Influence of Nanoparticles on the Dynamics and Clustering of Active Colloids Proximate to a Boundary

Active colloidal particles regularly interact with surfaces in applications ranging from microfluidics to sensing. Herein, experiments and simulations were conducted to illustrate the impact of nanoparticles on the propulsion dynamics and clustering behavior of micrometer scale catalytic active Janus colloids near a boundary. The addition of either negatively charged 20 nm polystyrene particles or polyethylene glycol (PEG) of molecular weight 6K and 600K decreased the apparent propulsion of a Janus colloid at infinite dilution to near zero. These experiments were extended to more concentrated systems in which the same active Janus colloids, in the absence of added nanoparticles, formed clusters. The extent of clustering tended to increase with fuel concentration. Similar to the case of Janus colloids at infinite dilution, the addition of polymers had a dramatic impact on clustering behavior. Following the addition of either 6K or 600K PEG, clustering was significantly mitigated, with the higher molecular weight polymer having a more dramatic effect. Complementary agent-based simulations considering the impact of hydrodynamics for active Janus colloids were conducted in the range of separation distances inferred from experiment. These simulations showed that propulsion speed decreased monotonically with decreasing average separation distance and also that clustering was reduced with decreasing propulsion speed. Taken together, these experiments and simulations demonstrate the impact of depletion and conductivity arising from the addition of nanoparticles on the dynamics and clustering of active colloids proximate to a boundary.