Wetting of flexible fibre arrays

The parameters critical in determining the behaviour of a fibrous medium wetted with a single liquid drop are identified as fibre flexibility, fibre geometry and drop volume. In a study of the behaviour of liquid droplets on flexible fibres, Camille Duprat and colleagues identify six parameters that control how a droplet wets the fibres, including droplet size and mechanical properties of the fibres. Depending on the parameters, the droplet can remain tightly spherical, bridging the fibres; partially wet the fibres; or fully wet them, causing the fibres to cling together. The authors demonstrate using a natural system — a goose feather — that by adjusting drop volume it is possible to control the matting of fibre arrays. From a technological perspective, careful control of these parameters offers the prospect of adjusting the adsorption of droplets for functional microstructured materials. Furthermore, drop volume can be used to control wetting in sprays such as hairsprays, and in the de-oiling of birds contaminated by oil spillage. Fibrous media are functional and versatile materials, as demonstrated by their ubiquity both in natural systems such as feathers1,2,3,4 and adhesive pads5 and in engineered systems from nanotextured surfaces6 to textile products7, where they offer benefits in filtration, insulation, wetting and colouring. The elasticity and high aspect ratios of the fibres allow deformation under capillary forces, which cause mechanical damage8, matting5,9 self-assembly10,11 or colour changes12, with many industrial and ecological consequences. Attempts to understand these systems have mostly focused on the wetting of rigid fibres13,14,15,16,17 or on elastocapillary effects in planar geometries18 and on a fibre brush withdrawn from an infinite bath19. Here we consider the frequently encountered case of a liquid drop deposited on a flexible fibre array and show that flexibility, fibre geometry and drop volume are the crucial parameters that are necessary to understand the various observations referred to above. We identify the conditions required for a drop to remain compact with minimal spreading or to cause a pair of elastic fibres to coalesce. We find that there is a critical volume of liquid, and, hence, a critical drop size, above which this coalescence does not occur. We also identify a drop size that maximizes liquid capture. For both wetting and deformation of the substrates, we present rules that are deduced from the geometric and material properties of the fibres and the volume of the drop. These ideas are applicable to a wide range of fibrous materials, as we illustrate with examples for feathers, beetle tarsi, sprays and microfabricated systems.