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Droplets on Compliant Substrates : Spontaneous Droplet Motion and Interaction
Several novel phenomena have emerged from recent research in the behavior of drops on compliant substrates. Here we present (a) spontaneous droplet motion on a substrate with periodic variation of compliance, and (b) interaction of droplets separated by a thin elastic film.
Droplet motion arises in many natural phenomena, ranging from the familiar gravity-driven slip and arrest of raindrops on windows, to the directed transport of droplets for water harvesting by plants and animals under dry conditions. Deliberate transportation and manipulation of droplets is also important in many technological applications, including droplet-based microfluidic chemical reactors and for thermal management. Droplet motion usually requires gradients of surface energy or temperature, or external vibration to overcome contact angle hysteresis. Here, we report a new phenomenon in which a drying droplet placed on a periodically compliant surface undergoes spontaneous, chaotic motion in the absence of surface energy gradients and external stimuli. By modeling the droplet as a mass-spring system on a substrate with periodically varying compliance, we show that the energy released during vaporization is partially stored by the compliant surface. It is released periodically via mechanical instabilities to drive the surprisingly large-scale spontaneous and concerted motion of the drop. Our finding offers a novel method by which the droplet’s internal chemical energy can be converted into mechanical kinetic energy.
The Laplace pressure of a droplet placed on one side of an elastic thin film can cause significant deformation in the form of a bulge on its opposite side. Here, we show that this deformation can be detected by other droplets suspended on the opposite side of the film leading to interaction between droplets separated by the solid (but deformable) film. The interaction is repulsive when the drops have a large overlap and attractive when they have a small overlap. Thus, if two identical droplets are placed right on top of each other (one on either side of the thin film) they tend to repel each other, eventually reaching an equilibrium configuration where there is a small overlap. This observation can be explained by analyzing the energy landscape of the droplets interacting via an elastically deformed film. We further demonstrate this idea by designing a pattern comprising a big central drop with satellite droplets. This phenomenon can lead to techniques for directed motion of droplets confined to one side of a thin elastic membrane by manipulations on the other side.