Dynamic Behavior of a Droplet across a Hydrophobic and Hydrophilic Boundary

Droplet-related functional surfaces attract considerable research interests due to many potential applications, including water harvesting, heat dissipation, and drug transport. The dynamic behavior of a droplet crossing a hydrophilic and hydrophobic boundary is studied experimentally and theoretically in this paper since such a phenomenon is involved in most of these functional surfaces. It is found that the whole crossing process is approximately divided into five stages according to the shape of the three-phase contact line and the droplet morphology. The force that drives the droplet moving from one stage to the next is analyzed, which depends mainly on the advancing and receding contact angles. Specially, the instant transport displacement and velocity in the main second stage are also predicted theoretically, which agree well with the experimental measurements. The results should be helpful for the in situ study of biological molecules, heat dissipation efficiency of functional surfaces, and the design of microfluidic systems.