  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.
