Title: Memory Formation in Particle Packings

Speaker: Dr. Sidney Nagel, University of Chicago

Abstract: A disordered solid created by packing spheres in a box has a myriad of stable configurations that defy straightforward enumeration and characterization. Since structure often determines function, each distinct arrangement potentially corresponds to different material behavior. The question naturally arises: How many mechanically stable ways are there to pack N identical spheres in a box? This has the spirit of a "Fermi estimation problem," but one for which we do not have an answer: we do not know how to account for rearranging the particles to produce packings with different connectivity. We therefore resort to measurement.

Stable packings of N identical soft spheres in d dimensions lie at the minima of a vast (Nd)-dimensional potential energy landscape. Starting from random positions, the particles can relax to mechanical equilibrium; all initial conditions that end at the same minimum belong to the same catchment basin. We find a very wide distribution of basin volumes. These results suggest how to understand the imprinting of memories in solids.

Bio: Sidney Nagel was educated at Columbia University (B.A. in 1969) and at Princeton University (Ph.D. in 1975). He did a postdoc at Brown and then moved to the University of Chicago in 1976 where he has remained and is now the Stein-Freiler Distinguished Service Professor in the Department of Physics and the James Franck and Enrico Fermi Institutes. Nagel is a condensed-matter experimentalist focusing on problems where the physics of disorder and far-from-equilibrium behavior play crucial roles. Of particular delight is when table-top experiments have analogs in disparate fields at wildly different scales. In particular, he has studied the glass transition, the phenomenology of granular material, and the physics of jamming; he has also focused on pattern and singularity formation in fluid interfaces, and the formation of stains left behind by evaporating liquid drops. A current interest is the investigation of memory formation in out-of-equilibrium matter. He has worked to make his science accessible to a broad public by emphasizing the inherent beauty of the phenomena he studies in the laboratory.

Zoom: https://northwestern.zoom.us/j/98030602428

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