Session F13: Drops: Elastic Surfaces and Fibers

To eject a droplet from a dampened, damped beam

At the scale of small insects, additional mass such as moisture deposited on wings from rain and dew fall alters flight dynamics. Successful flight, therefore, requires that surfaces are free of debris. Drawing from drying behaviors witnessed in mosquitoes, we investigate the release of droplets from flexible, high-amplitude cantilever beams at the mesoscale. These beams are sinusoidally displaced at their base across 85-400 Hz, producing surfaces accelerations of 2-120 gravities at droplet release.  We observe three principle droplet ejection modes: normal to beam ejection, tangential to beam ejection and droplet volume reduction through pinch-off. Experiments show that ejection modes are dependent on droplet and cantilever properties, and input motion in this highly coupled system. Predictions of ejection modes are accomplished by application of Euler elastica theory and droplet adhesion forces.   

Coiling of an elastic filament on a spherical bubble

When a thin elastic filament is placed on top of a spherical bubble, it buckles under the bubble's Laplace pressure. When the elasto-capillary length scale, defined as the length at which bending force is balanced by capillary force, is much smaller than the bubble size the filament is confined to the bubble surface. In this scale-separated regime we ask what shape does a filament take? We find in our experiments that the geodesic minimises bending energy up to a critical filament length for a fixed bubble size beyond which the weight of the filament affects its shape. As we increase the ratio of filament length to bubble radius it shifts from a geodesic to take a path approximated by a latitude at a given polar angle. The filament then starts coiling around the latitude at this fixed polar angle. We construct a geometric model that describes our experimental observations and obtain a two-parameter coiling phase-diagram with geometry described by the ratio of filament length to bubble size and the ratio of bending force to filament weight.   

Unduloid formation in colloidal membranes

Flat colloidal membranes composed of filamentous fd viruses were doped with shorter viruses of identical pitch and width to form metastable saddles. For sufficiently low long rod fractions in a range of osmotic pressures, these saddles coalesce into axisymmetric tubular shapes. We propose a simple steric model to explain how the introduction of short rods leads to a preferred curvature and employ an effective geometric theory to calculate these shapes, which are closely related to the unduloids studied in the theory of droplets.   

Elastocapillary contact between droplets and highly bendable membranes

Recently, there has been great interest in how the classic picture of Young contact is modified for soft solids: horizontal force balance must be supplemented by a vertical force balance. Previous theoretical work has largely focussed on the contact of droplets with semi-infinite soft solids or flexible elastic beams. However, several recent experiments have studied how the contact of a droplet on a thin membrane may be used experimentally to infer the state of stress within a membrane (under the assumption that a small droplet does not change this stress significantly). We consider the mechanics of an elastic membrane  subject to a pre-stress and wetted by a droplet, neglecting the effects of bending stiffness. We find that for `strong' pre-stress the tension at the contact line is only weakly perturbed from the value of the pre-stress. However, for `weak' pre-stress, the presence of the droplet significantly modifies the stress within the membrane: the tension at the contact line is significantly larger than the value prior to wetting. We define what is meant by `strong' and `weak' in these cases, and how these two cases may be distinguished experimentally when the level of pre-stress is not known a priori.   

Droplet Interactions with the Prickly Pear Cactus and Western Diamondback Rattlesnake of the Sonoran Desert

The Sonoran Desert is a unique xeric climate in which bimodal (summer, winter ) seasonal storms are the primary water source for plants and animals. To be able to subsist in this harsh environment, many species have developed specialized water storage and collection tactics. In this presentation, we will discuss our work on water droplet interactions involving two icons of the American Southwest: the prickly pear cactus and western diamondback rattlesnake. In the former, we discovered that seasonal hydration and dehydration cycles lead to the formation of multilevel micro-cracks of the exterior epidermis and related transition of the surface from super-hydrophobic to super-hydrophilic. While dramatically altering how droplets spread on the cacti overall, our results suggest that this seasonal epidermis microstructure is predominantly helping to regulate water vapor transport. In the second part of the talk, we will discuss how western diamondback rattlesnakes collect and drink rain droplets. This process, which is termed rain-harvesting, involves rain droplet impact onto the snake’s dorsal body, capillary transport on the skin’s surface, body shape adjustment, and suction-aided drinking. We will discuss both micro- and macroscopic aspects of these steps.