Capturing droplets with oil-impregnated fibers

Lubricant-impregnated surfaces (LIS) consist of a liquid lubricant stabilized within a textured surface by capillary forces. Droplets on LIS have the unique combination of a high lateral mobility (i.e., low contact angle hysteresis) and a high work of adhesion (i.e., difficult to detach). To date, virtually all reports characterizing the wettability of LIS have exclusively focused on the mobility of test droplets on planar substrates. Here, we pioneer the construct of lubricant-impregnated fibers (LIFs), which exhibit unique droplet dynamics due to the simultaneous exploitation of high mobility and high adhesion.

Superhydrophobic fibers of differing diameters were impregnated with silicone oil and held horizontally. Immiscible test droplets were released onto the top of a LIF over a wide range of Weber numbers ranging from gentle deposition (We = 0) to high-speed impact (We > 500). Due to the low contact angle hysteresis, droplets always slid toward the bottom of the fiber’s cross section. With increasing Weber number, a transition was observed from droplets remaining adhered to the fiber’s underside to an inertial detachment. Surprisingly, at yet larger Weber numbers, droplets once more adhered to the fiber. This is due to a new phenomenon we term “unsplashing,” where the impact splits a droplet into two which reduces their inertia, such that the droplet remains adhered after merging back together at the fiber’s bottom.