Driven and active colloids at fluid interfaces
Driven and active colloids at fluid interfaces
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DOI：
10.1017/jfm.2020.708
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发表时间：
2021-03-05
影响因子：
3.7
通讯作者：
Stebe, Kathleen J.
中科院分区：
文献类型：
--
作者：
Chisholm, Nicholas G.;Stebe, Kathleen J.
我们推导了由外驱动和活动（游动）胶体在平面流-液界面上产生的领先级远场流的表达式。我们认为胶体与界面相邻或粘附在界面上，有固定的接触线。假设雷诺数和毛细数远小于1，符合典型的微米尺度胶体涉及空气或烷烃-水界面。对于驱动胶体，该系统的点力（和/或扭矩）响应给出了一级流动。对于活性胶体，力偶极子（应力）响应发生在先导级。在清洁（无表面活性剂）界面，这些流体动力模式本质上是散装流体中常用Stokes多极子的一组限制。首先，被驱动的胶体施加平行于界面的stokeslet，而活跃的胶体则根据胶体的方向驱动不同方向的应力条。然后，我们考虑了不可压缩界面的存在如何改变这些模式，这是在表面活性剂存在下毛细管数小的胶体系统的典型情况。驱动胶体和活性胶体的导阶模式发生了显著的重构。对于驱动胶体，界面不可压缩性大大削弱了垂直于界面的远场流动；点力响应只驱动平行于界面的流动。然而，马兰戈尼应力诱导了一种新的偶极模式，在干净的界面上缺乏类似的模式。如果存在表面粘性应力，可能会在界面和周围流体上产生很长时间的流动。我们的研究结果对流体边界附近的胶体组装和对流质量输运增强具有重要意义。
We derive expressions for the leading-order far-field flows generated by externally driven and active (swimming) colloids at planar fluid-fluid interfaces. We consider colloids adjacent to the interface or adhered to the interface with a pinned contact line. The Reynolds and capillary numbers are assumed much less than unity, in line with typical micron-scale colloids involving air- or alkane-aqueous interfaces. For driven colloids, the leading-order flow is given by the point-force (and/or torque) response of this system. For active colloids, the force-dipole (stresslet) response occurs at leading order. At clean (surfactant-free) interfaces, these hydrodynamic modes are essentially a restricted set of the usual Stokes multipoles in a bulk fluid. To leading order, driven colloids exert Stokeslets parallel to the interface, while active colloids drive differently oriented stresslets depending on the colloid's orientation. We then consider how these modes are altered by the presence of an incompressible interface, a typical circumstance for colloidal systems at small capillary numbers in the presence of surfactant. The leading-order modes for driven and active colloids are restructured dramatically. For driven colloids, interfacial incompressibility substantially weakens the far-field flow normal to the interface; the point-force response drives flow only parallel to the interface. However, Marangoni stresses induce a new dipolar mode, which lacks an analogue on a clean interface. Surface-viscous stresses, if present, potentially generate very long-ranged flow on the interface and the surrounding fluids. Our results have important implications for colloid assembly and advective mass transport enhancement near fluid boundaries.