Hydrodynamic particle interactions in linear and radial viscosity gradients

TL;DR

This paper develops a perturbative method to calculate the mobility matrix of particles in fluids with spatially varying viscosity, revealing how viscosity gradients influence particle interactions and mobilities.

Contribution

It introduces a novel perturbative calculation scheme for particle mobility in non-uniform viscosity fields using the Lorentz reciprocal theorem and reflection methods.

Findings

Viscosity gradients induce long-range flow fields affecting particle motion.

Relative particle positioning in viscosity gradients influences their mobilities and interactions.

Temperature differences alter fluid viscosity, impacting particle self-mobilities and interactions.

Abstract

We present a versatile perturbative calculation scheme to determine the mobility matrix for two and more particles in a low Reynolds number fluid with spatially variant viscosity. Assuming an asymptotic non-constant viscosity perturbation superimposed on a constant viscosity background, we exploit the Lorentz reciprocal theorem and a reflection method to obtain the leading order correction to the mobility matrix. We apply our approach firstly to an interface-like linear viscosity gradient with an extension much larger than the length scale of the particle separation. We find that the viscosity gradient gives rise to additional long-range flow fields, for both particle translation and rotation, which decay by one order slower than their constant-viscosity counterparts. Also, we reveal that the relative positioning of two interacting particles in finite-size gradients, a natural precondition to avoid pathological negative viscosity, affects the mobilities of and interaction between both particles. Secondly, we apply our result to two interacting particles with temperatures different from that of the surrounding fluid, which alters both the fluid temperature and viscosity field. We find that both, the self-mobilities of the particles as well as their interactions, increase for hot particles and decrease for cold particles.