Search

The dynamics and pair trajectories of two self-propelled (active) colloids in a quiescent fluid for both axisymmetric and asymmetric cases are reported. The autonomous motions of the colloids are due to a catalytic chemical reaction taking place asymmetrically on their surfaces that generates a concentration gradient of interactive solutes around the particles and actuates particle propulsion. A combined analytical-numerical technique was developed to solve the coupled mass conservation equation for the solute and the hydrodynamics between the colloids in the Stokes flow regime. For axisymmetric motions, the translational swimming velocities of the particles in the near field can be enhanced or weakened (compared to their motions when they are far apart) depending on the relative orientations of their active sections. Moreover, it can be shown that different surface activities of two symmetric particles, e.g. an inert versus a catalytic particle or two catalytic particles, can also lead to a propulsion where the far-field swimming velocity for the inert and the catalytic particle attenuate as ∼1/Δ2 and ∼1/Δ5 respectively, Δ is the non-dimensional centre-to-centre distance. For asymmetric motions, our analysis indicates two possible scenarios for pair trajectories of catalytically active particles: either the particles approach, come into contact and assemble or they interact and move away from each other (escape). It is found that the direction of particle rotations is the key factor in determining the escape or assembly scenario due to an interplay between both hydrodynamic and phoretic effects. Based on the analysis, a phase diagram is sketched for the pair trajectory of the catalytically active particles as a function of reacting surface coverages and their initial relative orientations with respect to each other. We believe this study has important implications in elucidation of collective behaviours of autophoretically self-propelled colloids and would be certainly a guide for experimentalists to design and control active systems. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]