Viscophobic turning dictates microalgae transport in viscosity gradients

Gradients in fluid viscosity characterize microbiomes ranging from mucus layers on marine organisms 1 and human viscera 2,3 to biofilms 4 . While such environments are widely recognized for their protective effects against pathogens and their ability to influence cell motility 2,5 , the physical mechanisms regulating cell transport in viscosity gradients remain elusive 6–8 , 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 region 9,10 is stifled by a viscophobic turning motility. This deterministic cell rotation – consistent with a flagellar thrust imbalance 11,12 – 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.