Levitating droplet cluster is NOT the Leidenfrost effect

While writing about the ordered self-assembled levitating microdroplet clusters, I found that many people (including some reviewers in physics journals) tend to perceive the cluster by analogy with the Leidenfrost effect. In the Leidenfrost effect, a droplet, close to a solid surface significantly hotter than water’s boiling point, forms an insulating vapor layer keeping that liquid levitating without boiling rapidly.

Left: Leidenfrost effect (from Wikipedia). Right: a setup for levitating microdroplet cluster

Both pictures show self-assembled droplet clusters (images from Dr. Alexander Fedorets, who first discovered the phenomenon)

While in both cases levitating droplets are present, the Leidenfrost effect is very different from the droplet cluster.

1. The Leidenfrost effect occurs at high temperatures above the boiling point of water. Typically, the Leidenfrost temperature is about 200 ºC, while the droplet cluster forms at much lower temperatures, normally between 60-95 ºC.

2. The size of droplets in the Leidenfrost effect is large, typically, milimeter-sized, as opposed to small droplets of 0.01-0.2 mm in the droplet cluster.

3. The Leidenfrost effect occurs above a solid surface, while the droplet cluster forms above the liquid surface.

4. The droplet evaporates in the Leidenfrost effect, while droplets in the cluster typically grow due to the condensation.

5. There is no droplet self-assembly into a regular ordered structure (the cluster) in the Leidenfrost effect.

As a result, these two phenomena are very different. The Stefan flow due to evaporation plays a very significant role in the Leidenfrost effect, but it hardly can be significant for the droplet cluster (as we pointed out in our recent comment). Therefore, the instinct of those physicists, who work on the study of levitating droplets, to compare the cluster with the Leidenfrost effect, should be used with caution and consideration.