  Motivated by recent experiments demonstrating that motile algae get trapped
in draining foams, we study the trajectories of microorganisms confined in
model foam channels (section of a Plateau border). We track single
Chlamydomonas reinhardtii cells confined in a thin three-circle microfluidic
chamber and show that their spatial distribution exhibits strong corner
accumulation. Using empirical scattering laws observed in previous experiments
(scattering with a constant scattering angle), we next develop a two-dimension
geometrical model and compute the phase space of trapped and periodic
trajectories of swimmers inside a three-circles billiard. We find that the
majority of cell trajectories end up in a corner, providing a geometrical
mechanism for corner accumulation. Incorporating the distribution of scattering
angles observed in our experiments and including hydrodynamic interactions
between the cells and the surfaces into the geometrical model enables us to
reproduce the experimental probability density function of micro-swimmers in
microfluidic chambers. Both our experiments and models demonstrate therefore
that motility leads generically to trapping in complex geometries.
