Transitions in function at low Reynolds number: hair-bearing animal appendages

Many types of animals use appendages bearing arrays of hair-like structures to capture molecules (e.g. olfactory antennae, gills) or particles (e.g. suspension-feeding appendages) from the surrounding water or air, and to locomote or move <uid past themselves. The performance of these functions depends on how much of the <uid encountered by the array of hairs <ows through the gaps between the hairs rather than around the perimeter of the whole array. By modelling such arrays of hairs as rows of %nite width of cylinders operating at low Reynolds numbers, the <uid velocity %elds with respect to the hairs were calculated. Such models revealed a transition from non-leaky to leaky behaviour as Re was increased. The purpose of this paper is to provide a brief review of the features of this transition as revealed by models, and then to describe examples of how animals use the transition in <uid <ow to perform particular functions: rejection of captured material by copepods, and sni?ng by lobsters. Copyright ? 2001 John Wiley & Sons, Ltd.