The hydrodynamics of swimming microorganisms

TL;DR

This paper reviews the fundamental physics of microorganism swimming at low Reynolds numbers, covering flow phenomena, theoretical models, and recent research directions in biological and artificial microswimmers.

Contribution

It provides a comprehensive overview of the physical principles, classical theories, and emerging research areas related to microorganism locomotion in viscous fluids.

Findings

Flow singularities and resistance matrices are key to understanding microorganism swimming.

Classical models like resistive-force theory effectively predict flagellar propulsion.

Active research includes hydrodynamic interactions and artificial microswimmer design.

Abstract

Cell motility in viscous fluids is ubiquitous and affects many biological processes, including reproduction, infection, and the marine life ecosystem. Here we review the biophysical and mechanical principles of locomotion at the small scales relevant to cell swimming (tens of microns and below). The focus is on the fundamental flow physics phenomena occurring in this inertia-less realm, and the emphasis is on the simple physical picture. We review the basic properties of flows at low Reynolds number, paying special attention to aspects most relevant for swimming, such as resistance matrices for solid bodies, flow singularities, and kinematic requirements for net translation. Then we review classical theoretical work on cell motility: early calculations of the speed of a swimmer with prescribed stroke, and the application of resistive-force theory and slender-body theory to flagellar locomotion. After reviewing the physical means by which flagella are actuated, we outline areas of active research, including hydrodynamic interactions, biological locomotion in complex fluids, the design of small-scale artificial swimmers, and the optimization of locomotion strategies.