Viscophobic motility of biflagellated microalgae

Swimming cells often live in environments characterized by spatial gradients of rheological properties, including biofilms and mucus layers. In this work, we demonstrate experimentally that swimming biflagellate cells (Chlamydomonas reinhardtii) exhibit avoidance of high viscosity regions, stemming from a purely hydrodynamic effect. Microfluidic devices are used to generate a spatial gradient of a Newtonian polymer suspension, and video microscopy captures the cell motion. While cells are expected to accumulate in high viscosity due to a mechanical reduction in swimming speed, we observe enhanced cell concentration in low viscosity regions. A statistical analysis of cell motility reveals strongly curved swimming trajectories that are redirected toward low viscosity zones. This viscophobic behavior is explained by an orientation-dependent torque resulting from the differential force generated by the cells’ dual flagella, which sample different fluid viscosities within the gradient, and it is justified by a simple hydrodynamic model.