Direct observation of mechanical memory in athermal amorphous rafts and exploring its connections to yielding and hyperuniformity

Abstract

It is interesting to ask whether a system subjected to exactly the same mechanical perturbation repeatedly can subsequently retain memory of what it had undergone. The possibility of this kind of non-magnetic memory has attracted quite some interest in the recent past [1{3]. In dilute non-Brownian suspensions (size large enough for motion to re- main una ected by temperature uctuations), it was observed that on applying oscillatory perturbation of exactly the same nature repeatedly, the particles underwent time irreversible paths initially and later transitioned into having trajectories that were time reversible [4]. This was observed when the extent of shear was below a threshold value [4, 5]. This phe- nomenon indicates the adaptibility of a system to a particular mechanical perturbation. In other words, it is learning of a particular mechanical perturbation. However, memory would require being able to access the perturbation, learnt by the system, at a later stage. One experimentally feasible technique of applying mechanical perturbation is to apply oscillatory shear. In experiments that involve oscillatory shear, the quantity of interest is the displace- ment of the system that occurs in the direction of applied shear force at a boundary of the system, relative to the length of the system perpendicular to it.