Contact between particles and motile cells underpins a wide variety of biological processes, from nutrient capture and ligand binding, to grazing, viral infection and cell-cell communication. The window of opportunity for these interactions is ultimately determined by the physical mechanism that enables proximity and governs the contact time. Jeanneret et al. (Nat. Comm. 7: 12518, 2016) reported recently that for the biflagellate microalga Chlamydomonas reinhardtii contact with microparticles is controlled by events in which the object is entrained by the swimmer over large distances. However, neither the universality of this interaction mechanism nor its physical origins are currently understood. Here we show that particle entrainment is indeed a generic feature for microorganisms either pushed or pulled by flagella. By combining experiments, simulations and analytical modelling we reveal that entrainment length, and therefore contact time, 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. The existence of an optimal tracer size is predicted theoretically, and observed experimentally for C. reinhardtii. Spatial organisation of flagella, swimming speed, swimmer and tracer size influence entrainment features and provide different trade-offs that may be tuned to optimise microbial interactions like predation and infection.
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