  The ability to transfer microdroplets between fluid phases offers numerous
advantages in various fields, enabling better control, manipulation, and
utilization of small volumes of fluids in pharmaceutical formulations,
microfluidics, and lab-on-a-chip devices, single-cell analysis or droplet-based
techniques for nanomaterial synthesis. This study focuses on the stability and
morphology of a sessile oil microdroplet at the four-phase contact line of
solid-water-oil-air during the droplet transfer from underwater to air. We
observed a distinct transition in microdroplet dynamics, characterized by a
shift from a scenario dominated by Marangoni forces to one dominated by
capillary forces. In the regime dominated by Marangoni forces, the oil
microdroplets spread in response to the contact between the water-air interface
and the water-oil interface and the emergence of an oil concentration gradient
along the water-air interface. The spreading distance along the four-phase
contact line follows a power law relationship of $t^{3/4}$, 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. We identify the
crossover between these two regimes 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, we demonstrate how to use the four-phase
contact line for shaping oil microdroplets using a full liquid process by the
contact line lithography. The findings in this study may be also applied to
materials synthesis where nanoparticles, microspheres, or nanocapsules are
produced by microdroplet-based techniques.
