Self-Propulsion and a Push-Pull Mechanism in Sessile Droplets.

Self-propelled droplets on solid substrates can autonomously move by physical forces, such as surface tension. This phenomenon mimics natural processes, such as the crawling of living cells or the propulsion of oil droplets on water, and provides valuable insights that apply to both natural phenomena and microfluidic applications. In this study, the self-propulsion of the evaporating droplet by the surface tension gradient on a polymer-coated substrate is investigated. We have studied the internal dynamics and mechanism of droplet motion in sessile and 2D-confined droplets. We report that the asymmetric strength of the Marangoni vortices within the sessile droplet results in a push-pull mechanism that propels the droplet on the substrate. Furthermore, in a self-propelling droplet, the interfacial flow in the flattened droplet propagates at the droplet's free interface toward the droplet back, causing a continuous polar contraction and propulsion of the droplet. These findings provide essential insight into understanding the role of fluid physics in the self-propulsion of active droplets or living organisms.