Blooming of Emulsion Droplet During Evaporation

The evaporation of single-phase droplets has been well-documented in scientific literature. However, investigations on the evaporation of emulsion droplets, which is often encountered in industrial processes including inkjet printing, spray coating and herbicide applications are limited. This present study considers the evaporation of oil-in-water emulsion droplets on glass surfaces with varying wettability. The emulsion was prepared by mixing 5 cst Silicone oil at 10% w/w within deionised water (88% w/w) and adding surfactant Span 80 at 2% w/w to maintain stability. The mixture was homogenised for 5 mins resulting in an average drop size lower than 1 μm. The evaporation process was observed using an inverted microscope.

During the evaporation, the dispersed microdroplets close to the contact edge were found to coalesce and form a continuous phase that tended to spread out. The characteristics of the spreading film were influenced by the wettability of the solid surface. On hydrophilic surfaces a distinctive multi-layer structure emerged, featuring flower-like patterns. The spreading film was found to follow the law of r~t^1/3, where r is the contact radius of the film. This is much faster than the predictions of the well-known Tanner’s law. The fast spreading of the film is attributed to the strong slip and the disjoining pressure that dominates the spreading of the thin film front.