Abstract

I will demonstrate a new type of dynamical instability where a volatile droplet, when deposited on a floating swellable sheet, becomes asymmetric, lobed, and mobile. This phenomenon involves non-equilibrium swelling, evaporation and motion, working together to realize a self-excitable spatially extended oscillator. Solvent penetration from the droplet causes the film to swell locally and eventually buckle, changing shape and the drop responds by moving. Simultaneously, solvent evaporation from the swollen film causes it to regain its shape once the droplet has moved away. The process repeats and leads to complex pulsatile spinning and/or sliding movements. I will show a one-dimensional experiment that highlights the coupled dynamics of slow swelling of and evaporation from the film and the fast motion of the drop, a characteristic of excitable systems. Finally, I will provide a phase diagram for droplet excitability as a function of drop size and film thickness and scaling laws for the motions of the droplet.