Viscophobic turning dictates microalgae transport in viscosity gradients

Abstract

Gradients in fluid viscosity characterize microbiomes ranging from mucus layers on marine organisms1 and human viscera2,3 to biofilms4. While such environments are widely recognized for their protective effects against pathogens and their ability to influence cell motility2,5, the physical mechanisms regulating cell transport in viscosity gradients remain elusive6-8, primarily due to a lack of quantitative observations. Through microfluidic experiments, we directly observe the transport of model biflagellated microalgae (Chlamydomonas reinhardtii) in controlled viscosity gradients. We show that despite their locally reduced swimming speed, the expected cell accumulation in the viscous region9,10 is stifled by a viscophobic turning motility. This deterministic cell rotation - consistent with a flagellar thrust imbalance11,12 - reorients the swimmers down the gradient, causing their accumulation in the low viscosity zones for sufficiently strong gradients. Corroborated by Langevin simulations and a three-point force model of cell propulsion, our results illustrate how the competition between viscophobic turning and viscous slowdown ultimately dictates the fate of population scale microbial transport in viscosity gradients.