Effective temperatures of a driven, strongly anisotropic Brownian system

TL;DR

This study uses simulations to examine the concept of effective temperature in a driven, anisotropic colloidal system, revealing directional dependencies and questioning the universality of current effective temperature definitions.

Contribution

It provides a comparative analysis of effective temperatures derived from fluctuation-dissipation ratios and linear response in anisotropic systems, highlighting their differences.

Findings

Effective temperatures are approximately wave-vector independent at high shear rates.

Temperatures along the shear gradient are higher than along the vorticity direction.

Current formulas for effective temperatures may not be valid for strongly anisotropic, driven systems.

Abstract

We use Brownian Dynamics computer simulations of a moderately dense colloidal system undergoing steady shear flow to investigate the uniqueness of the so-called effective temperature. We compare effective temperatures calculated from the fluctuation-dissipation ratios and from the linear response to a static, long wavelength, external perturbation along two directions: the shear gradient direction and the vorticity direction. At high shear rates, when the system is strongly anisotropic, the fluctuation-dissipation ratio derived effective temperatures are approximately wave-vector independent, but the temperatures along the gradient direction are somewhat higher than those along the vorticity direction. The temperatures derived from the static linear response show the same dependence on the direction as those derived from the fluctuation-dissipation ratio. However, the former and the latter temperatures are different. Our results suggest that the presently used formulae for effective temperatures may not be applicable for strongly anisotropic, driven systems.