Shear-induced assembly and breakup in suspensions of magnetic Janus particles with laterally shifted dipoles.

Abstract

We employ Brownian dynamics simulations to investigate the shear-induced assembly and breakup of aggregates in dilute suspensions of magnetic Janus particles with laterally shifted dipoles. By systematically displacing the magnetic dipole from its geometric center, given by the dipolar shift <i>s</i>, and the strength of magnetic interactions relative to flow- and Brownian-induced forces, given by the Péclet number <i>Pe</i> and the dipolar coupling constant <i>λ</i>, respectively, distinct aggregation regimes are revealed. At low dipolar shifts (<i>s</i> ≤ 0.1) and low <i>Pe</i>, shear-enhanced diffusion promotes particle collisions, leading to faster aggregation of particles forming loop-like clusters that align with the flow. As <i>Pe</i> increases, these structures fragment into smaller aggregates and eventually disperse into gas-like arrangements. In contrast, particles with medium dipolar shifts (<i>s</i> ≥ 0.2) exhibit significant stability, forming compact vesicle- and micelle-like assemblies that resist shear-induced breakup even at high <i>Pe</i>, provided <i>λ</i> is sufficiently large. Orientational analysis indicates that particles maintain head-to-tail, head-to-side, and antiparallel alignments under shear, depending on <i>s</i> and <i>Pe</i>. The critical <i>Pe</i> required to induce cluster breakdown increases with both <i>s</i> and <i>λ</i>, underscoring the stabilizing influence of lateral dipole displacement and strong magnetic interactions. The transition to gas-like dispersion occurs when hydrodynamic and Brownian torques on the particles overcome the torques resulting from the interparticle interactions. Overall, these findings provide fundamental insights into the non-equilibrium self-assembly of anisotropic colloids, offering a framework for designing advanced materials with tunable structural and dynamic properties in microfluidics, drug delivery, and magnetorheological applications.