  In this letter we propose a kinematic model to show how collisions with a
surface and rotational Brownian motion give rise to the accumulation of
micro-swimmers near a surface. In this model, an elongated microswimmer
invariably travels parallel to the surface after hitting it from any incident
angle. It then swims away from the surface after some time, facilitated by
rotational Brownian motion. Simulations based on this model reproduce the
density distributions measured for the small bacteria E. coli and Caulobacter
crescentus, as well as for the much larger bull spermatozoa swimming in
confinement.
