Initial solidification dynamics of spreading droplets

TL;DR

This study investigates the early stages of solidification in spreading droplets on undercooled surfaces, revealing how nucleation and crystal growth influence contact-line arrest through advanced imaging and theoretical modeling.

Contribution

It combines TIR imaging with nucleation theory to elucidate the dynamics of droplet solidification and contact-line arrest under various undercooling conditions.

Findings

Nucleation rate and crystal growth speed are sensitive to substrate undercooling.

The temporal evolution of solid area fraction matches theoretical predictions.

Rapid crystal growth causes contact-line arrest at high undercooling.

Abstract

When a droplet is brought in contact with an undercooled surface, it wets the substrate and solidifies at the same time. The interplay between the phase transition effects and the contact-line motion, leading to its arrest, remains poorly understood. Here we reveal the early solidification patterns and dynamics of spreading hexadecane droplets. Total internal reflection (TIR) imaging is employed to temporally and spatially resolve the early solidification behaviour. With this, we determine the conditions leading to the contact-line arrest. We quantify the overall nucleation behaviour, \textit{i.e.} the nucleation rate and the crystal growth speed, and show its sensitivity to the applied undercooling of the substrate. By combining the Johnson-Mehl-Avrami-Kolmogorov nucleation theory and scaling relations for the spreading, we can calculate the temporal evolution of the solid area fraction, which is in good agreement with our observations. We also show that for strong enough undercooling it is the rapid growth of the crystals which determines the eventual arrest of the spreading contact line.