A universal entrainment dominates the interaction of flagellated microorganisms with micron-sized objects

The incessant activity of swimming microorganisms has a direct physical effect on surrounding microscopic objects, leading to enhanced diffusion far beyond the level of Brownian motion with possible influences on the spatial distribution of non-motile planktonic species and particulate drifters. Here we study in detail the effect on microparticles' dynamics of eukaryotic flagellates, represented by the freshwater microalga Chlamydomonas reinhardtii, and the marine species Tetraselmis subcordiformis and Oxyrrhis marina. Macro- and micro-scopic experiments reveal that microorganism-colloid interactions are dominated by rare close encounters leading to large displacements through direct entrainment. Simulations and theoretical modelling show that the ensuing particle dynamics can be understood in terms of a simple jump-diffusion process, combining standard diffusion with Poisson-distributed jumps. Entrainment length can be understood within the framework of Taylor dispersion as a competition between advection by the no-slip surface of the cell body and microparticle diffusion. Theory predicts the existence of an optimal tracer size which is observed experimentally for C. reinhardtii.