Shear-induced interfacial assembly of Janus particles

We investigate the hydrodynamics of spherical Janus particles at the interface between two immiscible fluids using a multicomponent lattice-Boltzmann method. The CahnHilliard model is used to evolve the composition for this binary system of incompressible fluids, while the particle-fluid interactions are taken into account by adding a supplemental force to recover the appropriate wettability at solid boundaries. We evaluate the capillaryinduced interactions between multiple Janus particles at a sheared interface and demonstrate the possibility of directing their assembly. In response to the flow, all particles approach a steady orientation resulting from the balance between shear-induced torque and the resistance due to preferred wetting. At sufficiently large shear rates leading to strong capillary dipoles, the particles rearrange and form chains normal to the shear direction. For the particle sizes considered, an intermediate window of surface coverage between 32% and 65% is found to give effective alignment with order parameters in the range of 0.7-1.0. An interesting feature of this directed assembly method is that the structure is preserved after removing the flow field: Janus particles only rotate to upright orientation without disintegrating the chains. This approach can enable directing a randomly oriented or distributed cluster of Janus particles into an ordered structure with controllable rheological properties.