Floor- or Ceiling-Sliding for Chemically Active, Gyrotactic, Sedimenting Janus Particles
Floor- or Ceiling-Sliding for Chemically Active, Gyrotactic, Sedimenting Janus Particles
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DOI：
10.1021/acs.langmuir.9b03696
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发表时间：
2020-06-30
期刊：
影响因子：
3.9
通讯作者：
Kretzschmar, Ilona
中科院分区：
文献类型：
--
作者：
Das, Sayan;Jalilvand, Zohreh;Kretzschmar, Ilona
化学活性颗粒由于其表面的催化化学反应而在没有外力和扭矩的情况下实现运动性（“自推进”），这改变了周围溶液的化学组成（称为“化学场”）并诱导溶液的流体动力学流动。通过将这些场的扭曲耦合回其运动，化学活性粒子经历与限制表面的有效相互作用。这种耦合可以导致丰富的行为，例如“滑动”的壁约束稳定状态的发生。大多数活性颗粒的密度与溶液不匹配，因此倾向于沉淀。此外，通常使用的Janus球，其由用帽状催化剂薄层装饰的惰性核材料组成，是回旋立构的（即，“底部重”）。它们是否可以在水平壁处表现出滑动状态取决于主动运动与重力驱动的沉降和对齐之间的相互作用，例如，使轴线沿着重力方向对齐的回转共规趋势被具有不同取向的竞争性活动驱动的对齐所克服。因此，重要的是要了解和量化的影响，这些重力引起的影响模型化学活性粒子在附近的墙壁移动的行为。对于模型回旋，自泳Janus粒子，在这里，我们研究理论上的滑动状态的发生在水平平面壁，要么低于（“地板”）或以上（“天花板”）的粒子。我们构建“状态图”，表征这种状态的发生作为沉降速度和颗粒的回旋响应的函数，以及颗粒的泳动性。我们表明，在某些情况下，滑动状态可能会同时出现在天花板和地板，而实验相关的参数空间的较大部分对应的粒子，将表现出滑动状态，只有在地板上或天花板上，或者根本没有滑动状态。这些预测与以前的实验研究的结果，以及与我们的专用实验进行的Pt涂层，聚苯乙烯核心，或二氧化硅核心Janus球浸泡在过氧化氢水溶液中进行了严格的比较。
Chemically active particles achieve motility without external forces and torques ("self-propulsion") due to catalytic chemical reactions at their surfaces, which change the chemical composition of the surrounding solution (called "chemical field") and induce hydrodynamic flow of the solution. By coupling the distortions of these fields back to its motion, a chemically active particle experiences an effective interaction with confining surfaces. This coupling can lead to a rich behavior, such as the occurrence of wall-bound steady states of "sliding". Most active particles are density mismatched with the solution and, thus, tend to sediment. Moreover, the often employed Janus spheres, which consist of an inert core material decorated with a cap-like, thin layer of a catalyst, are gyrotactic (i.e., "bottom-heavy"). Whether or not they may exhibit sliding states at horizontal walls depends on the interplay between the active motion and the gravity-driven sedimentation and alignment, such as the gyrotactic tendency to align the axis along the gravity direction being overcome by a competing, activity-driven alignment with a different orientation. It is therefore important to understand and quantify the influence of these gravity-induced effects on the behavior of model chemically active particles moving in the vicinity of walls. For model gyrotactic, self-phoretic Janus particles, here we study theoretically the occurrence of sliding states at horizontal planar walls that are either below ("floor") or above ("ceiling") the particle. We construct "state diagrams" characterizing the occurrence of such states as a function of the sedimentation velocity and of the gyrotactic response of the particle, as well as of the phoretic mobility of the particle. We show that in certain cases sliding states may emerge simultaneously at both the ceiling and the floor, while the larger part of the experimentally relevant parameter space corresponds to particles that would exhibit sliding states only either at the floor or at the ceiling-or there are no sliding states at all. These predictions are critically compared with the results of previous experimental studies, as well as with our dedicated experiments carried out with Pt-coated, polystyrene-core, or silica-core Janus spheres immersed in aqueous hydrogen peroxide solutions.