Durably Self-Sustained Droplet on a Fully Miscible Liquid Film

Droplets impacting onto a solid or liquid surfaceinducing wetting,floatation, splash, coalescence, etc. is ubiquitousin nature and industrial processes. Here, we report that liquiddroplets exhibit spherical caps upon contact with a fully miscibleliquidfilm of lower surface tension, despite the spontaneousmixing of the two liquids. Such a spherical cap on a continuousliquid surface sustains a long lifespan up to minutes beforeultimately merging into thefilm. Benefiting from large viscousforces in a thinfilm as a result of spatial confinement, the surfaceflow is substantially suppressed. Therefore, the surface tensiongradient responsible for this phenomenon is maintained because the normal diffusion offilm liquid into the droplet can timely dilutefilm liquid supplied by uphill Marangoniflow at the droplet surface. The presentfinding removes the conventional cognition thatdroplet coalescence is prompt on fully miscible continuous liquid surfaces, thus benefiting design of new types of microfluidicdevices.