Is hydrodynamic interaction enough for green algae to bounce back from a wall?*

The primary mechanism behind the interaction of swimming microorganisms with solid boundaries has been a matter of dispute. Based on series of visual observations on the behavior of green alga Chlamydomonas reinhardtii (widely known as a paradigm for puller-type swimmers), it has been believed that the scattering process is mainly governed by contact forces rather than hydrodynamic interactions. Here, via investigation of a model micro-swimmer designed in such a way that its flow field closely resembles the complex oscillatory flow of a C. reinhardtii cell, we show that the scattering from a wall can be purely hydrodynamic and that no mechanical force is needed for sensing and escaping the boundary.