Impact of Surface Preparation on the Propulsion of Catalytic Janus Colloids and Implications for Reproducibility

Abstract

Platinum-coated polystyrene (PS) Janus particles (JPs) propelled by the catalytic decomposition of hydrogen peroxide (H2O2) are a widely used model system for active matter, yet reported speeds for similar particles and fuel concentrations differ by more than an order of magnitude. Broad adoption of these materials requires increasing consistency in the preparation and achieving improved repeatability in their behavior. Herein, we focused on two routine, but rarely reported, preparation steps that we found to alter the particle speed. First, we found that pre-exposing the platinum cap to H2O2 before cell assembly increased the mean speed by a factor of 2.4. The size of this effect depended on batch history, where an aged batch responded strongly, while a freshly prepared batch sped up by only 16%. Second, we found that plasma cleaning of the glass substrate decreased the speed 2.7 times. In a 120 $μ$m cell the suppression was the same at both walls even when only one wall was treated. The suppression grew with the treated glass area per unit fluid volume, which pointed to species released into the bulk solution rather than to a change at the wall. Plasma-cleaned glass released more ions into water than untreated glass, and XPS detected an ionic metal fluoride on the plasma-cleaned surface. A model in which the released ions lower the speed through the solution conductivity reproduced the measured speeds in all four cell geometries. Of the 17 published studies compared here, pre-activation is reported in only four studies and substrate cleaning in only six studies. We propose a minimum reporting standard for the preparation and associated speed measurements of catalytic Janus particles.