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Crystallization in a Repeatedly Sheared Granular Medium 

Harry L. Swinney, University of Texas at Austin 

An understanding of how crystal formation is initiated within a homogeneous disordered state can provide insights into the old problems of glass formation and crystallization in fluids.  Experiments over the past half century have shown that granular materials under repeated small disturbances will reach a maximum volume fraction called random close packed (RCP). This talk will describe an experiment [1] and numerical simulations [2] on hard spheres that reveal, for repeated disturbances of sufficiently small amplitude, RCP is reached at a volume fraction 0.646±0.001. Further small repeated disturbances lead to the nucleation of crystallites. The crystallites that reach a critical size of 10-15 spheres grow steadily.  Pressure, repeated disturbances, and dissipation are necessary for the crystallization, but gravity and friction are not. 

[1] F. Rietz, C. Radin, H.L. Swinney, and M. Schröter, Phys. Rev. Lett. 120, 258003 (2018).

[2] W. Jin, C.S. O'Hern, C. Radin, M.D. Shattuck, and H.L. Swinney, Phys. Rev. Lett. 125, 258003(2020).