Granular materials are collections of macroscopic grains that interact via dissipative and nonlinear forces. These materials are of substantial importance in many industrial and natural processes while at the same time they offer a perfect test bed for new insights into problems in condensed-matter physics and materials science. Despite their seeming simplicity, granular matter behaves quite differently from the other familiar forms of matter and their rich and complex dynamic behaviour still present a major challenge in physics.
The main objectives of our lab’s research activities are: (1) a better understanding of the acoustics of granular media, (2) the design of granular-based structures and metamaterials for the control of acoustic propagation. In particular, applying a synergistic approach that combines experimental, theoretical and numerical studies, we pursue research at the following fields :
The dynamics of 1D magnetogranular phononic crystals composed of a chain of steel spherical beads inside a properly designed magnetic field. The latter is induced by an array of permanent magnets, located in a holder and at a distance from the chain. We display theoretical and experimental results of the band gap structure including all the six degrees of freedom of the beads, three translations and three rotations. We present experimental evidence of transversal-rotational modes of propagation while by changing the strength of the magnetic field, we tune the dynamic response of the granular chain. The combination of the non-contact tunability with the potentially strong nonlinear behavior of the granular systems, makes magneto-granular phononic crystals suitable nonlinear tunable mechanical metamaterials for the control of elastic wave propagation.
Hexagonal and honeycomb granular membranes
We theoretically study the dispersion properties of elastic waves in hexagonal and honeycomb monolayer granular membranes with either out-of-plane or in-plane particle motion. The particles interact predominantly via normal and transverse contact rigidities. When rotational degrees of freedom are taken into account, the bending and torsional rigidities of the intergrain contacts can control some of the phononic modes. The existence of zero-frequency modes, zero-group-velocity modes and their transformation into slow propagating phononic modes due to weak bending and torsional intergrain interactions are investigated. We also study the formation and manipulation of Dirac cones and multiple degenerated modes. This could motivate variety of potential applications in elastic waves control by manipulating the contact rigidities in granular phononic crystals.
Asymmetric Acoustic Propagation of Wave Packets Via the Self-Demodulation Effect
Experimental characterization of nonreciprocal elastic wave transmission in a single-mode elastic waveguide is presented . This asymmetric system is obtained by coupling a selection layer with a conversion layer: the selection component is provided by a phononic crystal, while the conversion is achieved by a nonlinear self-demodulation effect in a 3D unconsolidated granular medium. A quantitative experimental study of this acoustic rectifier indicates a high rectifying ratio, up to 106, with wide band (10 kHz) and an audible effect. Moreover, this system allows for wave-packet rectification and extends the future applications of asymmetric systems
Slow Relaxation and Aging Phenomena at the Nanoscale in Granular Materials
Granular matter exhibits a rich variety of dynamic behaviors, for which the role of thermal fluctuations is usually ignored. Thermal fluctuations can pronouncedly affect contacting nanoscale asperities at grain interfaces and brightly manifest themselves through the influence on nonlinear-acoustic effects. The proposed mechanism based on intrinsic bistability of nanoscale contacts comprises a wealth of slow-dynamics regimes including slow relaxations and aging as universal properties of a wide class of systems with metastable states.
Ultrasonic evaluation of the morphological characteristics of metallic powders in the context of mechanical alloying
An ultrasonic method has been proposed to characterize the morphological (geometrical) aspects of powders through the elastic modulus dependence of their packing on the factors of polydispersity, coordination number and particle shape. During the mechanical alloying process, the variation in geometrical characteristics of powders provides critical information. Ultrasonic parameters are shown to be sensitive not only to the average contact number per bead (i.e. the coordination number) but also to characteristics of the bead size distribution, when given the same sample preparation and confining pressure. These parameters, in turn, are sensitive to both the granular medium polydispersity and particle shapes. A non-monotonic behavior of the ultrasonic velocity (and of the derived compressional wave modulus) is observed throughout the alloying process, which thus offers possibilities for powder structure monitoring.
Nonlinear Hysteretic Torsional Waves
Hysteretic torsional pulses in a vertical granular chain made of cm-scale, self-hanged magnetic beads have been theoretically studied and experimentally reported in this work. As predicted by contact mechanics, the torsional coupling between two beads is found to be nonlinear hysteretic. This results in a nonlinear pulse distortion essentially different from the distortion predicted by classical nonlinearities and in a complex dynamic response depending on the history of the wave particle angular velocity. Both are consistent with the predictions of purely hysteretic nonlinear elasticity and the Preisach-Mayergoyz hysteresis model, providing the opportunity to study the phenomenon of nonlinear dynamic hysteresis in the absence of other types of material nonlinearities. The proposed configuration reveals a plethora of interesting phenomena including giant amplitude-dependent attenuation, short-term memory, as well as dispersive properties. Thus, it could find interesting applications in nonlinear wave control devices such as strong amplitude-dependent filters.
Power amplifiers, high power ultrasonic transducers, analysers, numerical oscilloscopes, conditionners, numerical filters, laser vibrometer, accelerometers, 5 experimental stations with PC and acquisition cards, anechoic room. Local technical support (Technicians / Engineers shared with other groups and material conception facilities), fatigue machine.
We are always pleased to invite post-doctoral researchers who have skills in one or several of the following domains :
Nonlinear acoustics, power ultrasonics, non destructive evaluation, physics of granular materials, acoustics of solids.
Skills in experimental studies are very welcome but we are also interested in theoretical and numerical skills. We do not receive recurrent post-doc fellowships.
This has to be investigated in each particular case. Please do not hesitate to contact us for any further information.