Active colloidal particles regularly interact with surfaces in applications ranging from microfluidics to sensing. Recent work has revealed the complex nature of these surface interactions for active particles. Herein, we summarize experiments and simulations that show the impact of charged nanoparticles on the propulsion of an active colloid near a boundary. Adding charged nanoparticles not only decreased the average separation distance of a passive colloid because of depletion attraction as expected but also decreased the apparent propulsion of a Janus colloid to near zero. 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. Although the trend found in experiments and simulations was in qualitative agreement, there was still a significant difference in the magnitude of speed reduction. The quantitative difference was attributed tothe influence of charged nanoparticles on the conductivity of theactive particle suspension. Follow-up experiments delineating theimpact of depletion and conductivity showed that both contribute tothe reduction of speed for an active Janus particle. The experimentaland simulated data suggests that it is necessary to consider the synergisticeffects between various mechanisms influencing interactions experiencedby an active particle near a boundary.
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