Floating bodies propelled by convective dissolution.

Self-propulsion can be achieved by modifying anisotropically the properties of the surrounding fluid. It has been shown that an asymmetrical submerged object, one side of which consists of a heating plate, can be propelled by the convection flow it generates [1]. In this work, we demonstrate an original propulsion mechanism in which convection is no longer of thermal origin but solutal, via the dissolution of an immersed soluble object [2]. To do this, we use boats just a few centimetres long, consisting of a plastic buoy and a sloping candy plate. Once placed in the water, these boats start to move forward, at speeds up to 5 mm/s and in a straight line, under the effect of the gravitational flow fed by their dissolution. We show that propulsion is mainly due to the flow beneath the boat, where the concentration boundary layer destabilizes to form plumes. We characterize the flow generated to determine the origin of the thrust. We propose also a model predicting the body speed depending on geometry and material properties and show that it captures the observed trends reasonably. The dynamics of dissolving solids demonstrated here applies equally well to solids undergoing phase change and may thus contribute to the drift of melting icebergs.

[1] Mercier et al, Self-propulsion of immersed objects via natural convection. Phys. Rev. Lett. 112, 204501 (2014)

[2] Sullivan et al, Turbulent solutal convection and surface patterning in solid dissolution. Phys. Rev. E 54, 486–495 (1996).