Assembly of single-patch colloidal Janus particles under steady shear flow via Brownian dynamics simulations is studied. In the absence of flow, by varying the Janus patch size and the range and strength of the anisotropic interaction potential, Janus colloids form different aggregates such as micelles, wormlike clusters, vesicles and lamellae. Under shear flow we observe rearrangement, deformation, and break-up of aggregates depending on the value of the Péclet number (Pe)—the ratio between viscous shear and Brownian forces. An enhanced aggregation at small Pe for micelles and vesicles is observed while a break-up into smaller micelles at intermediate Pe and finally a fluid-like structure at high Pe. In the case of lamellae, the structures initially break into smaller lamellae and finally break-up into a fluid-like structure at high Pe. In conditions where lamellar structures are formed, three orientation regimes are observed: (1) Random orientation for small Pe, (2) parallel orientation—lamellae with their normals parallel to the direction of the velocity gradient—at intermediate Pe and (3) perpendicular orientation—lamellae with their normals parallel to the vorticity direction—for large Pe. Our results suggest that the change of parallel to perpendicular lamellar orientation is caused by hydrodynamic torque on the particles overcoming Brownian rotary motion and torques resulting from interparticle interactions.
The different cluster sizes, morphologies, and alignments observed in colloidal Janus particles as functions of interaction range, Janus patch size, interaction strength, and shear rate, open new actuation routes for reconfigurable materials and applications.
Ubaldo M. Córdova-Figueroa is an Associate Professor of the Department of Chemical Engineering at the University of Puerto Rico - Mayagüez (UPRM). He obtained his MS and PhD in Chemical Engineering from the California Institute of Technology. His expertise is theoretical soft matter and fluid mechanics, resulting in publications in peer-reviewed journals such as Journal of Fluid Mechanics, Soft Matter, Physical Review Letters and Nature Chemistry. His work is focused on understanding the structure and rheology of patchy (Janus) colloidal particle suspensions exposed to flows and external fields, as well as the dynamics of active Janus particles in anisotropic fluids. In 2011 he obtained the NSF CAREER award and in 2013 was selected as Distinguished Professor in Chemical Engineering at UPRM. He serves as the Coordinator of the Research Academy for Faculty and Postdoctoral Fellows, a professional development program at UPRM designed to help early-career faculty and postdocs seeking funding for research.