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.