法国Langevin实验室Tourin主任与贾小平教授邀请报告

发布时间：2018-10-30

题目一: Time Reversal of waves in scattering media: from smart sonar to medical imaging

报告人：Prof. Arnard Tourin

Institut Langevin, ESPCI Paris, CNRS UMR, 1 rue Jussieu, 75005 Paris, France.

时间：10月30日上午，9:00-12:00

地点：科学楼5楼530会议室

联系人：许凯亮，他得安

摘要

Time reversal is an efficient way for focusing waves deeply inside heterogeneous media. In a typical time-reversal focusing experiment, the wave field radiated by a broadband source is recorded with a set of detectors located on a surface named a “Time Reversal Mirror”. The detectors then become sources sending back the time-reversed sequence of the recorded field. The result is a time-reversed field that focuses at the source location as if time were going backwards. Surprisingly, the more scattering the medium is, the thinner the focal spot is. This rather counterintuitive result has contributed to creating a new paradigm for the manipulation of waves in complex media: contrary to long-held beliefs, disorder is not only an impediment to focusing and imaging but can be turned into an ally for controlling waves. In this talk, I will be discussing some applications of time-reversal focusing to various fields ranging from underwater acoustics, wireless communications and non-destructive testing to medical imaging.

题目二：Multiscale acoustics of porous granular materials: from probing to pumping

报告人：Prof. Xiaoping Jia

Institut Langevin, ESPCI Paris, CNRS UMR, 1 rue Jussieu, 75005 Paris, France.

摘要

Granular materials are an assembly of macroscopic solid grains that interact with each other by dissipative contact forces. They are ubiquitous in everyday life, ranging from industrial applications to geophysical processes. Statics and dynamics of dense granular media strongly depend on the contact networks, resulting from the confining pressure [1]. Elastic wave propagation provides a powerful and sometimes unique probe of these inhomogeneous and fragile networks. The long-wavelength coherent waves give access to the elastic modulus whereas the short-wavelength scattered waves are sensitive to local rearrangements [2].

In this talk, we first describe the linear wave transport and acoustic probing of viscoelastic properties in dry and wet granular materials [3] as well as the shear banding [4]. Then, we discuss the nonlinear acoustic behaviours and show that beyond certain driving amplitude elastic waves not only serve as a probe but also as a pump to fluidize jammed granular solids [5,6]. Finally, we investigate the triggering of shear instability by the acoustic fluidization, such as granular avalanches and quicksands [7,8].

[1] H.M. Jaeger, S.R. Nagel & R.P. Behringer, Rev. Mod. Phys. 68, 1259 (1996)

[2] X. Jia, C. Caroli & B. Velicky, Phys. Rev. Lett. 82, 863 (1999); X. Jia, Phys. Rev. Lett. 82, 863 (2004)

[3] V. Langlois & X. Jia, Phys. Rev. E 89, 0223206 (2014); T. Brunet et al, Phys. Rev. Lett. 101, 138001 (2008)

[4] Y. Khidas & X. Jia, Phys. Rev. E 85, 051302 (2012)

[5] P. Johnson & X. Jia, Nature 437, 871 (2005); X. Jia et al, Phys. Rev. E 84, 020301(R) (2011)

[6] J. Brum, J.-L. Gennisson, M. Fink, A. Tourin, and X. Jia (submitted)

[7] J. Léopoldès, G. Conrad, and X. Jia, Phys. Rev. Lett. 110, 248301 (2013)

[8] S. Wildenberg et al, EPJ Web of Conferences 140, 03080 (2017)