Spontaneous Motion of Liquid Droplets on Soft Gradient Surfaces

Abstract: We report an experimental investigation of the spontaneous motion of liquid droplets on soft gels with a crosslinking gradient. By systematically adjusting the spatial difference in crosslinking density, we observed that millimeter-sized liquid droplets moved along the gradient of elastic modulus and even climbed tilted slopes against gravity. Unlike the wetting dynamics of micro-droplets, which are governed by elastocapillary effects, we demonstrated that the observed spontaneous movements of millimeter-sized droplets were attributed to the surface energy difference resulting from the variation in crosslinking density. Using {\em in-situ} confocal microscopy imaging, we analyzed the viscoelastic dissipation induced by the moving wetting ridges near dynamic contact lines. Based on the relationship between the crosslinking density and surface energy of soft gels, our findings reveal a new method for controlling droplet dynamics at soft and dissipative interfaces.