Airflow-driven water droplets moving from surface area with high wettability to low-wettability area

Surfaces containing various areas with different wettability have been used in many applications, and it is necessary to understand the droplet motion between areas where wettability is different. This study investigates the droplet motion under shear flow from an area with high surface wettability to a low-wettability area with the effect of contact angle hysteresis (CAH) considered. Several wettability-heterogeneous surfaces which contain areas with different wettability are fabricated from silicon wafers using the surface-hydrophobization technique. The mathematical expression of the increase of system potential energy when droplet moves from a high-wettability area to a low-wettability area is derived. The critical airflow velocities for droplets to cross the boundaries between different surface areas ( U c,b ) are measured, and are found to be affected by factors including droplet size, non-dimensional increase of system potential energy (Δ e p ), and the CAHs of both sides of the boundary. The effects of those factors on droplet boundary-crossing behaviors are investigated, and a map distinguishing droplet boundary-crossing behaviors is created in this study. The relations of U c,b between the investigated wettability-heterogeneous surfaces generally correspond to the relations of Δ e p between those surfaces. The study shows that a large Δ e p , together with a large CAH of the surface area on the low-wettability side, tends to result in droplet breakup as well as small droplet separating and staying on surface. The theories and findings in this study can serve as a reference for the design of the functional surfaces that can facilitate droplet removal particularly in the application of condensation. • Surfaces containing areas with different wettability are fabricated from Si wafers • The mathematical expression of the increase of system potential energy is derived • Critical airflow velocity for droplet to cross the wettability boundary is measured • Effects of CAH on droplet motion on wettability-heterogeneous surfaces are studied • Effects of influencing factors on droplet boundary crossing behaviors are studied