Cascade Freezing of Supercooled Water Droplet Collectives

TL;DR

This study reveals a cascade freezing mechanism in supercooled water droplet groups on nanotextured surfaces, driven by vapor-induced nucleation, impacting icephobic surface performance.

Contribution

It uncovers the vapor-mediated cascade freezing process among droplets, a phenomenon previously underexplored especially on nanotextured icephobic surfaces.

Findings

Cascade freezing causes rapid ice coverage of surfaces.

Vapor-induced nucleation is key to droplet freezing propagation.

Mechanism occurs even in low-pressure environments.

Abstract

Surface icing affects the safety and performance of numerous processes in technology. Previous studies mostly investigated freezing of individual droplets. The interaction among multiple droplets during freezing is investigated less, especially on nanotextured icephobic surfaces, despite its practical importance as water droplets never appear in isolation, but in groups. Here we show that freezing of a supercooled droplet leads to spontaneous self-heating and induces strong vaporization. The resulting, rapidly propagating vapor front causes immediate cascading freezing of neighboring supercooled droplets upon reaching them. We put forth the explanation that, as the vapor approaches cold neighboring droplets, it can lead to local supersaturation and formation of airborne microscopic ice crystals, which act as freezing nucleation sites. The sequential triggering and propagation of this mechanism results in the rapid freezing of an entire droplet ensemble resulting in ice coverage of the nanotextured surface. Although cascade freezing is observed in a low-pressure environment, it introduces an unexpected pathway of freezing propagation that can be crucial for the performance of rationally designed icephobic surfaces.