Koser Alison E, Keim Nathan C, Arratia Paulo E
Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2013 Dec;88(6):062304. doi: 10.1103/PhysRevE.88.062304. Epub 2013 Dec 10.
Many fascinating phenomena such as large-scale collective flows, enhanced fluid mixing, and pattern formation have been observed in so-called active fluids, which are composed of particles that can absorb energy and dissipate it into the fluid medium. For active particles immersed in liquids, fluid-mediated viscous stresses can play an important role on the emergence of collective behavior. Here, we experimentally investigate their role in the dynamics of self-assembling magnetically driven colloidal particles which can rapidly form organized hexagonal structures. We find that viscous stresses reduce hexagonal ordering, generate smaller clusters, and significantly decrease the rate of cluster formation, all while holding the system at constant number density. Furthermore, we show that time and length scales of cluster formation depend on the Mason number (Mn), or ratio of viscous to magnetic forces, scaling as t∝Mn and L∝Mn(-1/2). Our results suggest that viscous stresses hinder collective behavior in a self-assembling colloidal system.
在所谓的活性流体中，已经观察到许多引人入胜的现象，如大规模集体流动、增强的流体混合和图案形成。活性流体由能够吸收能量并将其耗散到流体介质中的颗粒组成。对于浸没在液体中的活性颗粒，流体介导的粘性应力在集体行为的出现中可以发挥重要作用。在这里，我们通过实验研究它们在自组装磁驱动胶体颗粒动力学中的作用，这些颗粒可以迅速形成有组织的六边形结构。我们发现，粘性应力会降低六边形有序性，产生更小的簇，并显著降低簇的形成速率，同时将系统保持在恒定的数密度。此外，我们表明簇形成的时间和长度尺度取决于梅森数（Mn），即粘性力与磁力的比值，其标度为t∝Mn和L∝Mn^(-1/2)。我们的结果表明，粘性应力会阻碍自组装胶体系统中的集体行为。
