I will present the results of three experiments at fluid interfaces. In the first, a soap bubble is gently deposited on a soap film (fig.1a). The bubble may interact with the underlying film in such a way as to decrease in size, leaving behind a smaller daughter bubble with approximately half the radius of its progenitor. The coalescence process occurs over a timescale comparable to the inertial-capillary time, which is the time for the capillary waves generated during the process to span the bubble. A simple model that captures the time variation of the bubble's size during the air evacuation phase is presented. Through dynamic similarity, the process for bubbles is remarkably similar to that for droplets. In the second experiment, a millimetric liquid droplet self-propels on a vibrating liquid bath via a resonant interaction with the wave field that it generates by bouncing on the bath’s surface. I will show that the motion of this walking droplet (or ‘walker’) can be controlled by varying the bath's bottom topography. Specifically, when the walker experiences a step change in the bath depth, it is deviated according to an effective Snell’s law (fig.1b). Beyond the analogy with light rays in geometrical optics, the walker exhibits behaviors reminiscent of phenomena described by the theory of electromagnetic radiation. In the third experiment, centimeter-sized bodies slide on the surface of water (fig.1c). I will show that their motion is dominated by skin friction due to the boundary layer that forms in the fluid beneath the body. A simple model that considers the boundary layer as quasi-steady is presented. The model is able to capture the experimental behaviour for a range of body sizes, masses, shapes and fluid viscosities. Furthermore, I will show that friction can be reduced by modification of the body's shape or bottom topography. These results are significant for understanding natural and artificial bodies moving at the air-water interface, and can inform the design of aerial-aquatic microrobots for environmental exploration and monitoring.
Fig.1. (a) A snapshot from the partial coalescence of a soap bubble with a soap film. (b) Trajectory of a walking droplet that experiences a step change in the bath depth. (c) A water slider.