Nonequilibrium mode-coupling theory for uniformly sheared underdamped systems

TL;DR

This paper develops a nonequilibrium mode-coupling theory for underdamped sheared systems, incorporating translational invariance and inertia effects, extending previous theories to include underdamped dynamics and resolving existing contradictions.

Contribution

It introduces a non-equilibrium MCT for underdamped systems that accounts for inertia and current fluctuations, extending and resolving issues in prior theories.

Findings

The theory satisfies translational invariance and wavevector alignment.

Current fluctuations grow during the α-relaxation, affecting yield stress.

Inertia effects influence the response to shear rate perturbations.

Abstract

Nonequilibrium mode-coupling theory (MCT) for uniformly sheared underdamped systems is developed, starting from the microscopic thermostatted SLLOD equation, and the corresponding Liouville equation. Special attention is paid to the translational invariance in the sheared frame, which requires an appropriate definition of the transient time-correlators. The derived MCT equation satisfies the alignment of the wavevectors, and is manifestly translationally invariant. Isothermal condition is implemented by the introduction of the current fluctuation in the dissipative coupling to the thermostat. This current fluctation grows in the $\alpha$-relaxation regime, which generates a deviation of the yield stress in the glassy phase from the overdamped case. Response to a perturbation of the shear rate demonstrates an inertia effect which is not observed in the overdamped case. Our theory turns out to be a non-trivial extension of the theory by Fuchs and Cates (J. Rheol. 53(4), 2009) to underdamped systems. Since our starting point is identical to that by Chong and Kim (Phys. Rev. E 79, 2009), the contradictions between Fuchs-Cates and Chong-Kim are resolved.