Designing Plastrons for Underwater Bubble Capture: From Model Microstructures to Stochastic Nanostructures.

Bubbles and foams are often removed via chemical defoamers and/or mechanical agitation. Designing surfaces that promote chemical-free and energy-passive bubble capture is desirable for numerous industrial processes, including mineral flotation, wastewater treatment, and electrolysis. When immersed, super-liquid-repellent surfaces form plastrons, which are textured solid topographies with interconnected gas domains. Plastrons exhibit the remarkable ability of capturing bubbles through coalescence. However, the two-step mechanics of plastron-induced bubble coalescence, namely, rupture (initiation and location) and subsequent absorption (propagation and drainage) are not well understood. Here, the influence of 1) topographical feature size and 2) gas fraction on bubble capture dynamics is investigated. Smaller feature sizes accelerate rupture while larger gas fractions markedly improve absorption. Rupture is initiated solely on solid domains and is more probable near the edges of solid features. Yet, rupture time becomes longer as solid fraction increases. This counterintuitive behavior represents unexpected complexities. Upon rupture, the bubble's moving liquid-solid contact line influences its absorption rate and equilibrium state. These findings show the importance of rationally minimizing surface feature sizes and contact line interactions for rapid bubble rupture and absorption. This work provides key design principles for plastron-induced bubble coalescence, inspiring future development of industrially-relevant surfaces for underwater bubble capture.

Imparting scalephobicity with rational microtexturing of soft materials.

Crystallization fouling, a process where scale forms on surfaces, is widespread in nature and technology, negatively affecting energy and water industries. Despite the effort, rationally designed surfaces that are intrinsically resistant to it remain elusive, due in part to a lack of understanding of how microfoulants deposit and adhere in dynamic aqueous environments. Here, we show that rational tuning of coating compliance and wettability works synergistically with microtexture to enhance microfoulant repellency, characterized by low adhesion and high removal efficiency of numerous individual microparticles and tenacious crystallites in a flowing water environment. We study the microfoulant interfacial dynamics in situ using a micro-scanning fluid dynamic gauge system, elucidate the removal mechanisms, and rationalize the behavior with a shear adhesive moment model. We then demonstrate a rationally developed coating that can remove 98% of deposits under shear flow conditions, 66% better than rigid substrates.