Photothermocapillary Oscillators.

We present a new class of tunable light-driven oscillators based on mm-scale objects adsorbed at fluid interfaces. A fixed light source induces photothermal surface tension gradients (Marangoni stresses) that drive nanocomposite hydrogel discs away from a stable apex position atop a drop of water. The capillary forces on the disc increase with surface curvature; thus, they act to restore the disc to its original position. As the disc reenters the light source it again experiences Marangoni propulsion, leading to sustained oscillation for appropriate conditions. Propulsive forces can be modulated with incident light intensity, while the restoring force can be tuned via surface curvature-i.e., drop volume-providing highly tunable oscillatory behaviors. To our knowledge, this is the first example where Marangoni and capillary forces combine to incite sustained motion. As such, a model was developed that describes this behavior and provides key insights into the underlying control parameters. We expect that this simple approach will enable the study of more complex and coupled oscillatory systems.