From the fixative properties of hairsprays to the stickiness of filaments on beetles' feet, the wetting of flexible fibres with droplets of liquid is a universal phenomenon — but one we know surprisingly little about. On page 510 of this issue, Duprat et al. formulate rules to describe how mists of droplets interact with flexible fibre arrays (C. Duprat, S. Protière, A. Y. Beebe & H. A. Stone Nature 482, 510–513; 2012).
The researchers began with the simplest possible model: the interactions of water droplets with a pair of closely aligned, flexible glass fibres that were clamped at one end but free to bend at the other. They observed that a droplet deposited close to the clamped ends adopts one of three forms: it could remain as a tight, spherical bridge between the filaments, or, depending on the conditions, it could either partially or completely spread along the fibres, in the latter case causing them to coalesce.
On further investigation, Duprat et al. found that six physical parameters control droplet shape and spreading; they include fibre geometry, the distance between the fibres, and the fibres' mechanical properties. The authors also identified a critical droplet volume above which fibres do not coalesce, and a second critical volume at which droplet capture by fibres is maximized.
The team went on to explore the wetting of a natural fibre array by spraying a goose feather with oil droplets and observing the effects on the barbules (filaments projecting from each barb of a feather). They found that their theoretical model held up — small droplets spread along the barbules and caused barbule clumping, whereas larger droplets did not spread and could be easily dislodged — despite the roughness of the feather's barbules and the chemical affinity between the droplet and the barbules' surfaces.
Duprat and colleagues' discoveries suggest that the mechanical properties and spatial organization of biological fibre arrays may have evolved to optimize interactions with liquid droplets, and so enhance functions such as adhesion, dew collection and self-cleaning. The work also offers opportunities for improving the performance of technological wetting systems — for example, droplet volumes in sprays could be engineered to fine-tune their wetting interactions with relevant fibres.