Drop motion induced by repeated stretching and relaxation on a gradient surface with hysteresis

The motion of a droplet can be induced by periodically compressing and extending it between two similar gradient surfaces possessing significant wetting hysteresis. The shape fluctuation of the drop during repeated compression-extension cycles leads to its ratchetlike motion toward the region of higher wettability. A simple model requiring the volume preservation of the drop during the compression-extension cycles is sufficient to account for the effect and predict drop velocity across the surface when drop size and cycle frequency are specified. In connection with this study, we also report a variation of the standard vapor phase adsorption method of preparing a chemically graded surface that allows for good control over the steepness and the length of the active zone. The method can be used to produce a linear or a radial gradient, both of which are employed here to drive droplet motion along these patterns. This type of discrete droplet motion can be used to move drops on surfaces to transport materials within miniaturized digital fluidic devices.