  We explore in this paper the phenomenon of photofocusing: a coupling between
flow vorticity and biased swimming of microalgae toward a light source that
produces a focusing of the microswimmer suspension. We combine experiments that
investigate the stationary state of this phenomenon as well as the transition
regime with analytical and numerical modeling. We show that the experimentally
observed scalings on the width of the focalized region and the establishment
length as a function of the flow velocity are well described by a simple
theoretical model.
