Dynamics of microdroplets over the surface of hot water

TL;DR

This study investigates the behavior of microdroplets forming white membranes on hot water, revealing their composition, levitation, and the collective dynamics leading to membrane splitting triggered by surface disturbances.

Contribution

The paper provides detailed observations of monodispersed microdroplets, their levitation, and the collective wave-like splitting phenomena on hot water surfaces, using high-speed microscopy.

Findings

Membranes consist of ~10 μm droplets

Droplets levitate 10-100 μm above water surface

Splitting propagates as a wave front at 1-2 m/s

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.