Forward, reverse, and no motion of Marangoni surfers under confinement
Forward, reverse, and no motion of Marangoni surfers under confinement
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DOI：
10.1103/physrevfluids.5.084004
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发表时间：
2020-08-12
影响因子：
2.7
通讯作者：
Masoud, Hassan
中科院分区：
文献类型：
--
作者：
Kang, Saeed Jafari;Sur, Samrat;Masoud, Hassan
我们研究了跨越有限深度液体层和半无限气体层之间界面的化学活性粒子的迁移率。表面活性剂从颗粒中不对称地释放，局部降低界面表面张力。通常认为，表面张力的不均匀分布和相关的马兰戈尼流导致活跃冲浪者的推进力与释放方向相反，其中表面张力较高。这被认为是向前运动。然而，我们最近的理论分析（在活性剂可忽略不计的惯性和扩散主导的传输的限制下）表明，对于某些形状的冲浪者和足够浅的液体层，这种趋势可能会逆转。在这里，我们超越了斯托克斯体系，研究了马兰戈尼驱动的薄圆柱盘和扁球体的运动，以获得广泛的释放率和渗出化学物质的扩散率，从而控制有效的雷诺数和佩克莱特数。我们考虑了液膜厚度所代表的不同程度的限制，并表明冲浪者确实可以经历向前、向后或阻止的运动。我们还确定了这些移动模式与作用在冲浪者身上的力以及其附近的流动结构之间的联系。出乎意料的是，我们发现负压是冲浪者所经历的流体力的主要贡献者，并且这种吸力主要负责反向马兰戈尼推进力。报告的结果基于密切证实的数值计算和实验测量。
We examine the mobility of a chemically active particle straddling the interface between a liquid layer of finite depth and a semi-infinite layer of gas. A surface-active agent is released asymmetrically from the particle that locally lowers the interfacial surface tension. It is commonly presumed that the uneven distribution of the surface tension and the associated Marangoni flow lead to the propulsion of the active surfer opposite to the release direction, where the surface tension is higher. This is considered forward motion. However, our recent theoretical analysis-in the limits of negligible inertia and diffusion-dominated transport of the active agent-has shown that this trend may be reversed for certain shapes of the surfer and shallow enough liquid layers. Advancing beyond the Stokes regime, here, we study the Marangoni-driven motion of thin cylindrical disks and oblate spheroids for a wide range of release rates and diffusivity of the exuded chemical species, that control the effective Reynolds and Peclet numbers. We consider various degrees of confinement represented by the thickness of the liquid film, and show that indeed the surfers can undergo a forward, a backward, or an arrested motion. We also identify the links between these modes of mobility and the forces acting on the surfers as well as the flow structure in their vicinity. Rather unexpectedly, we discover that negative pressure is the primary contributor to the fluid force experienced by the surfer and that this suction force is mainly responsible for the reverse Marangoni propulsion. The reported results are based on closely corroborating numerical calculations and experimental measurements.