Surfing Mists

Watch your hot cup of coffee or tea carefully, and you may notice a white mist of tiny micron-sized droplets hovering near the surface. These microdroplets are a little understood part of evaporation. They form over a heated liquid, levitating on vapor that diffuses out from them and reflects off the liquid surface. (This is similar to the Leidenfrost effect, but the authors note it occurs at much lower temperatures. Unrelated research has suggested the Leidenfrost effect can occur at lower temperatures when there is very little surface roughness.)

One of the particularly peculiar behaviors of these tiny levitating microdroplets is that they can exist over dry surfaces as well. The image above shows microdroplets migrating from a liquid surface (right) to a dry surface (center and left). When the droplets near the contact line, they encounter a strong upward flow due to increased evaporation there. This launches the droplets upward and they sail to the dry area. There, their vapor layers continue creating levitation and provide a cushion between them and their neighbors, causing the drops to self-organize into arrays. (Image credit: D. Zaitsev et al.; via Physics World; submitted by Kam-Yung Soh)

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