Effective Viscosity of a Dilute Suspension of Membrane-bound Inclusions

TL;DR

This paper studies how the effective viscosity of a dilute suspension of membrane-bound disks depends on particle size and membrane-bulk fluid interactions, revealing unique two-dimensional hydrodynamic effects.

Contribution

It extends classical hydrodynamics of suspensions to membrane-bound particles, highlighting the impact of membrane coupling and particle size on viscosity.

Findings

Particle size relative to hydrodynamic length scale affects viscosity.

Membrane hydrodynamics differ significantly from bulk fluid suspensions.

Mathematical tools for mixed boundary value problems are elucidated.

Abstract

When particulate suspensions are sheared, perturbations in the shear flows around the rigid particles increase the local energy dissipation, so that the viscosity of the suspension is effectively higher than that of the solvent. For bulk (three-dimensional) fluids, understanding this viscosity enhancement is a classic problem in hydrodynamics that originated over a century ago with Einstein's study of a dilute suspension of spherical particles. \cite{Einstein1} In this paper, we investigate the analogous problem of the effective viscosity of a suspension of disks embedded in a two-dimensional membrane or interface. Unlike the hydrodynamics of bulk fluids, low-Reynolds number membrane hydrodynamics is characterized by an inherent length scale generated by the coupling of the membrane to the bulk fluids that surround it. As a result, we find that the size of the particles in the suspension relative to this hydrodynamic length scale has a dramatic effect on the effective viscosity of the suspension. Our study also helps to elucidate the mathematical tools needed to solve the mixed boundary value problems that generically arise when considering the motion of rigid inclusions in fluid membranes.