Levitating and Self-Organizing Droplets

Fluid dynamics is an ongoing topic with many surprising aspects. One of these is the Leidenfrost effect. It comes from the name of the German scientist who described it for the first time: Johann Gottlob Leidenfrost. It occurs when a liquid, in contact with an object significantly warmer than the liquid's boiling temperature, produces a vapor layer which surrounds the object and thus isolates it from the direct thermal exchanges with the object.

Like many fluid dynamic aspects, this phenomenon is poorly understood and yet it has important implications for the thermodynamics of evaporation – and could also have a range of applications in water cooling microchips or moving chemical molecules.

In Russia, Researcher Oleg A. Kabov from Institute of Thermophysics and his team have also been studying this interesting effect [1]. In particular, they looked at levitating droplets of liquid condensate and their organization into ordered arrays.

Array of levitating droplets over the liquid-gas interface viewed from the top.

The team report the geometric characteristics of levitation over a dry heated solid surface are remarkably similar to the case of droplets levitating over liquid-gas interfaces. They proposed a simple model  that predicts the mechanisms of both droplet levitation and interdroplet interaction leading to pattern formation over a dry surface. The mechanism is based on the diffusion around a droplet which leads to fluid motion known as the Stefan flow. In this case, the flow symmetry is broken in the presence of the wall and the reflection of the Stefan flow off the substrate causes the observed levitation phenomenon. Moreover, the outflow of vapour creates a repulsive interaction between droplets, which causes multiple droplets to form regular arrays.

In the US, another research team from the University of Bath made a very interesting application for controlling the droplets movement [2]. The explanations and the surprising results are shown in the following video.

Continue reading: 
http://physicsworld.com/cws/article/news/2017/sep/07/levitating-droplets-go-with-the-flow

[1] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.094503

[2] http://www.bath.ac.uk/research/news/2015/05/19/leidenfrost-thermostat/

   
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