Self-propulsion of symmetric chemically active particles: Point-source model and experiments on camphor disks
Self-propulsion of symmetric chemically active particles: Point-source model and experiments on camphor disks
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DOI：
10.1103/physreve.99.062605
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发表时间：
2019-06-17
影响因子：
2.4
通讯作者：
Detcheverry, Francois
中科院分区：
文献类型：
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作者：
Boniface, Dolachai;Cottin-Bizonne, Cecile;Detcheverry, Francois
被液体介质或界面包围的固体不可变形粒子可以通过改变其局部环境来推动自己。这种非机械性游泳在自泳者和界面游泳者中都起作用，前者的自身产生的场梯度在其表面引起滑动速度，后者利用表面张力的不平衡。在这两类系统中，具有内在不对称性的游泳者受到了最多的关注，但完全各向同性的粒子也有可能自我推进。理论上已经为自流系统建立了基本的对称性破缺不稳定性，但对于固体粒子，还没有在实验上观察到。对于界面游泳运动员，一些实验工作指出了这种机制，但对它的理解仍然不完整。这项工作的目标就是填补这一空白。在前面的建议的基础上，我们首先开发了一个点源模型，该模型可以普遍应用于界面或磷酸盐游泳者。使用这张近似但统一的图像，我们表明它们在非常不同的区域运行，并获得对推进速度及其与游泳者大小和不对称性的依赖关系的解析预测。接下来，我们给出了在界面樟脑盘上的实验表明，它们确实在平流主导的区域内自推进，在该区域内固有的不对称性是无关的，并且游泳速度随着尺寸的增大而亚线性地增加。最后，我们结合实验讨论了点源模型的优点和局限性，并指出其更广泛的相关性。
Solid undeformable particles surrounded by a liquid medium or interface may propel themselves by altering their local environment. Such nonmechanical swimming is at work in autophoretic swimmers, whose self-generated field gradient induces a slip velocity on their surface, and in interfacial swimmers, which exploit unbalance in surface tension. In both classes of systems, swimmers with intrinsic asymmetry have received the most attention but self-propulsion is also possible for particles that are perfectly isotropic. The underlying symmetry-breaking instability has been established theoretically for autophoretic systems but has yet to be observed experimentally for solid particles. For interfacial swimmers, several experimental works point to such a mechanism, but its understanding has remained incomplete. The goal of this work is to fill this gap. Building on an earlier proposal, we first develop a point-source model that may be applied generically to interfacial or phoretic swimmers. Using this approximate but unifying picture, we show that they operate in very different regimes and obtain analytical predictions for the propulsion velocity and its dependence on swimmer size and asymmetry. Next, we present experiments on interfacial camphor disks showing that they indeed self-propel in an advection-dominated regime where intrinsic asymmetry is irrelevant and that the swimming velocity increases sublinearly with size. Finally, we discuss the merits and limitations of the point-source model in light of the experiments and point out its broader relevance.