Shear-Induced Alignment of Janus Particle Lamellar Structures

Control over the alignment of colloidal structures plays a crucial role in advanced reconfigurable materials. In this work, we study the alignment of Janus particle lamellar structures under shear flow via Brownian dynamics simulations. Lamellar alignment (orientation relative to flow direction) is measured as a function of the Péclet number (Pe)the ratio of the viscous shear to the Brownian forcesthe particle volume fraction, and the strength of the anisotropic interaction potential made dimensionless with thermal energy. Under conditions where lamellar structures are formed, three orientation regimes are observed: (1) random orientation for very small Pe, (2) parallel orientationlamellae with their normals parallel to the direction of the velocity gradientfor intermediate values of Pe, and (3) perpendicular orientationlamellae with their normals parallel to the vorticity directionfor large Pe. To understand the alignment mechanism, we carry out a scaling analysis of competing torques between a pair of particles in the lamellar structure. Our results suggest that the change of parallel to perpendicular orientation is independent of the particle volume fraction and is caused by the hydrodynamic and Brownian torques on the particles overcoming the torques resulting from the interparticle interactions. This initial study of shear-induced alignment on lamellar structures formed by Janus colloidal particles also opens the door for future applications where a reversible actuator for structure orientation is required.

对胶体结构取向的精准调控，在先进可重构材料领域具有核心关键作用。本研究通过布朗动力学（Brownian dynamics）模拟，系统探究了剪切流环境中Janus颗粒（Janus particle）层状结构的取向行为。层状结构的取向（即相对于流动方向的方位）以佩克莱特数（Péclet number, Pe，粘性剪切力与布朗力的比值）、颗粒体积分数，以及以热能为基准无量纲化的各向异性相互作用势强度作为调控变量进行表征。在可稳定形成层状结构的条件下，共观测到三类取向模式：(1) 极低佩克莱特数下的随机取向；(2) 中等佩克莱特数区间的平行取向——此时层状结构的法线平行于速度梯度方向；(3) 高佩克莱特数下的垂直取向——此时层状结构的法线平行于涡量方向。为阐明该取向调控的内在物理机制，我们对层状结构内颗粒对之间的竞争力矩开展了标度分析。研究结果显示，从平行取向到垂直取向的转变与颗粒体积分数无关，其本质是颗粒所受流体力学力矩与布朗力矩共同克服了颗粒间相互作用产生的力矩。此项针对Janus胶体颗粒形成的层状结构的剪切诱导取向的初步研究，亦为未来需要结构取向可逆驱动装置的应用场景开辟了全新的研究路径。