Propulsion generated by diffusion-driven flow

Few would expect the process of diffusion to generate useful work. But when a buoyant wedge-shaped object is placed in a fluid with a vertically stratified density gradient, the upward flow driven by diffusion can be translated into a measurable horizontal propulsion. Buoyancy-driven flow, which is flow driven by spatial variations in fluid density1, lies at the heart of a variety of physical processes, including mineral transport in rocks2, the melting of icebergs3 and the migration of tectonic plates4. Here we show that buoyancy-driven flows can also generate propulsion. Specifically, we find that when a neutrally buoyant wedge-shaped object floats in a density-stratified fluid, the diffusion-driven flow at its sloping boundaries generated by molecular diffusion produces a macroscopic sideways thrust. Computer simulations reveal that thrust results from diffusion-driven flow creating a region of low pressure at the front, relative to the rear of an object. This discovery has implications for transport processes in regions of varying fluid density, such as marine snow aggregation at ocean pycnoclines5, and wherever there is a temperature difference between immersed objects and the surrounding fluid, such as particles in volcanic clouds6.