Trainable amorphous matter: tuning yielding by mechanical annealing

Abstract

Living organisms can demonstrate highly adaptable and sophisticated responses using memory resulting from repeated exposure to external conditions or training. However, realizing similar adaptability in mechanical responses in inanimate, physical materials presents an outstanding challenge in several fields, including soft matter, materials science, and in the domain of soft robotics, to name a few. Our study focuses on disordered solids, which are model systems that resemble granular matter, foam and other disordered, soft solids. Here, combining bulk rheology, in-situ optical imaging, and numerical simulations, we demonstrate how training via cyclic shear can encode memories that tune the yield point in a unique way and over unprecedented ranges. Our study reveals that such tunability is intricately linked to the plasticity, non-affine deformations, and formation of shear bands. Remarkably, our numerical simulations illustrate that systems with identical internal energies, prepared via different protocols (mechanical or thermal), can display markedly different rheological responses, indicating that energy alone does not determine mechanical behavior. Moreover, while the yield strain increases with training amplitude, the material simultaneously softens, contrasting with the thermal case where both quantities increase monotonically with increasing annealing. Our results open up possibilities for memory-induced tuning of mechanical response in trainable amorphous matter, independently or in combination with thermal annealing, far beyond the material--feature space achievable via the latter alone.