Structural predictor for nonlinear sheared dynamics in simple glass-forming liquids

Abstract

Glass-forming liquids subjected to sufficiently strong shear universally exhibit striking nonlinear behavior; for example, a power-law decrease of the viscosity with increasing shear rate. This phenomenon has attracted considerable attention over the years from both fundamental and applicational viewpoints. However, the out-of-equilibrium and nonlinear nature of sheared fluids have made theoretical understanding of this phenomenon very challenging and thus slower to progress. We find here that the structural relaxation time as a function of the two-body excess entropy, calculated for the extensional axis of the shear flow, collapses onto the corresponding equilibrium curve for a wide range of pair potentials ranging from harsh repulsive to soft and finite. This two-body excess entropy collapse provides a powerful approach to predicting the dynamics of nonequilibrium liquids from their equilibrium counterparts. Furthermore, the two-body excess entropy scaling suggests that sheared dynamics is controlled purely by the liquid structure captured in the form of the two-body excess entropy along the extensional direction, shedding light on the perplexing mechanism behind shear thinning.

Scaling of relaxation and excess entropy in plastically deformed amorphous solids

“…The comparatively high peaks in g(r ) at t = 19.2 s compared to t = 16.7 s suggest a more ordered structure in the former case. The differences in peak height at different times were rather small and were in accord with measurements in sheared molecular glasses (14,16,37). From the time-dependent g(r ) data, we computed S2 versus t,…”

“…We used nonaffine particle motions to evaluate sample relaxation behavior (16,37). At time t, the self-part of the intermediate scattering function is…”

“…If the scaling relation still holds, then, by implication, sample static structure can provide information about relaxation induced by plastic deformation. Previously, viscosity, diffusion coefficients, and relaxation times have been found to obey a simple excess entropy scaling law, τα ∼ f (S ex ), for a wide variety of materials (20-30) spanning different particle type, size, density, interaction, temperature, material phase, and even shear rate (16,24). Importantly, S ex is well approximated by the two-body contribution, S2, which is readily determined from scattering or imaging experiments (15,19,27).…”

“…Excess entropy accurately predicts transport coefficients of simple and complex fluids in equilibrium using their static structure (15,(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30). Recently, in computer simulations, S ex has been applied to supercooled liquids under steady-state shear; the shear-dependent relaxation time of the supercooled liquids was found to scale with S ex (16), thereby revealing a simple structural connection to shear-thinning induced relaxation. This intriguing discovery has not been tested experimentally.…”

“…In a different vein, thermodynamic predictors based on excess entropy (S ex ) have shown promise for explaining nonlinear mechanical phenomena in complex fluids (12)(13)(14)(15)(16)(17). Excess entropy concepts were developed from studies of liquids rather than solids; they facilitate comparison of macroscopic systemaveraged structural and dynamical quantities.…”