Droplet emission induced by ultrafast spreading on a superhydrophilic surface†
A novel phenomenon is reported, wherein, if a mother droplet spreads faster than its critical velocity on a superhydrophilic surface, a smaller daughter droplet is emitted in a process that is controlled both dynamically and thermodynamically. Using a high-speed camera, we directly observed that while the lower part of the droplet spread, the upper part remained unchanged at first, and a temporary meniscus bridge formed in between. This meniscus bridge became unstable owing to capillary pressure, and broke when the lower part spread faster than its critical velocity, emitting the upper part as a daughter droplet. Through modeling and varying experimental parameters such as contact velocity, contact angle and surface structure, we determined that droplet emission could only be achieved by ultrafast spreading on superhydrophilic surfaces. The insights from these observations are helpful for the creation and application of special wettable surfaces.