Multiple transient memories are a remarkable and counterintuitive example of how information can be stored in driven disordered systems. A system with transient memories will ‘learn’ multiple driving amplitudes that can subsequently be read out. However, most of the memories will be forgotten after many driving cycles. Surprisingly, if noise is added, all of the memories are retained for much longer. This memory behavior has been observed in traveling charge-density waves and was predicted in simulations of sheared suspensions [Keim & Nagel, Phys. Rev. Lett. 107, 2011].

Left: 2D slice of a suspension of ~100 μm PMMA spheres in a viscous fluid, illuminated with a laser sheet. To train a single memory of 1.2, the suspension was sheared 200 times between strain amplitudes γ=0 and γ=1.2. Middle: Subtraction of pictures taken before and after a shear of amplitude γ = 1.2. The trajectories are reversible so the result is monotone. Right: Corresponding image subtraction for γ = 1.4, indicating a memory of 1.2 < γ < 1.4.
Left: 2D slice of a suspension of ~100 μm PMMA spheres in a viscous fluid, illuminated with a laser sheet. To train a single memory of 1.2, the suspension was sheared 200 times between strain amplitudes γ=0 and γ=1.2. Middle: Subtraction of pictures taken before and after a shear of amplitude γ = 1.2. The trajectories are reversible so the result is monotone. Right: Corresponding image subtraction for γ = 1.4, indicating a memory of 1.2 < γ < 1.4.

We have shown behavior in experiment that is consistent with multiple transient memories that become stabilized by noise. We apply low Reynolds-number cyclic shear to a neutrally buoyant, non-Brownian suspension. Starting from a random configuration, the particle trajectories are irreversible at first but (as has been shown before [Corte et al., Nat. Phys. 4, 2008]) eventually find a configuration where they retrace their paths exactly during each cycle. This comprises a memory of the driving, which is read out by measuring the degree of particle reversibility versus strain amplitude. We can also encode multiple memories, wherein smaller memories are forgotten when larger shear is applied. Finally, when noise is added, all the memories are retained for much longer.

Publications:

Multiple transient memories in experiments on sheared non-Brownian suspensions. J. D. Paulsen, N. C. Keim, and S. R. Nagel, Physical Review Letters 113, 068301 (2014).  pdf  link  bibtex

Multiple transient memories in sheared suspensions: Robustness, structure, and routes to plasticity. N. C. Keim, J. D. Paulsen, and S. R. Nagel, Physical Review E 88, 032306 (2013).  pdf  link  bibtex

Commentary:

Commentary in Journal Club for Condensed Matter Physics.  pdf  link

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