The Diffusion Fish

Abstract

It is a remarkable fact that an asymmetric, neutrally-buoyant object can spontaneously swim in a stratified fluid. The effect, which we have recently discovered, occurs as a consequence of diffusion-driven flow, which was first studied in the context of transport along rock fissures and oceanic boundary flows. Diffusion-driven flow requires a discontinuity in diffusivity at an inclined boundary, in which case flux continuity across the boundary generates a thin buoyancy layer adjacent to the wall boundary in which convection occurs. On this basis, we have carefully designed and built a triangular wedge – the “diffusion-fish” – which can propel itself in a stratified fluid. Here we present a series of experimental results demonstrating this remarkable effect, and discuss the potential applications to locomotion and mixing in the oceans.