Influence of Bubble Lifetime on the Drying of Catalytically Active Sessile Droplets

When colloidal droplets evaporate, suspended particles are redistributed by a competition between evaporation-driven capillary advection, interfacial Marangoni stresses and particle mobility, leading to diverse deposition patterns relevant to coating and self-assembly. While these mechanisms are well understood for passive suspensions, their interplay in chemically active colloidal systems remains less explored. Here, we investigate the drying dynamics of droplets containing catalytic polystyrene-platinum (PS-Pt) Janus particles in the presence of hydrogen peroxide (H2O2) fuel. H2O2 undergoes catalytic decomposition at the Pt hemisphere, resulting in the formation of oxygen (O2). By systematically varying H2O2 concentration, surface wettability and open versus confined drying conditions, we identify distinct transport regimes governed by the relative magnitudes of capillary flow and gas bubble-induced Marangoni convection. While time-resolved contact-angle measurements reveal substrate-dependent evaporation modes, an increase in catalytic activity promotes O2 bubble generation that locally reverses or disrupts outward particle transport. Closed drying conditions further modify evaporation rates and prolong bubble residence times, leading to transitions from peripheral accumulation to spatially uniform or centrally concentrated deposits. Bubble-induced Marangoni flow, controlled here by tuning substrate wettability and environmental conditions, therefore emerges as the dominant mechanism governing the evaporation dynamics and dried morphologies of catalytically active Janus particle droplets.

💡 Research Summary

This paper investigates how the lifetime of oxygen bubbles generated by catalytic polystyrene‑platinum (PS‑Pt) Janus particles influences the drying dynamics and final deposit patterns of colloidal droplets containing hydrogen peroxide (H₂O₂) fuel. By systematically varying three experimental parameters—(1) H₂O₂ concentration (0.1–10 wt %), (2) substrate wettability (hydrophilic silicon vs. hydrophobic PDMS), and (3) drying environment (open to air versus sealed in a Petri dish)—the authors dissect the competition between evaporation‑driven capillary flow and bubble‑induced Marangoni convection.

In the absence of fuel, droplets behave as classic passive colloidal systems: capillary flow drives particles outward, producing a coffee‑ring deposit. Substrate wettability modulates the pinning strength and particle orientation: on hydrophilic surfaces the contact line pins briefly, particles reorient with their Pt caps contacting the substrate, and the droplet quickly transitions to a constant‑contact‑angle (CCA) mode; on hydrophobic surfaces the contact line remains pinned longer, the Pt side stays exposed, and a thicker outer ring forms.

When H₂O₂ is added, catalytic decomposition at the Pt hemisphere produces dissolved O₂ that nucleates into bubbles. At low fuel levels (≤1 wt %), bubbles are rare and the drying sequence resembles the passive case, with a brief CCR (constant‑contact‑radius) stage followed by CCA. Above 1 wt %, bubbles appear repeatedly. Their nucleation, growth, and rapid collapse generate localized surface‑tension gradients, launching Marangoni circulations that oppose the outward capillary flux. Consequently, the classic CCA stage disappears; instead the droplet spends an extended “mixed” regime where both contact angle and footprint shrink simultaneously, followed by a fast‑evaporation final stage.

Substrate wettability crucially determines where bubbles reside and how long they persist. Hydrophilic substrates, with strong pinning and shallow droplet curvature, cause bubbles to form near the three‑phase line and burst quickly, limiting their influence to brief perturbations. Hydrophobic PDMS, with a higher contact angle and a more curved interface, allows bubbles to stay near the droplet apex for longer periods. The sustained Marangoni flow then circulates the entire droplet, pulling particles toward the centre and yielding a centrally concentrated (“core”) deposit rather than a peripheral ring.

The drying environment further modulates bubble dynamics. In an open configuration, oxygen can dissolve into the ambient air, shortening bubble lifetime. In a sealed Petri dish, gas exchange is suppressed; bubbles remain longer, occupy a larger fraction of the droplet volume, and slow the overall evaporation rate. When a bubble finally collapses, the sudden pressure release reorganizes internal flows, erasing the ring‑type pattern and producing either a uniform film or a centrally focused deposit, depending on the substrate.

Particle orientation adds another feedback loop. PS‑Pt Janus particles have a hydrophilic PS side and a catalytic Pt side. On hydrophilic substrates the PS side preferentially adheres, anchoring particles at the contact line, while the Pt side remains exposed to the liquid‑air interface, maximizing O₂ generation. On hydrophobic substrates the opposite orientation occurs, enhancing bubble formation and Marangoni stresses. Thus, the anisotropic wetting of the particles couples directly to bubble generation and flow fields.

Overall, the study identifies three governing mechanisms: (i) bubble lifetime controls the balance between capillary and Marangoni flows; (ii) substrate wettability and confinement dictate bubble location, persistence, and particle‑substrate interactions; (iii) the intrinsic polarity of Janus particles mediates where catalytic activity occurs. By quantifying these effects through time‑resolved contact‑angle measurements, high‑speed imaging, and post‑drying SEM/AFM analysis, the authors provide design rules for tailoring deposit morphologies in applications such as microreactors, inkjet printing, and self‑assembled nanostructures. Controlling bubble lifetime—either by adjusting fuel concentration, sealing the drying chamber, or engineering surface chemistry—offers a practical route to switch between ring‑like, uniform, or centrally concentrated patterns on demand.