Funnel ratchets in biology at low Reynolds number: choanotaxis

We present an example of a ratchet phenomena in biology consisting of swimming bacteria in an enclosed structure separated by a wall of funnels. The funnels serve to concentrate bacteria that are motile, in spite of the fact that the motion occurs at low Reynolds numbers and would be expected to be time reversible invariant. We discuss some of the basic ideas which underly the phenomena including spatial and temporal coherence of the motion of bacteria and interaction of the bacteria with surfaces.