论文标题
流体界面的驱动和活跃胶体
Driven and active colloids at fluid interfaces
论文作者
论文摘要
我们得出了由平面流体流体界面上外部驱动和主动(游泳)胶体产生的前阶远场流的表达式。我们考虑与界面相邻的胶体或用固定的接触线粘附在界面上。雷诺和毛细血管数的假定远小于统一,这与涉及空气或烷烃水接口的典型微米尺度胶体一致。对于驱动的胶体,前阶流程由该系统的点(和/或扭矩)响应给出。对于主动胶体,力 - 二极管(应力)响应发生在领先顺序。在清洁(无表面活性剂)界面时,这些流体动力模式本质上是散装流体中通常的stokes多物种的受限组。为了领先的顺序,驱动的胶体施加平行于界面的stokeslet,而活性胶体则根据胶体的方向驱动不同方向的压力。然后,我们考虑如何通过不可压缩界面的存在来改变这些模式,这是在存在表面活性剂的情况下在毛细管数小的胶体系统的典型情况。驱动和主动胶体的前阶模式急剧重组。对于驱动的胶体,界面不可压缩性基本上会削弱远与界面正常的远场流动。点力响应仅驱动与接口平行的流动。但是,Marangoni强调会引起新的偶极模式,这在干净的界面上缺乏类似物。表面粘应力(如果存在)可能会在界面和周围流体上产生非常长的流动。我们的结果对胶体组装和对流质量运输的增强具有重要意义。
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.