Direct measurement of the flow field around swimming microorganisms

TL;DR

This study directly measures the flow fields around swimming microorganisms Volvox and Chlamydomonas, revealing dominant flow contributions and challenging previous assumptions about their hydrodynamic signatures.

Contribution

First detailed experimental flow measurements around freely-swimming microorganisms, showing the dominance of Stokeslet flow in Volvox and a new flow model for Chlamydomonas.

Findings

V. carteri flow dominated by Stokeslet due to density excess

C. reinhardtii flow differs from typical stresslet models

Flow features resemble sedimenting particle suspensions

Abstract

Swimming microorganisms create flows that influence their mutual interactions and modify the rheology of their suspensions. While extensively studied theoretically, these flows have not been measured in detail around any freely-swimming microorganism. We report such measurements for the microphytes Volvox carteri and Chlamydomonas reinhardtii. The minute ~0.3% density excess of V. carteri over water leads to a strongly dominant Stokeslet contribution, with the widely-assumed stresslet flow only a correction to the subleading source dipole term. This implies that suspensions of V. carteri have features similar to suspensions of sedimenting particles. The flow in the region around C. reinhardtii where significant hydrodynamic interaction is likely to occur differs qualitatively from a "puller" stresslet, and can be described by a simple three-Stokeslet model.