Berkeley Fluids Seminar

Wednesday, September 20, 2017

Fanuc Room, Etcheverry Hall

Suraj Shankar (Syracuse University/KITP)

Topological sound and flocking on curved surfaces

Abstract: Flocking, the self-organized and spontaneous motion of a large collection of self-propelled entities, is ubiquitous in the natural world. Such collective motion is often found on many scales from bird flocks to cohesive tissue flows during growth and development. On a sphere, orientationally ordered phases of matter including flocks, are forced to be inhomogeneous, often developing topological defects due to constraints imposed by the underlying geometry of the surface. Additionally the presence of spontaneous flow in an ordered polar flock allows it to support long-wavelength propagating sound modes which get gapped by the curvature of the sphere The spontaneous breaking of time reversal symmetry due to collective motion and the presence of a finite energy threshold to excite sound modes together gives rise to topologically protected sound modes that propagate unidirectionally and are equatorially localized on the sphere. These excitations are analogous to edge states in electronic quantum Hall systems or well-known equatorial waves in ocean and atmospheric flows, and provide unidirectional channels for information transport in the flock, robust against disorder and backscattering.

Bio: Suraj Shankar is a PhD candidate in Theoritical Physics at Syracuse University, New York. He is working with Prof. Cristina Marchetti and Prof. Mark Bowick on statistical mechanics of active matter and also mechanics of kirigami, the art of cutting and folding paper, to design mechanical meta-materials. He has been a Kavli Institufe of Theoritical Physics (KITP) Fellow in Fall 2017. This research that he is presenting, topological features of flocking models, has also been covered in APS synopsis.

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Acknowledgments

Prof. Graham Fleming (Vice Chancellor for Research, UC Berkeley)

Prof. Eliot Quataert on behalf of The Theoretical Astrophysics Center and the Astronomy Department (UC Berkeley)

Prof. Philip S. Marcus on behalf of the Mechanical Engineering Department (UC Berkeley)

Prof. Michael Manga (Earth and Planetary Science, UC Berkeley)

Prof. Evan Variano (Civil and Environmental Engineering, UC Berkeley)