Synergistic Benefits of Micro/Nanostructured Oil-Impregnated Surfaces in Reducing Fouling while Enhancing Heat Transfer

Wetting Ridge Growth Dynamics on Textured Lubricant-Infused Surfaces

Understanding droplet-surface interactions has broad implications in microfluidics and lab-on-a-chip devices. In contrast to droplets on conventional textured air-filled superhydrophobic surfaces, water droplets on state-of-the-art lubricant-infused surfaces are accompanied by an axisymmetric annular wetting ridge, the source and nature of which are not clearly established to date. Generally, the imbalance of interfacial forces at the contact line is believed to play a pivotal role in accumulating the lubricant oil near the droplet base to form the axisymmetric wetting ridge. In this study, we experimentally characterize and model the wetting ridge that plays a crucial role in droplet mobility. We developed a geometry-based analytical model of the steady-state wetting ridge shape that is validated by using experiments and numerical simulations. Our wetting ridge model shows that at steady state (1) the radius of the wetting ridge is ≈30% higher than the droplet radius, (2) the wetting ridge rises halfway to the droplet radius, (3) the volume of the wetting ridge is half (≈50%) of the droplet volume, and (4) the wetting ridge shape does not depend on the oil viscosity used for impregnation. The insights gained from this work improve our state-of-the-art mechanistic understanding of the wetting ridge dynamics.

Mitigating Algal Competition with Fouling-Prevention Coatings for Coral Restoration and Reef Engineering

Coral reefs are undergoing unprecedented degradation due to rising ocean temperatures, acidification, overfishing, and coastal pollution. Despite conservation efforts, including marine protected areas and sustainable fishing practices, the magnitude of these challenges calls for innovative approaches to repair and restore coral reefs. In this study, we explore the application of bioinspired materials to address the challenge of algal competition, a key bottleneck for effective restoration approaches. We develop and optimize slippery liquid-infused porous surfaces (SLIPS), as a fouling-prevention coating tailored for coral reef restoration and engineering. Through aquarium experiments and in situ trials on O'ahu, Hawai'i, we assess the effectiveness of these coatings in mitigating algal competition and facilitating coral growth. Our results demonstrate that PDMS-based SLIPS coatings significantly reduce algal coverage compared to commercial aragonite-based surfaces, with up to 70% reduction observed over a 12-week deployment period in situ. We also develop coral-guards, which are slippery substrates customized for coral fragment outplanting. Coral-guards facilitate tissue growth of Stylophora pistillata fragments, without competitive turf algal growth. These approaches hold promise for advancing restoration efforts, including the engineering of hybrid reefs and targeted coral gardening approaches.

Visualization and Experimental Characterization of Wrapping Layer Using Planar Laser-Induced Fluorescence

Droplets on nanotextured oil-impregnated surfaces have high mobility due to record-low contact angle hysteresis (∼1-3°), attributed to the absence of solid-liquid contact. Past studies have utilized the ultralow droplet adhesion on these surfaces to improve condensation, reduce hydrodynamic drag, and inhibit biofouling. Despite their promising utility, oil-impregnated surfaces are not fully embraced by industry because of the concern for lubricant depletion, the source of which has not been adequately studied. Here, we use planar laser-induced fluorescence (PLIF) to not only visualize the oil layer encapsulating the droplet (aka wrapping layer) but also measure its thickness since the wrapping layer contributes to lubricant depletion. Our PLIF visualization and experiments show that (a) due to the imbalance of interfacial forces at the three-phase contact line, silicone oil forms a wrapping layer on the outer surface of water droplets, (b) the thickness of the wrapping layer is nonuniform both in space and time, and (c) the time-average thickness of the wrapping layer is ∼50 ± 10 nm, a result that compares favorably with our scaling analysis (∼50 nm), which balances the curvature-induced capillary force with the intermolecular van der Waals forces. Our experiments show that, unlike silicone oil, mineral oil does not form a wrapping layer, an observation that can be exploited to mitigate oil depletion of nanotextured oil-impregnated surfaces. Besides advancing our mechanistic understanding of the wrapping oil layer dynamics, the insights gained from this work can be used to quantify the lubricant depletion rate by pendant droplets in dropwise condensation and water harvesting.