Flow characteristics of Chlamydomonas result in purely hydrodynamic scattering.

It has long been believed that eukaryotic flagellated swimming cells feel solid boundaries through direct ciliary contact. Specifically, based on observations of behavior of green alga Chlamydomonas reinhardtii it has been reported that it is their "flagella [that] prevent the cell body from touching the surface" [Kantsler et al., Proc. Natl. Acad. Sci. USA 110, 1187 (2013)PNASA60027-842410.1073/pnas.1210548110]. Here, via investigation of a model swimmer whose flow field closely resembles that of C. reinhardtii, we show that the scattering from a wall can be purely hydrodynamic and that no mechanical or flagellar force is needed for sensing and escaping the boundary.