  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.
