Department of Chemical and Biological Engineering, Colorado School of Mines , Golden, Colorado 80401, United States.
Langmuir. 2018 Jan 23;34(3):952-960. doi: 10.1021/acs.langmuir.7b02793. Epub 2017 Oct 17.
As demonstrated in biological systems, breaking the symmetry of surrounding hydrodynamic flow is the key to achieve autonomous locomotion of microscopic objects. In recent years, a variety of synthetic motors have been developed based on different propulsion mechanisms. Most work, however, focuses on the propulsion of individual motors. Here, we study the collective behaviors of colloidal dimers actuated by a perpendicularly applied AC electric field, which controls the electrohydrodynamic flow at subparticle levels. Although these motors experience strong dipolar repulsion from each other and are highly active, surprisingly, they assemble into a family of stable planar clusters with handedness. We show that this type of unusual structure arises from the contractile hydrodynamic flow around small lobes but extensile flow around the large lobes. We further reveal that the collective behavior, assembled structure, and assembly dynamics of these motors all depend on the specific directions of electrohydrodynamic flow surrounding each lobe of the dimers. By fine-tuning the surface charge asymmetry on particles and salt concentration in solution, we demonstrate the ability to control their collective behaviors on demand. This novel type of active assembly via hydrodynamic interactions has the potential to grow monodisperse clusters in a self-limiting fashion. The underlying concept revealed in this work should also apply to other types of active and asymmetric particles.
在生物系统中，打破周围流体动力流的对称性是实现微观物体自主运动的关键。近年来，已经基于不同的推进机制开发了各种合成马达。然而，大多数工作都集中在单个马达的推进上。在这里，我们研究了在垂直施加的交流电场作用下胶体二聚体的集体行为，该电场控制亚颗粒水平的电流体动力学流动。尽管这些马达彼此之间受到强烈的偶极排斥并且非常活跃，但令人惊讶的是，它们组装成一系列具有手性的稳定平面簇。我们表明，这种类型的异常结构源自围绕小叶片的收缩性流体动力学流动，但围绕大叶片的流动是伸展性的。我们进一步揭示，这些马达的集体行为、组装结构和组装动力学都取决于围绕二聚体每个叶片的特定电流体动力学流动方向。通过微调颗粒表面上的电荷不对称性和溶液中的盐浓度，我们证明了按需控制其集体行为的能力。这种通过流体动力相互作用的新型主动组装方式有可能以自限制的方式生长单分散簇。这项工作揭示的基本概念也应该适用于其他类型的主动和不对称颗粒。
