow control. Large arrays of more than 30 highly miniaturized continuous and pulsed microjets with exit velocities up to 230 m/s & 3 kHz oscillating frequency, and synthetic microjets up to 55 m/s & 700 Hz were fabricated (Fig. 8) [1-2]. To our knowledge they are the only MEMS based micro-jet actuators fulfilling functional specifications for real aeronautic or automobile applications. In parallel, we elaborated hot wire micro-sensors with sizes down to 50 mm (one order of value improved spatial resolution relatively to commercial devices). They provide a sub-millimitric multiparameter measurement (pressure, velocity, and temperature) with a robustness compatible with flow velocities >100 m/s and electric parameters directly compatible with commercial electronic modules (Fig. 9) [3-6]. The devices have been successfully applied in wind tunnels for separation control on airplane wings, S-ducts, and at the rear end of a reduced scale car model (Ahmed body) with demonstration of 8% drag reduction in the latter experiment.

3 patents have been deposited (2 with international extensions). The experiments were made in tight long term collaborations with the European aeronautic community and car constructors (ONERA, Dassault, MBDA, EADS, Rolls Royce, SNECMA, MTU, PSA, & Renault) in the framework of the European FP6 ADVACT project, 2 PEA projects from the French Délégation Générale de l’Armement (DGA), projects from the French aeronautic foundation (FNRAE) and EADS foundation, 2 projects from the French National Center for Technological Research ‘Aérodynamique et aéroacoustique des véhicules terrestres’ (CNRT-R2A) and the French GDR ‘Separation Control’.

Fig. 8: Set of 12 self-oscillating pulsed microvalves with their packaging mounted at the exit of a Nozzle engine for acoustic noise control measurements at LMFA Lyon.

Fig. 9: Left- In plane micro-hot wire for high flow rate measurement in pulsatile flow. Right- Array of very localized out of plane micro-hot-wires made on nano-crysttaline diamond.

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