From Passive to Active: Self-Propelled Colloids in Coatings Formulation and Film Formation.

Coatings are widely used in protective and functional applications but are fundamentally limited by the passive nature of their formulation ingredients. This leads to a critical lack of control over the spatial distribution of ingredients and prevents the optimization of key functional properties. Here, we demonstrate the use of active, self-propelled constituents in coatings formulations. By introducing active Janus colloids (JCs), we show how they overcome sedimentation and chemical gradients to accumulate at both top and bottom coating interfaces. We find that balancing the timescales of fuel depletion and evaporation-induced assembly is key to control the JCs distribution within the dried coating. JCs at the top coating surface have an orientational bias, with the sub-equatorial orientation being the most common. While our study is a proof-of-concept demonstration on a model system, it establishes a framework for harnessing active Janus colloids to influence coating microstructure. This work could lay the foundation for future studies developing functional coatings with tuned microstructure enabled by orientation-biased active JCs.