Curvature driven propulsion of floating films: Part 2.

Thin films readily buckle and wrinkle to form a variety of three-dimensional shapes of general curvature. Motivated by the self-propulsion of floating films on a curved water bath, we perform Surface Evolver simulations to probe the energy of placing a thin, easily-wrinkled film on a curved liquid interface. Our simulations use a coarse-grained approach where the film is modeled using a triangulated surface with edges that can compress but not stretch [1]. The system energy is comprised of the gravitational energy of the liquid, and the surface energy of the liquid that is not covered by the film. By measuring the energy as we vary the location of the film, we show how the film experiences a force directed away from a curved meniscus towards a central flat region of the liquid interface, consistent with our experiments using ultrathin polymer films. Although these behaviors are perhaps reminiscent of the capillary migration of solid particles [2], the phenomenon studied here is intrinsic to elastic films that are easily wrinkled yet hard to stretch. (This is part 2 of a 2-talk series.)

[1] Paulsen et al., Nature Materials 14, 1206 (2015).

[2] Cavallaro et al., PNAS 108, 52 (2011).