Light-driven capillary assembly and motion of hydrogel nanocomposite disks at the air/water interface

Modulation of capillary forces by active changes in shape is an approach used by a number of insects to achieve propulsion at air-water interfaces. Learning from nature, we have designed light-responsive sub-millimeter hydrogel disks that exhibit: (1) attraction, leading to formation of well-defined assemblies; (2) repulsion; or (3) sustained rotation, depending on the pattern of illumination. Fabrication of light-responsive hydrogel disks containing patterned gold nanoparticles (Au NPs) was achieved by photo-chemical reduction of Au³⁺ ions to metallic Au NPs within micro-patterned temperature-responsive hydrogels. Photothermal deswelling of illuminated Au NP-containing hydrogel regions induced out-of-plane buckling and deformation of the air/water interface, with a characteristic response time of seconds. This provided a versatile means to control capillary interactions in both time and space, thereby leading to a diverse range of behaviors at the air/water interface.