Scaling of relaxation and excess entropy in plastically deformed amorphous solids

Significance

When stressed sufficiently, many solids plastically deform and flow. This plastic deformation induces irreversible structural changes, which are sometimes used in practice to manipulate microstructure of materials to achieve desired mechanical properties. Unfortunately, our limited fundamental understanding of the interdependence of plastic flow and microstructure represents a design barrier for improvement of strength, hardness, and ductility in amorphous solids, where constituent particles are haphazardly arranged. Here, we study plastic flow and its influence on the microstructure of disordered colloidal solids. Video images, with single-particle resolution, reveal connections between bulk mechanical response and microstructure during plastic deformation. Specifically, structural relaxation induced by plastic flow depends on strain rate at earlier times and predicts microscopic structural features at later times.

Abstract

When stressed sufficiently, solid materials yield and deform plastically via reorganization of microscopic constituents. Indeed, it is possible to alter the microstructure of materials by judicious application of stress, an empirical process utilized in practice to enhance the mechanical properties of metals. Understanding the interdependence of plastic flow and microscopic structure in these nonequilibrium states, however, remains a major challenge. Here, we experimentally investigate this relationship, between the relaxation dynamics and microscopic structure of disordered colloidal solids during plastic deformation. We apply oscillatory shear to solid colloidal monolayers and study their particle trajectories as a function of shear rate in the plastic regime. Under these circumstances, the strain rate, the relaxation rate associated with plastic flow, and the sample microscopic structure oscillate together, but with different phases. Interestingly, the experiments reveal that the relaxation rate associated with plastic flow at time $t$ is correlated with the strain rate and sample microscopic structure measured at earlier and later times, respectively. The relaxation rate, in this nonstationary condition, exhibits power-law, shear-thinning behavior and scales exponentially with sample excess entropy. Thus, measurement of sample static structure (excess entropy) provides insight about both strain rate and constituent rearrangement dynamics in the sample at earlier times.