Dewetting Dynamics on Lotus Leaves: Adhesion-Controlled Droplet Jumping

Abstract

Water condensation on superhydrophobic (SHP) lotus leaf and dragonfly wing surfaces has been directly examined using a high-speed camera mounted on an optical microscope to achieve millisecond and sub-micrometer resolution, and with a high-speed/digital single-lens-reflex camera on a microlens for reaching millisecond and sub-millimeter resolutions. Our measurements reveal that water droplets as small as a few hundred nanometers in diameter start to form on the tips of nanoscale tubules/pillars on the SHP surfaces and remain tightly pinned at their initial nucleation sites. As condensation proceeds, the growing droplets undergo one of the following distinct events upon coalescing with a neighboring droplet: (1) a short displacement of a few micrometers if the merged droplets are less than similar to 10 mu m in diameter, or (2) a jumping motion from the surface if the merged droplet diameter exceeds similar to 15 mu m. However, the overwhelming majority of the jumping droplets were only 20-30 mu m in diameter and long-distance sliding events were rarely observed, regardless of the droplet size and the origin of driving force. These results demonstrate that the adhesion forces exerted by the superhydrophobic surfaces are strong enough to inhibit the coalescence-driven jumping motion of small coalescing droplets and to significantly limit or suppress the sliding/rolling motion of large ones, respectively. This finding, together with the higher steady-state number density of larger and stickier droplets identified on the 1-tiered dragonfly wing, also suggests that the observed upper limit of similar to 1 mm for the diameter of droplets jumping from the lotus leaf is in large part due to the adhesion force exerted by the superhydrophobic surface, in addition to the weakened surface tension relative to the inertial and gravitational forces on the large droplet.