Directed motion of two-component droplets on wedge-shaped composite copper surfaces without back-end pinning

Abstract

The motion of three-phase contact line of a droplet is always halted by the contact angle hysteresis on a common solid surface. In this work, the Ag/Cu surface and Cu(OH)2/Cu surface with water contact angle of 25.0° were fabricated and found favorable for two-component droplet moving. The droplet, water with propylene glycol in irregular shape could restore to a circle (top view) on these prepared surfaces. Inspired by the shape restoration, the directed motion of two-component droplet without back-end pinning was achieved on both the wedge-shaped Ag/Cu and Cu(OH)2/Cu composite surfaces. The two-component droplet moves in a way of the front-end spreading with the subsequent back-end shrinking. The needle-like Cu(OH)2 microstructure is more conducive to the front-end spreading, while the spheroidal Ag particles on Cu substrate is in favor of the back-end shrinking. In addition, the segmented Ag@Cu(OH)2/Cu wedge-shaped composite surfaces with Ag film on the narrow end of the track and Cu(OH)2 on the wide end could enhance the droplet moving. Finally, a micro-chemical reactor was designed capable of driving two dispersed droplets to move in a specific direction, converge, and then react with each other.