  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.
