Following or Against Topographic Wettability Gradient: Movements of Droplets on a Micropatterned Surface.

Two kinds of possible movements of droplets (i.e., following or against the topographic wettability gradient) on a micropatterned surface are investigated by using a particle-based method: many-body dissipative particle dynamics (MDPD). The displacement along the wettability gradient and contact angles on both sides of the droplet are analyzed. The results show that the migration trajectory of the droplet is determined by the coexistence of Cassie and Wenzel states and the unbalanced Young's force, which are related to the impact velocity, pillar height, and surface tension. The droplet remains in the Cassie state and advances spontaneously following the wettability gradient under a small impact velocity, high pillar height, and large surface tension. On the contrary, when the coexistence of Cassie and Wenzel states appears, the contact line on the side of the Cassie state retracts and that on the side of the Wenzel state pins, inducing droplet movement against the wettability gradient. Additionally, the critical impinging velocity, which determines the migration direction of the droplet, also depends on the pillar height and surface tension. The outcomes are helpful in designing surfaces with topographical wettability gradients for droplet transportation.