Experimental study of morphology evolution of sessile compound droplets

Compound droplets are widely utilized in diverse industrial and scientific fields; accurate manipulation during their fabrication is critical for their performance. In this paper, morphology evolution is systematically investigated by combining experimental observations with theoretical analysis. When Neumann triangle conditions at three-phase interfaces are not satisfied, a unique metastable quasi-collar configuration emerges, where the liquid lens submerges into the sessile droplet rather than spreading on it. Furthermore, our findings reveal the critical role of the volume ratio of the liquid lens and the sessile droplet in the compound droplet, as well as the viscosity of the sessile droplet. Successful migration of the liquid lens to the center of a sessile droplet requires their volume ratio to be above a position-dependent critical value. The migration velocity variation is non-monotonic with the viscosity of a sessile droplet, peaking at an intermediate viscosity. This trend is due to the competition between the driving force and viscous force in the compound droplet. This study deepens the understanding of compound droplet dynamics and provides guidelines for the precise manipulation of compound droplets in multiphase fluid systems.