Propulsion generated by diffusion-driven flow

Buoyancy-driven flow, which is flowdriven by spatial variations in fluid density(1), lies at the heart of a variety of physical processes, including mineral transport in rocks(2), the melting of icebergs(3) and the migration of tectonic plates(4). 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 pycnoclines(5), and wherever there is a temperature difference between immersed objects and the surrounding fluid, such as particles in volcanic clouds(6).