Bubble-driven flow transitions in evaporating active droplets on structured surfaces

Abstract

The evaporation of particle-laden droplets on engineered surfaces is important in printing, sensing, and interfacial assembly. While the influence of substrate topography on passive particle deposition is well established, the combined effects of active particle dynamics, catalytic gas generation, and surface structuring remain unexplored. Here, we investigate the drying of aqueous droplets containing Janus particles (polystyrene–platinum, PS-Pt) on topographically patterned substrates in the presence of hydrogen peroxide (H2O2) fuel. The catalytic decomposition of H2O2 at the Pt cap produces oxygen bubbles within the droplet, introducing strong, transient hydrodynamic perturbations that compete with evaporation-driven capillary flows and contact line interactions, thereby disrupting classical coffee-ring flow. We demonstrate that the final deposit morphology can be systematically tuned by controlling the topography and wettability of the substrate. Hydrophobic micropatterned substrates with feature sizes commensurate with particle diameter promote asymmetric deposits driven by bubble-induced vortices, whereas hydrophilizing the same surface suppresses bubble nucleation and yields uniformly distributed particle deposits. On nanopatterned substrates, bubble pinning within substrate grooves generates highly irregular deposits with chain-like particle assemblies. These results demonstrate how the coupling of catalytic activity, bubble dynamics, and substrate topography efficiently steer particle transport at the nanometer and micrometer scales. Such control of particle deposition during active droplet evaporation provides a new route for reaction-driven directed assembly.