  When a volatile droplet is deposited on a floating swellable sheet, it
becomes asymmetric, lobed and mobile. We describe and quantify this phenomena
that involves nonequilibrium swelling, evaporation and motion, working together
to realize a self-excitable spatially extended oscillator. Solvent penetration
causes the film to swell locally and eventually buckle, changing its 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. We use a one-dimensional experiment to highlight the slow swelling
of and evaporation from the film and the fast motion of the drop, a
characteristic of excitable systems. Finally, we provide a phase diagram for
droplet excitability as a function of drop size and film thickness and scaling
laws for the motion of the droplet.
