Two-step shear thinning in deeply supercooled liquids

Abstract Nonlinear rheological phenomena are often observed in complex fluids and soft materials. For instance, although supercooled glassy liquids behave as a Newtonian fluid with a shear-rate independent viscosity under slow flows, they exhibit a significant reduction in viscosity when exposed to fast shear flows. Despite being fundamentally and practically important, the mechanism behind this phenomenon, known as shear thinning, has remained unsolved for decades. In the past, various theories including the mode coupling theory (MCT) have been put forward, which, however, propose conflicting scenarios. To resolve this conflict, we use computer simulations to study the Gaussian core model that mimics soft materials like star polymers and dendrimers. Remarkably, we discover that two distinct scenarios, "advection" and "distortion", emerge at different shear rates, which results in a two-step shear thinning phenomenon. We extend the schematic model of the MCT to account for distortion and advection mechanisms simultaneously, and theoretically explain the two-step thinning. Our results gain a consensus between competing theories, and establish a comprehensive understanding of the shear thinning in supercooled liquids. The two-step shear thinning can occur in various soft materials; in particular, star polymers and dendrimers can be utilized to validate our numerical and theoretical results experimentally.