Chaigne Martin, Berhanu Michael, Kudrolli Arshad
Laboratoire Matière et Systèmes Complexes, Université Paris Cité, CNRS (UMR 7057), F-75013 Paris, France.
Department of Physics, Clark University, Worcester, MA 01610.
Proc Natl Acad Sci U S A. 2023 Aug 8;120(32):e2301947120. doi: 10.1073/pnas.2301947120. Epub 2023 Jul 31.
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.
我们表明，漂浮在流体中的无约束不对称溶解固体能够沿直线移动，这是由于附着在其倾斜表面上的密度流所致。观察到由附着在浮标上的厘米级糖和盐板组成的船形固体在水中快速移动，其速度高达5毫米/秒，该速度由溶解表面的倾斜角度和方向决定。虽然对称的船缓慢漂移，但观察到不对称的船在其阻力与溶解产生的推力相匹配之前，会沿着一条直线快速加速，直至达到终端速度。通过可视化物体周围的流动，我们表明船的速度始终与密度流的水平分量方向相反。我们从测量的运动学中推导出作用在物体上的推力，并表明推进机制与附着密度流产生的不平衡动量一致。我们得到了一个取决于几何形状和材料特性的物体速度解析公式，并表明它合理地捕捉到了观察到的趋势。我们的分析表明，重力流设定了与我们的观察结果一致的物体速度尺度，并且我们估计速度可以随着倾斜溶解表面长度的立方根缓慢增长。这里展示的溶解固体动力学同样适用于经历相变的固体，并且除了解开活性物质中实现运动的原始策略之外，还可能增强融化冰山的漂移。
