A proof of Einstein's effective viscosity for a dilute suspension of spheres

TL;DR

This paper provides a rigorous mathematical proof of Einstein's formula for the effective viscosity of dilute suspensions of spheres, including boundary effects and explicit bounds, with potential for generalization.

Contribution

It offers the first rigorous proof of Einstein's effective viscosity formula for dilute sphere suspensions, including boundary effects and explicit bounds.

Findings

Einstein's formula is validated as a first-order asymptotic expansion.

Explicit upper and lower bounds on effective viscosity are derived.

The proof can be extended to other particle shapes and self-propelled particles.

Abstract

We present a mathematical proof of Einstein's formula for the effective viscosity of a dilute suspension of rigid neutrally--buoyant spheres when the spheres are centered on the vertices of a cubic lattice. We keep the size of the container finite in the dilute limit and consider boundary effects. Einstein's formula is recovered as a first-order asymptotic expansion of the effective viscosity in the volume fraction. To rigorously justify this expansion, we obtain an explicit upper and lower bound on the effective viscosity. A lower bound is found using energy methods reminiscent of the work of Keller et al. An upper bound follows by obtaining an explicit estimate for the tractions, the normal component of the stress on the fluid boundary, in terms of the velocity on the fluid boundary. This estimate, in turn, is established using a boundary integral formulation for the Stokes equation. Our proof admits a generalization to other particle shapes and the inclusion of point forces to model self-propelled particles.