In coastal deserts, moist air can frequently blow over dry land, without ever coalescing or precipitating down on the surface. These regions often also suffer economic hardship and lack of infrastructure, such that harnessing airborne moisture can be a basis for life. Humans have spent some decades developing meshes to intercept and collect fog. The living world has spent eons whimsically mixing physical and chemical mechanisms. A Namib desert beetle, which collects fog from the morning wind atop sand dunes near the coast, has raised the prospect of transformative biomimetic solutions, but nearly all research has focused on the surface transport mechanism, particularly with respect to wettability. We are investigating the fluid dynamics of the other crucial step: droplet interception before coalescence and collection, focusing on the role of surface morphology (ie. bumps like those of beetle elytra) and non-trivial flow geometries, to find new angles on improved collection efficiency, and insight into the antithetical problem of icing in flight and wind power.

Physics of termite mound respiration and moisture conservation

Many termite species live in large colonies and harvest fungus to supplement nutrition. Living underground keeps them cool and protected, but creates a serious issue for respiration. The tall mounds that protrude above the nests are effectively lungs, which harness oscillations of ambient temperature to drive bulk flow between the nest and mound, while gases exchange via diffusion through the porous walls. Mound architecture amplifies transient thermal gradients along connected conduits to facilitate the internal convection, in an inspirational example of emergent engineering.

Understanding the physical mechanisms by which their design manages to respire metabolic gases while conserving moisture is our present focus.

Wrinkling and crumpling of thin sheets

When a thin sheet is compressed in one thin direction, it readily buckles outward, demonstrating the relative ease in storing energy in bending rather than in-plane stresses. If the sheet is adhered to a soft (or liquid) substrate, the same Euler instability leads to a pattern of smooth wrinkles, caused by competition between the bending modulus of the sheet and the stiffness (or weight) of the substrate. This buckling motif fundamentally differs from that observed when one crumples a piece of paper: pointy cusps connected by ridges. The sharpness in the crumpled shape illustrate the phenomenon of stress focusing, in which the sheet chooses to concentrate elastic energy in small regions in response to confinement from the boundaries. By confining a flat sheet to a slightly, variably curved, liquid meniscus and optically measuring its deformation, we tested predictions for wrinkle extent in the simplest case of non-uniform confinement, and discovered a generic route by which a featureless sheet assumes a crumpled shape.