Soft Matter Seminar: Transient Memories in Experiments on Sheared Non-Brownian Suspensions
Tuesday, April 16, 2013 – 11:00am
Joseph D. Paulsen
University of Chicago
A novel kind of memory has been observed in simulations of cyclically sheared non-Brownian suspensions . We are conducting experiments designed to see these effects in the lab. In this type of memory, a system remembers a set of shear amplitudes but forgets almost all of them later on, even as they are continually applied. If noise is added, the system can store all memories indefinitely. While exceedingly counterintuitive, these properties can be understood from simple considerations. Furthermore, the phenomenon is expected to be generic—the same effect is seen in simulations and experiments on traveling charge-density waves . We use neutrally buoyant, non-Brownian suspensions at low Reynolds number, which we shear cyclically in a Couette cell geometry. Starting from a random configuration, the particle trajectories are irreversible at first but (as has been shown before ) eventually settle into a configuration where they retrace their paths exactly during each cycle. We show that the resulting configuration comprises a memory of the driving amplitude, which can be read out by measuring the degree of particle reversibility versus shear amplitude. Furthermore, we form multiple memories in which smaller memories are forgotten when larger shear is applied. Finally, we find that noise stabilizes all memories over longer periods of time. We are currently exploring whether the forgetting is sufficiently gradual, so that one memory erodes away while another takes over. This key point distinguishes multiple transient memories from other classes of memory.
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Host: Daniel Blair