Micro-Structured Surface Ratchets for Droplet Transport

We report a new paradigm for droplet transport that exploits asymmetric hysteresis—the difference in the sensitivity to texture of the advancing and receding contact angles on super-hydrophobic surfaces. Texture is engineered to create contiguous "surface ratchets" that pin one edge more than the other, thereby guiding the droplet in one direction when vibrated. This paper presents the principle, fabricated structure, experimental results that validate the hypothesis—showing droplets moving with velocities greater than 12 mm/s—and preliminary understanding of the physics of ratcheting.