Curvature-Driven Propulsion of Floating Films

Small floating objects often clump together, due to the long-range capillary interactions between their boundary menisci. Previous studies of this “Cheerios effect” [1] have focused on rigid solids, but much less is known about analogous behaviors for floating objects that are easily deformed by surface tension. Here the behaviors may be much more subtle, and can be strongly influenced by geometric incompatibilities between the solid and the fluid. We focus on a model system where a thin polymer film (~100 nm) is confined to the curved water interface in an overfilled petri dish. We measure the trajectories of initially planar films and curved shells released from various starting positions. By altering the geometry and thickness of the films and the viscosity of the fluid, we build an empirical description of this system, including regimes where the drag force is dominated by either the liquid viscosity or inertia. In all cases, the spontaneous translation is directed towards a region where the curvature of the meniscus is similar to the rest curvature of the film. This suggests a novel route to fast, parallel sorting of interfacial shells by their rest curvature.

[1] Vella & Mahadevan, Am. J. Phys. 73, 814 (2005)