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.

Droplet Navigation by Photothermal Pumping in an Optofluidic System.

Droplets with guided motion have potential applications as microreactors and delivery vehicles. Directing long-range migration powered solely by light is particularly advantageous since light can be applied remotely, patterned with a photomask, and readily translated to irradiate specified locations. Herein, we describe a universal platform that allows fast directional navigation and collective merging of droplets controlled by either ultraviolet or visible light. The guided motion of water and oil droplets follows density-driven convective flows arising from photothermal conversion at a light-absorbing amphiphobic substrate. Because of the relatively high photothermal efficiency, a low-intensity light beam can be employed. Further, we demonstrate that the moving droplets can function as carriers and on-demand reaction chambers.