How liquid-liquid phase separation induces active spreading

TL;DR

This paper reveals how liquid-liquid phase separation actively influences the spreading of droplets, challenging classical laws and showing early nucleation driven by surface forces, with broad implications across science and technology.

Contribution

It uncovers the coupling between phase separation and droplet spreading, demonstrating active forces beyond classical capillarity and early nucleation in wetting precursor films.

Findings

Phase separation couples strongly with droplet spreading.

Classical Cox-Voinov law does not apply in this context.

Surface forces induce early nucleation in precursor films.

Abstract

The interplay between phase separation and wetting of multicomponent mixtures is ubiquitous in nature and technology and recently gained significant attention across scientific disciplines, due to the discovery of biomolecular condensates. It is well understood that sessile droplets, undergoing phase separation in a static wetting configuration, exhibit microdroplet nucleation at their contact lines, forming an oil ring during later stages. However, very little is known about the dynamic counterpart, when phase separation occurs in a non-equilibrium wetting configuration, i.e., spreading droplets. Here we report that liquid-liquid phase separation strongly couples to the spreading motion of three-phase contact lines. Thus, the classical Cox-Voinov law is not applicable anymore, because phase separation adds an active spreading force beyond the capillary driving. Intriguingly, we observe that spreading starts well before any visible nucleation of microdroplets in the main droplet. Using high-speed ellipsometry, we further demonstrate that surface forces cause an even earlier nucleation in the wetting precursor film around the droplet, initiating the observed wetting transition. We expect our findings to enrich the fundamental understanding of phase separation processes that involve dynamical contact lines and/or surface forces, with implications in a wide range of applications, from oil recovery or inkjet printing to material synthesis and biomolecular condensates.