Freezing-induced topological transition of double-emulsion

TL;DR

This study investigates how freezing induces a topological transition in double-emulsion droplets, revealing a size-dependent stability that impacts the understanding of solidification in hierarchical emulsions.

Contribution

We demonstrate that freezing can trigger a topological transition in double-emulsions, and identify a critical size below which the transition does not occur.

Findings

Freezing causes a topological transition from W/O/W to O/W emulsion.

A critical inner droplet size of approximately 19 μm prevents the transition.

High-speed imaging characterizes the destabilization process during freezing.

Abstract

Solidification of complex liquids is pertinent to numerous natural and industrial processes. Here, we examine the freezing of a W/O/W double-emulsion, i.e., water-in-oil compound droplets dispersed in water. We show that the solidification of such hierarchical emulsions can trigger a topological transition; for example, in our case, we observe the transition from the stable W/O/W state to a (frozen) O/W single-emulsion configuration. Strikingly, this transition is characterised by sudden expulsion of the inner water drop from the encapsulating oil droplet. We propose that this topological transition is triggered by the freezing of the encapsulating oil droplet from the outside in, putting tension on the inner water drop thus, destabilizing the W/O/W configuration. Using high-speed imaging we characterize the destabilization process. Interestingly, we find that below a critical size of the inner drop, $R_{\mathrm{in,crit}} \approx 19 \, \mu\mathrm{m}$, the topological transition does not occur any more and the double-emulsion remains stable, in line with our interpretation.