Dynamic Oscillation and Motion of Oil-in-Water Emulsion Droplets

The interplay of interfacial tensions on droplets results in a range of self-powered motions that mimic those of living systems and serve as a tunable model to understand their complex non-equilibrium behavior. Spontaneous shape deformations and oscillations are crucial features observed in nature but difficult to incorporate in synthetic artificial systems. Here, we report sessile oil-in-water emulsions that exhibit rapid oscillating behavior. The oscillations depend on the nature and concentration of the surfactant, the chemical composition of the oil, and the wettability of the solid substrate. The rapid changes in the contact angle per oscillation are observed using side-view optical microscopy. We propose that the changes in the interfacial tension of the oil droplets is due to the partitioning of the surfactant into the oil phase and the movement of self-emulsified oil out of the parent droplets giving rise to the rhythmic variation in droplet contact-line. The ability to control and understand droplet oscillation can help model similar oscillations in out-of-equilibrium systems in nature and reproduce biomimetic behavior in artificial systems for various applications, such as microfluidic lab-on-a-chip and adaptive materials. The intersection of surfactant partitioning, spontaneous emulsification, and substrate wettability generates a unique type of motion-droplet oscillation-rarely reported in the literature for oil-in-water emulsion systems. The effect of varying the concentration of the surfactant, substrate properties, and the chemical nature of the oil and surfactant on the oscillating droplets was studied.image