Self-Directed Movements of Droplets on Radially Patterned Surfaces Based on Self-Assembled Monolayers

This paper demonstrates macroscopic (millimetre-scale), self-directed transport of droplets on radially patterned silicon surfaces to induce wettability gradients based on self-assembled monolayers (SAMs). A difference in wettability between the opposite sides of the droplet edge creates a Laplace pressure gradient inside the droplet, forcing the droplet to move forward. Circulating wedge-shaped features with alternate hydrophobic and hydrophilic regions were photographically patterned and combined with selective coating of silanol- and thiol-based SAMs to transport the droplets right to the centre passively. Positions and velocities of the droplets were measured using high speed CCD camera