Dissolution-driven propulsion of floating solids

TL;DR

This paper demonstrates that floating asymmetric dissolving solids can achieve rectilinear motion driven by attached density currents, with potential applications in active matter and natural phenomena like melting icebergs.

Contribution

It introduces a novel propulsion mechanism based on dissolution-induced density currents and provides an analytical model linking geometry, material properties, and speed.

Findings

Asymmetric dissolving solids reach speeds up to 5 mm/s.

Boat velocity is opposite to the horizontal component of the density current.

The model predicts speed scales with the cube-root of the dissolving surface length.

Abstract

We show that unconstrained asymmetric dissolving solids floating in a fluid can move rectilinearly as a result of attached density currents which occur along their inclined surfaces. Solids in the form of boats composed of centimeter-scale sugar and salt slabs attached to a buoy are observed to move rapidly in water with speeds up to 5 mm/s determined by the inclination angle and orientation of the dissolving surfaces. While symmetric boats drift slowly, asymmetric boats are observed to accelerate rapidly along a line before reaching a terminal velocity when their drag matches the thrust generated by dissolution. By visualizing the flow around the body, we show that the boat velocity is always directed opposite to the horizontal component of the density current. We derive the thrust acting on the body from its measured kinematics, and show that the propulsion mechanism is consistent with the unbalanced momentum generated by the attached density current. We obtain an analytical formula for the body speed depending on geometry and material properties, and show that it captures the observed trends reasonably. Our analysis shows that the gravity current sets the scale of the body speed consistent with our observations, and we estimate that speeds can grow slowly as the cube-root of the length of the inclined dissolving surface. The dynamics of dissolving solids demonstrated here applies equally well to solids undergoing phase change, and may enhance the drift of melting icebergs, besides unraveling a primal strategy by which to achieve locomotion in active matter.