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 fluid past themselves. The performance of these functions depends on how much of the fluid encountered by the array of hairs flows through the gaps between the hairs rather than around the perimeter of the whole array. By modelling such arrays of hairs as rows of finite width of cylinders operating at low Reynolds numbers, the fluid velocity fields 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 fluid flow to perform particular functions: rejection of captured material by copepods, and sniffing by lobsters. Copyright © 2001 John Wiley & Sons, Ltd.