(63c) Cap-Weight and Surface Wettability Effects on Janus Particle Self-Propulsion Near Boundaries

Janus particles exhibiting active motion have promising applications in drug delivery, water purification, biosensing, and microscale cargo transport. Among these, platinum-coated polystyrene Janus particles in hydrogen peroxide are widely studied for their self-propulsion behavior. In this study, we investigate the interplay between gravitational torque and active motion by varying the thickness of the platinum coating from 5 nm to 35 nm. Additionally, we explore the effect of surface hydrophilicity and hydrophobicity on particle motion by altering the slip length at the boundary. The self-propulsion force is controlled by adjusting the hydrogen peroxide concentration from 1 wt./vol % to 3 wt./vol %. We systematically analyze the velocities of Janus particles with different cap thicknesses (5 nm, 10 nm, 20 nm, and 35 nm) on surfaces with varying wettability (hydrophilic or hydrophobic) under two peroxide concentrations (1% and 3%). Our results show that the highest velocities are observed for particles with the thinnest cap (5 nm) in the highest peroxide concentration (3 wt./vol %) on hydrophobic surfaces. Conversely, particles with the thickest cap (35 nm) in the lowest peroxide concentration (1 wt./vol %) on hydrophilic surfaces exhibit the lowest velocities and display quenching behavior. The reduced active propulsion in particles with heavier caps leads to their cap aligning toward the boundary, minimizing motion and enhancing quenching effects. Hydrophobic surfaces, on the other hand, promote greater active propulsion, reducing the likelihood of quenching. These findings highlight the balance between active self-propulsion, gravitational effects of cap weight, and boundary slip length in determining particle velocities and quenching behavior. This study advances the understanding of Janus particle dynamics near boundaries, contributing to the broader application of these particles in various microfluidic and biomedical systems.