Dynamics of microdroplets over the surface of hot water

Abstract

When drinking a cup of coffee under the morning sunshine, you may notice white membranes of steam floating on the surface of the hot water. They stay notably close to the surface and appear to almost stick to it. Although the membranes whiffle because of the air flow of rising steam, peculiarly fast splitting events occasionally occur. They resemble cracking to open slits approximately 1 mm wide in the membranes, and leave curious patterns. We studied this phenomenon using a microscope with a high-speed video camera and found intriguing details: i) the white membranes consist of fairly monodispersed small droplets of the order of 10 $\mu\,{\rm m}$; ii) they levitate above the water surface by 10$\sim$100 $\mu{\rm m}$; iii) the splitting events are a collective disappearance of the droplets, which propagates as a wave front of the surface wave with a speed of 1$\sim$2 m/s; and iv) these events are triggered by a surface disturbance, which results from the disappearance of a single droplet.