On the Bridge Formation Process for Emulsion Wetting: Novel Insights.

The final stage of emulsion wetting is characterized by the spontaneous emergence and radial growth of a fluid bridge formation between a fluid drop and a rigid surface. Contrasting traditional spinodal wetting, experiments prove that bridge formation occurs between a parent drop and one of several drop fluid islands formed on a rigid surface (i.e., coalescence-induced wetting). Bridge formation is observed to be a non-unique event, and a necessary but insufficient wetting criterion (mere establishment of a drop fluid bridge does not guarantee this). If the bridge is smaller than a critical length, it is reabsorbed into the bulk liquid, and any discontinuities in the film are healed. Radial bridge propagation velocity follows a unidirectional model, which may potentially augment wetting timescales. Complementing experiments, direct numerical simulations will aim to provide detailed insights on (i) key length and time scales for emulsion wetting, (ii) confirm and supplement qualitative wetting trends with simple thermodynamic models to quantitatively describe island growth kinetics, and (iii) obtain critical insights on the role(s) of hydrodynamic (gravity-induced descent) and non-hydrodynamic (disjoining pressure) contributions to obtain a unified picture of coalescence-induced wetting. This work serves as a foundation for future studies aimed at completely capturing emulsion bridge formation, leading to a systematic design of emulsions of commercial interest with pre-designed shelf lives.