Shaping a Surface Microdroplet by Marangoni Forces along a Moving Contact Line of Four Immiscible Phases

The ability to transfer microdroplets between fluid phases offers numerous advantages in various fields. This study focuses on the stability and morphology of a sessile oil microdropletduring the transfer from underwater to air. A distinct transition in microdroplet dynamics is observed, characterized by a shift from a scenario dominated by Marangoni forces to one dominated by capillary forces. In the former regime, the oil microdroplets spread in response to the contact between the water‐air interface and the water‐oil interface. The spreading distance follows a power law relationship of t3/4, reflecting the balance between Marangoni forces and viscous forces. On the other hand, in the capillarity‐dominated regime, the oil microdroplets remain stable at the contact line and after being transferred into the air. The crossover between these two regimes is identified in the parameter space defined by three factors: the approaching velocity of the solid‐water‐air contact line (v cl ), the radius of the oil microdroplet (r o ), and the radius of the water drop (r w ). Furthermore, how to use the four‐phase contact line for shaping oil microdroplets is demonstrated using a full liquid process by the contact line lithography.