Seminar 20 -S4, Tuesday 07 May 2024, 14:00 (London Time)

Speaker: Lianchao WANG (Department of Mechanical Engineering, City University of Hong Kong)

Title: Non-reciprocal and Non-Newtonian Mechanical Metamaterials

Abstract: Non-Newtonian liquids are characterized by stress and velocity-dependent dynamical response. In elasticity, and in particular, in the field of phononics, reciprocity in the equations acts against obtaining a directional response for passive media. Active stimuli-responsive materials have been conceived to overcome it. Significantly, Milton and Willis have shown theoretically in 2007 that quasi-rigid bodies containing masses at resonance can display a very rich dynamical behavior, hence opening a route toward the design of non-reciprocal and non-Newtonian metamaterials. In this paper, we design a solid structure that displays unidirectional shock resistance, thus going beyond Newton’s second law in analogy to non-Newtonian fluids. We design the mechanical metamaterial with finite element analysis and fabricate it using three-dimensional printing at the centimetric scale (with fused deposition modeling) and at the micrometric scale (with two-photon lithography). The non-Newtonian elastic response is measured via dynamical velocity-dependent experiments. Reversing the direction of the impact, we further highlight the intrinsic non-reciprocal response.

Biography: Lianchao Wang is a Ph.D. in the mechanics of solids. Lianchao was awarded his first Ph.D. diploma at the University Bourgogne Franche-Comté (France) and his second one at Harbin Institute of Technology (China) in July and September 2023, respectively. Currently, he works as a postdoc researcher at the Department of Engineering, City University of Hong Kong. His research is on the static and/or dynamic mechanical properties of porous structures or metamaterials.

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Seminar 23 – S4, Tuesday 28 May 2024, 14:00 (London Time)

Speaker: Thomas Brunet (Institute of Mechanical Engineering, University of Bordeaux, France)

Title: Soft Acoustic Metamaterials: from 3D locally resonant metafluids to soft porous gradient index metasurfaces

Abstract: Soft acoustic metamaterials are classes of functional materials for acoustics achieved by means of soft matter techniques such as microfluidics, chemical formulation or self-assembly [1]. In this talk, I will review our achievements in that field that allowed us to fabricate the first 3D acoustic metamaterial with a negative index [2]. To do that, we took benefit from the strong low-frequency Mie-type resonances of “ultra-slow” micro-beads randomly dispersed in a water-based gel. The soft porous silicone rubber material used to make these soft porous particles, not only exhibits ultra-low sound speeds (~40m/s), but also shows a strong dependence of its sound speed on the porosity [3], providing thus a material with an acoustic index varying over a wide range of values. By assembling thin stripes made of these soft porous silicone rubbers with different porosities, we have recently proposed to fabricate soft gradient-index metasurfaces, alternatively to our 3D locally resonant metafluids, for wavefront shaping. The ability of these flat and ultra-thin acoustic lenses will be demonstrated through various underwater ultrasound experiments such as beam steering, ultrasound focusing and vortex beam generation [4]. Finally, I will report on 3D underwater ultrasound focusing experiments performed with a quasi-flat high-index acoustic lens [5] whose focal length can be tuned by hardening or softening the material thanks to UV light [6].

Biography: After completing his PhD in physics about acoustics of granular media at University Paris-Est in 2006, Thomas Brunet spent three years as a postdoctoral researcher at Institut des NanoSciences de Paris where he became interested in phononic crystals. In 2010, he joined Bordeaux INP as Associate Professor of Mechanical Engineering and, since then, he has extended his activities on acoustic waves in complex media to various systems of disordered resonant scatterers. In particular, Thomas Brunet’s researches aim at developing a new class of soft acoustic metamaterial-based devices combining soft matter techniques with microfluidics. He is also interested in fundamental aspects of the acoustic wave transport in strongly scattering media, such as sound diffusion and localization effects.

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