X-ray Microscopy Study of Freezing Sessile Droplets

TL;DR

This study uses in situ X-ray imaging to analyze the shape and internal structure of freezing sessile droplets, revealing how bubble formation affects tip angles and validating geometric theories of freezing dynamics.

Contribution

It introduces high-resolution X-ray tomography to quantify internal bubble effects and confirms the universal tip angle in bubble-free conditions, advancing understanding of freezing behavior.

Findings

Tip angle varies with substrate temperature due to bubble formation.

Effective ice-water density ratio accounts for trapped bubbles.

Universal tip angle observed in bubble-free frozen droplets.

Abstract

A sessile water droplet on a cold substrate freezes into a shape with a sharp apex because of water's expansion upon freezing, yielding a universal tip angle across various conditions. Using \textit{in situ} X-ray imaging, we report that this angle changes with substrate temperature, and the deviation originates from bubble formation during freezing. Three-dimensional tomography enables direct quantification of the effective ice-water density ratio, accounting for trapped bubbles. Incorporating this effective density ratio reconciles the temperature-dependent tip angles. We also confirm that a bubble-free frozen droplet in a vacuum chamber exhibits the universal tip angle. Furthermore, X-ray imaging allows us to measure the three-phase boundary angles \textit{in situ}, thereby validating the geometric theory behind tip formation. These findings advance our understanding of the freezing dynamics associated with multiphase systems and highlight the capabilities of high-resolution X-ray imaging in ice research.