Self-propulsion of dissolving or melting bodies

Self-propulsion can be achieved by anisotropically modifying the properties of the surrounding fluid. It has been shown that an asymmetric submerged object with one side consisting of a heating plate can be propelled by the convection flow it produces [1]. However, convective flow can also be generated without an additional heat source by the progressive phase change of a solid immersed in a fluid, which progressively dissolves or melts and locally modifies the density field, generating gravity currents. We have recently experimentally demonstrated an original propulsion mechanism in which the convection is no longer thermal, but solutal, due to the dissolution of an immersed soluble object [2]. We use boats of a few centimeters length, consisting of a plastic buoy and an inclined candy plate, and observe a terminal velocity of up to 5 mm/s. We have also proposed a model that predicts the boat speed as a function of geometry and material properties, and show that it reasonably captures the observed trends. We then studied the propulsion for ice melting, using a rectangular ice wedge of a few tens of centimeters. For a freshwater bath of temperature above 4°C, we show that the cooling of the bath generates by thermal convection similar gravity currents that propels the asymmetric ice blocks to velocity of few millimeters per second in a bath at a temperature about 20°C. We further show that in a saltwater bath with salinity similar to that of the ocean, the ice block moves robustly in the same direction as in fresh water, suggesting that this propulsion mechanism may be relevant to iceberg drift in sufficiently warm oceans.

[1] Mercier, M. J., Aderkani, A.M., Allshouse, M. R., Doyle B. & Peacock T., Self-propulsion of immersed objects via natural convection. Phys. Rev. Lett. 112, 204501 (2014).
[2] Chaigne, M., Berhanu, M., & Kudrolli, A., Dissolution-driven propulsion of floating solids. Proceedings of the National Academy of Sciences, 120(32), e2301947120 (2023).