Abstract
A disordered material that cannot relax to equilibrium, such as an amorphous or glassy solid, responds to deformation in a way that depends on its past. In experiments we train a 2D athermal amorphous solid with oscillatory shear, and show that a suitable readout protocol reveals the shearing amplitude. When shearing alternates between two amplitudes, signatures of both values are retained only if the smaller one is applied last. We show that these behaviors arise because individual clusters of rearrangements are hysteretic and dissipative, and because different clusters respond differently to shear. These roles for hysteresis and disorder are reminiscent of the return-point memory seen in ferromagnets and many other systems. Accordingly, we show how a simple model of a ferromagnet can reproduce key results of our experiments and of previous simulations. Unlike ferromagnets, amorphous solids' disorder is unquenched; they require "training" to develop this behavior.
Highlights
A disordered material that cannot relax to equilibrium, such as an amorphous or glassy solid, responds to deformation in a way that depends on its past
These findings represent new possibilities for describing and exploiting these materials’ complex history dependence, but they prompt new questions: What is the mechanism for memory formation and readout? What can memory reveal about the physics of amorphous solids more broadly? How should one place this behavior among examples of memory in other systems, and what explains the contrast with a more dilute system of particles?. In this Rapid Communication, we describe experiments with the two-dimensional amorphous solid in Fig. 1(a), showing the readout of stored memories, consistent with other systems [7,16,17,18]. We propose that these memory results are approximately consistent with a behavior called return-point memory (RPM) that is exhibited by many hysteretic systems [6,20,21]
By observing the motions of particles, and considering a simple example of RPM, we have shown how our material’s memory arises from the hysteresis of individual rearranging clusters, each of which responds differently to global deformations
Summary
Our experiment consists of polystyrene sulfate latex particles (Invitrogen), with diameters 3.7 μm (Lot 1839598) and 5.4 μm (Lot 1818113) in roughly equal numbers [Fig. 1(a)], adsorbed at the interface between decane The needle is 230 μm in diameter and 32 mm long; it protrudes from the ends of the channel to keep this yield-stress material from forming solid “plugs” there. This means that the working sample is approximately 18 mm long and 1.5 mm wide on each side of the needle. We subtract the average motion of a region of nearby material (radius Rdisp = 8.5a), to avoid spurious signals due to small motions of the camera or variation of the needle position, yielding rlocal [14,27].
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