Spontaneous motion of liquid droplets on soft gel surfaces with non-uniform cross-linking densities

We report an experimental investigation of the spontaneous motion of liquid droplets on soft gels with a cross-linking gradient. By systematically adjusting the spatial difference in cross-linking density, we observed that millimeter-sized liquid droplets moved along the elastic modulus gradient and even climbed inclined 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 driven by the surface energy difference resulting from the variations in cross-linking density. Using in situ confocal microscopy imaging, we analyzed the viscoelastic dissipation induced by the moving wetting ridges near dynamic contact lines. Our findings provide a novel strategy for controlling droplet dynamics on soft and dissipative interfaces, based on the relationship between cross-linking density and surface energy of soft gels.