Rectified Motion of Liquid Drops on Gradient Surfaces Induced by Vibration

When a liquid drop (1−2 μL) is placed on a surface possessing a continuous gradient of wettability, it moves toward the more wettable part of the gradient with typical speeds of 1−2 mm/s. This low speed arises because the driving force due to surface tension is reduced by contact angle hysteresis. The hysteresis force acting on a drop on a gradient surface is, however, spatially asymmetricits magnitude against the gradient being larger than that along the gradient. If a periodic force is applied to a drop resting on such a gradient surface, the force against the gradient is rectified whereas it is enhanced along the gradient. This half-wave rectification of periodic force causes 1−2 μL size drops to move with enhanced speeds of 5−10 mm/s.