Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, 400 W. 13th St., Rolla, Missouri 65409, USA.
Soft Matter. 2019 Mar 13;15(11):2439-2446. doi: 10.1039/c8sm02522c.
Manipulation of droplets based on physical properties (e.g., size, interfacial tension, electrical, and mechanical properties) is a critical step in droplet microfluidics. Manipulations based on magnetic fields have several benefits compared to other active methods. While traditional magnetic manipulations require spatially inhomogeneous fields to apply forces, the fast spatial decay of the magnetic field strength from the source makes these techniques difficult to scale up. In this work, we report the observation of lateral migration of ferrofluid (or magnetic) droplets under the combined action of a uniform magnetic field and a pressure-driven flow in a microchannel. While the uniform magnetic field exerts negligible net force on the droplet, the Maxwell stresses deform the droplet to achieve elongated shapes and modulate the orientation relative to the fluid flow. Hydrodynamic interactions between the droplets and the channel walls result in a directional lateral migration. We experimentally study the effects of field strength and direction, and interfacial tension, and use analytical and numerical modeling to understand the lateral migration mechanism.
基于物理性质（例如尺寸、界面张力、电学和力学性质）的液滴操控是液滴微流控中的关键步骤。与其他主动方法相比，基于磁场的操控具有若干优势。虽然传统的磁场操控需要非均匀的空间磁场来施加力，但磁场强度从源处的快速空间衰减使得这些技术难以扩展。在这项工作中，我们报告了在微通道中，在均匀磁场和压力驱动流的共同作用下，观察到铁磁流体（或磁性）液滴的横向迁移。虽然均匀磁场对液滴施加的净力可以忽略不计，但麦克斯韦应力使液滴变形为拉长形状，并调节相对于流体流动的方向。液滴与通道壁之间的流体动力学相互作用导致液滴定向横向迁移。我们实验研究了场强和方向以及界面张力的影响，并使用分析和数值建模来理解横向迁移机制。
