top of page
Interfacial Wetting Phenomena of Bioinspired Surfaces
William S. Y. Wong
Adapted with permission from Wiley.
The rational design of surfaces at the molecular level is essential toward realizing many engineering applications. However, molecular-scale defects affect processes such as triboelectrification, scaling, and condensation. These defects are often detectable via contact angle hysteresis (CAH) measurements. Liquid-like surfaces exhibit extremely low CAH (≤ 5°) and rely on the use of highly flexible molecular species such as long-chain alkyls or siloxanes. Their low glass transition temperatures lead to the so-termed self-smoothing behavior, reducing sensitivity to defects formed during fabrication. However, utilizing rigid molecular species such as perfluoroalkyl chains often results in higher hysteresis (10° to 60°) as defects are not self-smoothed after fabrication. Consequently, state-of-the-art perfluoroalkylated surfaces often show sub-optimal interfacial properties. Here, a customizable chemical vapor deposition process creates molecularly-thick, low-defect surfaces from trichloro(1H,1H,2H,2H-perfluorooctyl)silane. By implementing moisture-exposure controls, highly homogenous surfaces with root-mean-square roughness below 1 nm are fabricated. CAH is achieved down to ≈ 4° (average: 6°), surpassing the state-of-the-art by ≈ 5°. Reduction of CAH (26° to 6°) results in condensation suppression, decreasing surface droplet density by one order and surface droplet coverage by 40%. This work guides the synthesis of high-quality surfaces from tri-functional perfluoroalkylsilanes with liquid-like properties despite their molecular rigidity.
WilliamSYWong © 2024
Koochak, P., Kiseleva, M. S., Lepikko, S., Latikka, M., Ras R. H. A., Wong, W. S. Y.* Smoothening Perfluoroalkylated Surfaces: Liquid-Like Despite Molecular Rigidity?
Advanced Materials Interfaces 2024 (in press).
link
Adapted with permission from Wiley.
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, i.e. froth removal in mineral flotation, wastewater treatment, and electrolysis. When immersed, super-liquid-repellent surfaces form plastrons, i.e. 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, we show how (1) topographical feature size and (2) gas fraction influences the bubble capture dynamics. Smaller feature sizes accelerate rupture while larger gas fractions markedly improve absorption. Rupture is initiated solely on solid domains and more probable near the edges of solid features. Yet, rupture time becomes longer as solid fraction increases. This is counterintuitive to expectations and represents unexpected complexities. Upon rupture, the bubble’s moving liquid-solid contact line influences its absorption rate and equilibrium state. Our findings show the importance of rationally minimizing surface feature size 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.
WilliamSYWong © 2024
Wong, W. S. Y.*, Naga, A., Neef, Tobias, Karunakaran, B., Poulikakos, D., and Ras R. H. A. Designing Plastrons for Underwater Bubble Capture: From Model Microstructures to Stochastic Nanostructures.
Advanced Science 2024 (in press).
link
Reprinted (adapted) with permission from ACS. Copyright 2024 American Chemical Society.
Wetting is typically defined by the relative liquid to solid surface tension/energy, which are composed of polar and non-polar sub-contributions. Current studies often assume that they remain invariant, i.e. surfaces are wetting-inert. Complex wetting scenarios, such as adaptive or reactive wetting processes, may involve time-dependent variations in interfacial energies. To maximize differences in energetic states, we employ low-energy perfluoroalkyls integrated with high-energy silica-based polar moieties, grown on low-energy polydimethylsiloxane. To this end, we tune the hydrophilic-like wettability on these perfluoroalkyl-silica-polydimethylsiloxane surfaces. Drop contact behaviors range from invariantly hydrophobic at ca. 110° to rapidly spreading at ca. 0° within 5s. Unintuitively, these vapor-grown surfaces transits towards greater hydrophilicity with increasing perfluoroalkyl deposition. Notably, this occurs as the silica-perfluoroalkyl deposition also leaves behind embedded polar moieties. We highlight how surfaces having such chemical heterogeneity are inherently wetting-reactive. By creating an abrupt wetting transition composed of reactive- and inert- domains, we introduce spatial-dependency. Drops contacting the transition spread before retracting, occurring over the timescale of a few seconds. This phenomenon contradicts current understanding, exhibiting a uniquely (1) decreasing advancing contact angle, and (2) increasing receding contact angle. To explain the behavior, we model such time- and space- dependent reactive wetting using first order kinetics. In doing so, we explore how reactive and recovery mechanisms govern the characteristic timescales of spreading and retracting sessile drops.
WilliamSYWong © 2024
Wong, W. S. Y.*, Kiseleva M. S., and Naga, A. Polarity-Induced Reactive Wetting: Spreading and Retracting Sessile Water Drops.
Langmuir 2024, 40, 13562.
link
Adapted with permission from Wiley.
Super-liquid-repellent surfaces, which find key applications in anti-fouling and self-cleaning, feature high liquid contact angles and low sliding angles. While achieving repellency for water is easily accomplished with hydrocarbon functionalities, achieving repellency for many low-surface-tension liquids (down to 30 mN/m) still requires perfluoroalkyls - a persistent environmental pollutant and bioaccumulation hazard. In this study, we investigate the scalable room-temperature synthesis of stochastic nanoparticle surfaces with fluoro-free moieties. We benchmark silicone (dimethyl and monomethyl) and hydrocarbon surface chemistries against perfluoroalkyls, evaluating their performance using model low-surface-tension liquids (ethanol-water mixtures). Our findings reveal that both hydrocarbon- and dimethyl-silicone-based functionalization can achieve super-liquid-repellency down to 40-41 mN/m and 32-33 mN/m, respectively (compared to 27-32 mN/m for perfluoroalkyls). Notably, the dimethyl silicone variant demonstrates superior fluoro-free liquid repellency, likely attributed to its denser dimethyl molecular configuration. More importantly, our results indicate that perfluoroalkyls are not necessary for many real-world scenarios requiring super-liquid-repellency. We also show that effective super-repellency of different surface chemistries can be adequately predicted using empirically verified phase diagrams. These findings encourage a liquid-centric design approach, where we tailor surfaces to target specific liquid properties. Herein, key guidelines are provided for achieving functional yet sustainably designed super-liquid-repellency.
WilliamSYWong © 2023
Wong, W. S. Y, M. S. Kiseleva, S. Zhou, M. Junaid, L. Pitkänen, and R. H. A. Ras* Design of Fluoro-Free Surfaces Super-Repellent to Low-Surface-Tension Liquids
Adv. Mater. 2023, 2300306.
link
Reprinted (adapted) with permission from ACS. Copyright 2022 American Chemical Society.
When a water drop slides over a hydrophobic surface, it usually acquires a positive charge and deposits the negative countercharge on the surface. Although the electrification of solid surfaces induced after contact with a liquid is intensively studied, the actual mechanisms of charge separation, so-termed slide electrification, are still unclear. Here, slide electrification is studied by measuring the charge of a series of water drops sliding down inclined glass plates. The glass was coated with hydrophobic (hydrocarbon/fluorocarbon) and amine-terminated silanes. On hydrophobic surfaces, drops charge positively while the surfaces charge negatively. Hydrophobic surfaces coated with a mono-amine (3-aminopropyltriethyoxysilane) lead to negatively charged drops and positively charged surfaces. When coated with a multiamine (N-(3-trimethoxysilylpropyl)diethylenetriamine), a gradual transition from positively to negatively charged drops is observed. We attribute this tunable drop charging to surface-directed ion transfer. Some of the protons accepted by the amine-functionalized surfaces (−NH2 with H+ acceptor) remain on the surface even after drop departure. These findings demonstrate the facile tunability of surface-controlled slide electrification.
WilliamSYWong © 2022
Wong, W. S. Y, Bista, P., Li, X., Veith, L., Sharifi-Aghili, A., Weber, S. A. L., and Butt, H-J. Tuning the Charge of Sliding Water Drops.
Langmuir 2022, 38, 6224.
link
Adapted with permission from Nature Publishing Group under Open Access Rights.
The presence of supercooled water in polar regions causes anchor ice to grow on submerged objects, generating costly problems for engineered materials and life-endangering risks for benthic communities. The factors driving underwater ice accretion are poorly understood, and passive prevention mechanisms remain unknown. Here we report that the Antarctic scallop Adamussium colbecki appears to remain ice-free in shallow Antarctic marine environments where underwater ice growth is prevalent. In contrast, scallops colonized by bush sponges in the same microhabitat grow ice and are removed from the population. Characterization of the Antarctic scallop shells revealed a hierarchical micro-ridge structure with sub-micron nano-ridges which promotes directed icing. This concentrates the formation of ice on the growth rings while leaving the regions in between free of ice, and appears to reduce ice-to-shell adhesion when compared to temperate species that do not possess highly ordered surface structures. The ability to control the formation of ice may enable passive underwater anti-icing protection, with the removal of ice possibly facilitated by ocean currents or scallop movements. We term this behavior cryofouling avoidance. We posit that the evolution of natural anti-icing structures is a key trait for the survival of Antarctic scallops in anchor ice zones.
WilliamSYWong © 2022
Wong, W. S. Y, Hauer, L., Cziko, P., and Meister, K. Cryofouling Avoidance in the Antarctic Scallop Adamussium colbecki.
Communications Biology 2022, 5, 83.
link
Reprinted (adapted) with permission from Elsevier. Copyright 2021.
The rapid development of user interface products (touch-operated smartphones and computers) has changed how we interact with technology. Fingerprints (oil contamination) are often left on touch interfaces, significantly impairing users' visual experience. Currently, oil-repellent and oleophilic coatings are used to minimize fingerprint oil remnants on glass surfaces. However, these methods are still limited by real-world exposure conditions. Inspired by the heterogeneous wetting of Stenocara beetles, a heterogeneous-wetting glass surface is designed. The glass surface is made up of nanopillars consisting of oleophilic top surfaces with oleophobic sidewalls and bases. We term this construct contrasted wetting. This configuration combines the anti-fingerprint properties of both oleophobicity and oleophilicity. Surfaces show remarkable wettability contrast that enables excellent optical and anti-fingerprint properties. The synergistic integration of oleophobic and oleophilic characteristics contributes to the non-uniform distribution of wetting/adhesion force. Fingerprint oil detaching from the surface is heterogeneously ruptured. This is further aided by the physical air gaps between nanostructures. Due to the preferential wettability of oleophilic pillar tops, remnant oil splits up into fragmented thin films (due to oleophilic spreading). Fragmentation of thin, flat films prevents light scattering with minimal reflection/refraction, thus allowing fingerprints to appear transparent. The heterogeneous glass also maintained stable anti-fingerprint properties after ten standard cycles of finger touch and wiping, and even up to 1000 cycles of continuous wiping, illustrating good mechanical robustness. Our findings provide a new route towards the future design of anti-fingerprint glass.
WilliamSYWong © 2021
Wang, W., Gu, W., Liu, P., Liu, J., Wang X., Liu J., Yu X.*, Wong, W. S. Y.* and Zhang, Y.* Heterogeneously-wetting glass with enhanced anti-fingerprint properties.
Chemical Engineering Journal 2021 (j.cej.2021.132902).
link
Adapted with permission from Wiley.
The use of superhydrophobic/superamphiphobic surfaces demands the presence of a stable plastron, i.e., a film of air between micro- and nanostructures. Without actively replenishing the plastron with gases, it eventually disappears during immersion. The air diffuses into the immersion liquid, i.e., water. Current methods for sustaining the plastron under immersion remain limited to techniques such as electrocatalysis, electrolysis, boiling, and air-refilling. These methods are difficult to implement at scale, are either energy-consuming, or require continuous monitoring of the plastron (and subsequent intervention). Here, the concept of passive on-demand recovery of the plastron via the use of a chemical reaction (effervescence) is presented. A superhydrophobic nanostructured surface is layered onto a wetting-reactive, gas-forming (effervescent) sublayer. During extended exposure to moisture, the effervescent layer must be protected by a moisture-absorbent, water-soluble polymer. Under prolonged immersion, partial collapse of the Cassie-state induces contact of water with the effervescent layer. This induces the local formation of gases from effervescence, which restores the Cassie-state. These facile and scalable design principles offer a new route toward intervention-free and immersion-durable superhydrophobic/superamphiphobic surfaces.
WilliamSYWong © 2021
Wong, W. S. Y* and Vollmer, D.* Effervescence-Inspired Self-Healing Plastrons for Long-Term Immersion Stability.
Advanced Functional Materials 2021, 32, 2107831.
link
Adapted with permission from Nature Publishing Group under Open Access Rights.
Wet and dry foams are prevalent in many industries, ranging from the food processing and commercial cosmetic sectors to industries such as chemical and oil-refining. Uncontrolled foaming results in product losses, equipment downtime or damage and cleanup costs. To speed up defoaming or enable anti-foaming, liquid oil or hydrophobic particles are usually added. However, such additives may need to be later separated and removed for environmental reasons and product quality. Here, we show that passive defoaming or active anti-foaming is possible simply by the interaction of foam with chemically or morphologically modified surfaces, of which the superamphiphobic variant exhibits superior performance. They significantly improve retraction of highly stable wet foams and prevention of growing dry foams, as quantified for beer and aqueous soap solution as model systems. Microscopic imaging reveals that amphiphobic nano-protrusions directly destabilize contacting foam bubbles, which can favorably vent through air gaps warranted by a Cassie wetting state. This mode of interfacial destabilization offers untapped potential for developing efficient, low-power and sustainable foam and froth management.
WilliamSYWong © 2021
Wong, W. S. Y.*, Naga, A., Hauer, L., Baumli, P., Bauer, H., Hegner, K. I., D‘Acunzi, M., Kaltbeitzel, A., Butt, H-J. and Vollmer, D.* Super Liquid Repellent Surfaces for Anti-Foaming.
Nature Communications 2021, 12, 5358
link
Adapted with permission from Wiley.
Controlling bubble motion or passively bursting bubbles using solid interfaces is advantageous in numerous industrial applications including flotation, catalysis, electrochemical processes, and microfluidics. Current research has explored the formation, dissolution, pinning, and rupturing of bubbles on different surfaces. However, the ability to tune and control the rate of bubble bursting is not yet achieved. Scaling down surface-induced bubble bursting to just a few milliseconds is vital. In this work, the hierarchical structure of superamphiphobic surfaces is tuned in order to rapidly rupture contacting bubbles. Surfaces prepared using flame spray pyrolysis show ultrafast bubble bursting (down to 2 ms) and superior durability. The coatings demonstrate excellent mechanical and chemical stability even in the presence of surface-active species. Air from the ruptured bubble is absorbed into the aerophilic Cassie-state. Long-term applicability is achieved by preventing air accumulation in the plastron simply by connecting the plastron to the environment. The bubble rupture time is reduced by approximately a factor of 3 compared to previously reported values.
WilliamSYWong © 2021
Hegner, K. I., Wong, W. S. Y.* and Vollmer, D.* Ultrafast Bubble Bursting by Superamphiphobic Coatings.
Advanced Materials 2021, 33, 2101855.
link
Reprinted (adapted) with permission from ACS. Copyright 2021 American Chemical Society.
Frost is ubiquitously observed in nature whenever warmer and more humid air encounters colder than melting point surfaces (e.g. morning dew frosting). However, frost formation is problematic as it damages infrastructure, roads, crops, and the efficient operation of industrial equipment (i.e., heat exchangers, cooling fins). While lubricant-infused surfaces offer promising antifrosting properties, underlying mechanisms of frost formation and its consequential effect on frost-to-surface dynamics remain elusive. Here, we monitor the dynamics of condensation frosting on micro- and hierarchically structured surfaces (the latter combines micro- with nano- features) infused with lubricant, temporally and spatially resolved using laser scanning confocal microscopy. The growth dynamics of water droplets differs for micro- and hierarchically structured surfaces, by hindered drop coalescence on the hierarchical ones. However, the growth and propagation of frost dendrites follow the same scaling on both surface types. Frost propagation is accompanied by a reorganization of the lubricant thin film. We numerically quantify the experimentally observed flow profile using an asymptotic long-wave model. Our results reveal that lubricant reorganization is governed by two distinct driving mechanisms, namely: (1) frost propagation speed and (2) frost dendrite morphology. These in-depth insights into the coupling between lubricant flow and frost formation/propagation enable an improved control over frosting by adjusting the design and features of the surface.
WilliamSYWong © 2021
Hauer, L.^, Wong, W. S. Y.^, Donadei, V., Hegner, K. I., Kondic, L. and Vollmer, D. How Frost Forms and Grows on Lubricated Micro- and Nanostructured Surfaces.
ACS Nano 2021, 15, 4658‐4668
link
Reprinted (adapted) with permission from ACS. Copyright 2020 American Chemical Society.
Slippery lubricant-infused surfaces (SLIPS) have shown great promise for anti-frosting and anti-icing. However, small length scales associated with frost dendrites exert immense capillary suction pressure on the lubricant. This pressure depletes the lubricant film and is detrimental to the functionality of SLIPS. To prevent lubricant depletion, we demonstrate that interstitial spacing in SLIPS needs to be kept below those found in frost dendrites. Densely packed nanoparticles create the optimally sized nanointerstitial features in SLIPS (Nano-SLIPS). The capillary pressure stabilizing the lubricant in Nano-SLIPS balances or exceeds the capillary suction pressure by frost dendrites. We term this concept capillary balancing. Three-dimensional spatial analysis via confocal microscopy reveals that lubricants in optimally structured Nano-SLIPS are not affected throughout condensation (0 °C), extreme frosting (−20 °C to −100 °C), and traverse ice-shearing (−10 °C) tests. These surfaces preserve low ice adhesion (10–30 kPa) over 50 icing cycles, demonstrating a design principle for next-generation anti-icing surfaces.
WilliamSYWong © 2020
Wong, W. S. Y.*, Hegner, K., Donadei, V., Hauer, L., Naga, A. and Vollmer, D.* Capillary Balancing: Designing Frost-Resistant Lubricant-Infused Surfaces.
Nano Letters 2020, 20, 8508-8515
link
Reprinted (adapted) with permission from Elsevier. Copyright 2020.
Superamphiphobic coatings have attracted tremendous interest from both academia and industry owing to their potential for self-cleaning and anti-fouling. However, many state-of-the-art superamphiphobic coatings are unable to preserve their super-repellent properties after prolonged liquid immersion. This drawback limits practical applications. Herein, we highlight the immersion-stable performance of a nanostructurally-densified superamphiphobic coating possessing super liquid repellency to water and various low-surface-tension liquids. They exhibit excellent functional stability even after prolonged immersion in mixed synthetic and vegetable oils. Moreover, they also persist exemplarily under heated-immersion conditions. These findings highlight the potential of using nanostructurally-densified superampiphobic coatings as anti-fouling surfaces in a kitchen environment, with potential applications in petroleum extraction and oil transportation.
WilliamSYWong © 2020
Jiao, X., Li, M., Yu, X., Wong, W. S. Y.* and Zhang, Y.* Oil-Immersion Stable Superamphiphobic Coatings for Long-term Super Liquid-Repellency.
Chemical Engineering Journal 2020, 420, 127606.
link
Reprinted (adapted) with permission from ACS. Copyright 2020 American Chemical Society.
The contact angles of a drop depends on its contact line velocity. Typically, velocity dependent contact angles are modelled using molecular kinetic or hydrodynamic theory. On polydimethylsiloxane (PDMS) and other soft polymers, these theories do not describe dynamic contact angles adequately. Here, we test if spontaneous changes in surface chemistry during wetting, so-termed wetting adaptation, can explain the velocity dependence of contact angles. Therefore, we measured advancing and receding contact angles of sessile water drops on cross-linked PDMS as a function of contact line velocity. Our results show that wetting adaptation theory provides a quantitative description of dynamic advancing contact angles. Experiments indicate that PDMS adapts to the presence of water by an enrichment of free oligomers at the interface. This work sheds new light on soft elastomeric materials, highlighting how wetting contact can induce lubrication which alters effective interfacial energies and dynamic contact angles.
WilliamSYWong © 2020
Wong, W. S. Y., Hauer, L., Naga, A., Kaltbeitzel, A., Baumli, P., D‘Acunzi, M., Berger, R., Vollmer, D. and Butt, H-J. Adaptive Wetting of Polydimethylsiloxane.
Langmuir 2020, 36, 7236-7245.
link
Reprinted (adapted) with permission from ACS. Copyright 2020 American Chemical Society.
Superamphiphobic surfaces are commonly associated with superior anticontamination and antifouling properties. Visually, this is justified by their ability to easily shed off drops and contaminants. However, on micropillar arrays, tiny droplets are known to remain on pillars’ top faces while the drop advances. This raises the question of whether remnants remain even on nanostructured superamphiphobic surfaces. Are superamphiphobic surfaces really self-cleaning? Here we investigate the presence of microdroplet contaminants on three nanostructured superamphiphobic surfaces. After brief contact with liquids having different volatilities and surface tension (water, ethylene glycol, hexadecane, and an ionic liquid), confocal microscopy reveals a “blanket-like” layer of microdroplets remaining on the surface. It appears that the phenomenon is universal. Notably, when placing subsequent drops onto the contaminated surface, they are still able to roll off. However, adhesion forces can gradually increase by up to 3 times after repeated liquid drop contact. Therefore, we conclude that superamphiphobic surfaces do not warrant self-cleaning and anticontamination capabilities at sub-micrometric length scales.
WilliamSYWong © 2020
Wong, W. S. Y.*, Corrales, T. P., Naga A., Baumli P., Kaltbeitzel A., Kappl M., Papadopoulos P., Vollmer, D. and Butt, H-J.* Microdroplet Contaminants: When and Why Superamphiphobic Surfaces are Not Self-Cleaning.
ACS Nano 2020,14, 3836-3846.
link
Reprinted (adapted) with permission from ACS. Copyright 2019 American Chemical Society.
Superhydrophobic, superoleo(amphi)phobic and superomniphobic materials are universally important in the fields of science and engineering. Despite rapid advancements, gaps of understanding still exist between each distinctive wetting state. The transition of superhydrophobicity-to-super(oleo-, amphi- and omni-)phobicity typically requires the use of re-entrant features. Today, re-entrant geometry induced super(amphi-/omni-)phobicity is well supported by both experiment and theory. However, owing to geometrical complexities, the concept of re-entrant geometry forms a dogma that limits the industrial progress of these unique states of wettability. Moreover, a key fundamental question remains unanswered: Are extreme surface chemistry enhancements able to influence super liquid-repellency? Here, this was rigorously tested via an alternative pathway that does not require explicit designer re-entrant features. Highly controllable and tunable vertical network polymerization-functionalization was used to achieve fluoroalkyl densification on nanoparticles. For the first time, relative fluoro-functionalization densities are quantitatively tuned and correlated to super liquid-repellency performance. Step-wise tunable superamphiphobic nanoparticle films with a Cassie-Baxter state (contact angle > 150º, sliding angle < 10º) against various liquids is demonstrated. This was tested down to very low surface tension liquids, to a minimum of ca. 23.8 mN/m. Such findings could eventually lead to the future development of super(amphi)omniphobic materials that transcends the sole use of re-entrant geometry.
WilliamSYWong © 2019
Wong, W. S. Y.* Surface Chemistry Enhancements for the Tunable Super Liquid Repellency of Low Surface Tension Liquids.
Nano Letters 2019, 19, 1892-1901.
link
Reprinted (adapted) with permission from ACS. Copyright 2018 American Chemical Society.
Despite the rapid advent of superomniphobic materials, methods for accurately analyzing ultra low-energy solid-liquid interactions remain undependable. For instance, universally employed models such as the pendant droplet often fail to provide representative information of the wetting properties of superomniphobic surfaces. The delicate balance between the forces acting at the droplet–surface and droplet–needle interfaces can easily result in heavily distorted droplet profiles. Here, we introduce a Cassie-levitating droplet model which overcomes the limitations of the pendant droplet model, allowing a distortion-free assessment of the interactions between super(amphi)omniphobic materials and low surface tension liquids. Comparative analysis in wetting of low surface tension fluids such as hexadecane (∼27.47 mN/m) on superamphiphobic surfaces via the Cassie-levitating and pendant droplet models reveals up to 70° (800%) deviations in the estimated contact angle hysteresis. A theoretical framework is developed to assess experimentally observed profile distortions against ideal gravity-induced sagging of droplet shapes during dynamic droplet expansion and contraction cycles. Notably, pendant droplets were devastated by up to 50% distortion while the Cassie-levitating ones suffered from less than just 10% deviations. We believe that the Cassie-levitating droplet model bears ample potential for the characterization of the rapidly emerging family of superomniphobic materials, setting the basis for their future engineering in numerous emerging applications.
WilliamSYWong © 2018
Wong, W. S. Y.* and Tricoli, A. Cassie-Levitated Droplets for Distortion-Free Low-Energy Solid–Liquid Interactions
ACS Applied Materials and Interfaces 2018, 10, 13999-14007.
link
Adapted with permission from Wiley.
In nature, cellular membranes perform critical functions such as endo- and exo-cytosis through smart fluid gating processes mediated by non-specific amphiphilic interactions. Despite considerable progress, artificial fluid gating membranes still rely on laborious stimuli-responsive mechanisms and triggering systems. Here, we present a room temperature gas-phase approach for dynamically switching a porous material from a superhydrophobic to a superhydrophilic wetting state and back. This was realized by the reversible attachment of bipolar amphiphiles, which promote surface wetting. Application of this reversible amphiphilic functionalization to an impermeable nanofibrous membrane induces a temporary state of superhydrophilicity resulting in its pressure-less permeation. This mechanism allows for rapid smart fluid gating processes that can be triggered at room temperature by variations in the environment of the membrane. Owing to the universal adsorption of volatile amphiphiles on surfaces, this approach is applicable to a broad range of materials and geometries enabling facile fabrication of valve-less flow systems, fluid-erasable microfluidic arrays and sophisticated microfluidic designs.
WilliamSYWong © 2017
Wong, W. S. Y., Gengenbach, T., Nguyen H. T., Gao X., Craig V.S.J. and Tricoli, A. Dynamically Gas-Phase Switchable Super(de)Wetting States by Reversible Amphiphilic Functionalization: A Powerful Approach for Smart Fluid Gating Membranes
Advanced Functional Materials 2017, 28, 1704423.
link
Adapted with permission from Wiley.
Manipulation of nanoliter droplets is a key step for many emerging technologies such as lab-on-a-chip microfluidics, 3D printing or advanced inkjet systems. Despite much progress, the contamination-free generation, deposition and manipulation of nanoliter droplets by compact low-cost devices remains elusive. In the present study, inspired by butterflies' capability for manipulating minute amounts of fluids, we engineered a superamphiphobic bionic proboscis (SAP) system that surpasses synthetic and natural designs. The scalable fabrication of SAPs with tunable inner diameters down to 50 µm is performed using rapid gas-phase nanotexturing of the outer and inner surfaces of readily available hypodermic needles. Optimized SAPs achieve contamination-free manipulation of water and oil droplets down to a liquid surface tension of 26.56 mN/m at a minimum configurable volume of 10 nL. The unique potential of this layout is finally demonstrated by the rapid and careful control of in-air synthesized core-shell droplets with pre-configurable compositions. These findings provide a new low-cost tool for the high-precision manipulation of nanoliter droplets, offering a powerful alternative to established thermal- and electrodynamic-based devices.
WilliamSYWong © 2017
Wong, W. S. Y., Liu, G. and Tricoli, A. Superamphiphobic Bionic Proboscis for Contamination-Free Manipulation of Nano and Core–Shell Droplets.
Small 2017, 13, 1603688.
link
Reprinted (adapted) with permission from ACS. Copyright 2017 American Chemical Society.
The occurrence of re-entrant micro-textures in nature that are capable of repelling both water and oil (superamphiphobicity) is exceedingly rare. A type of hexapod, the springtail (Folsomia candida) possesses a series of mushroom-like structures on its exoskeleton, and has been shown to be capable of oil-decontamination. However, the artificial creation of similar synthetic structures requires much greater effort and has so far only been achieved using non-scalable templating or lithographical techniques. These current top-down methods are also limited by directional line-of-sight fabrication mechanisms that cannot be applied to highly uneven, curved, and enclosed surfaces. Alternatively, current bottom-up self-assembly techniques that exist are far from perfect, and typically suffer from poor optical transparency and morphological unpredictability. Here, we present an approach that enables the rapid synthesis of ultra-transparent superhydrophobic and -oleophobic coatings on many variable surface types. This was achieved through the spontaneous formation of a multi re-entrant morphology during the tunable self-assembly of nanoparticle aerosols. A mathematical model was then developed to explain and describe the self-assembly dynamics involved, providing important insights for the rational engineering of such functional materials. These findings represent a significant advance in imparting superamphiphobicity to a so-far inapplicable family of materials and geometries for multifunctional applications.
WilliamSYWong © 2017
Wong, W. S. Y., Liu, G., Nasiri, N., Hao, C., Wang, Z. and Tricoli, A. Omnidirectional Self-Assembly of Transparent Superoleophobic Nanotextures.
ACS Nano 2016, 11, 587-596.
link
Reprinted with permission from AAAS.
One of the innate fundamentals of living systems lies in their ability to respond towards distinct stimuli by various self-organization behaviors. Despite extensive progress, the engineering of spontaneous motion in man-made inorganic materials still lacks the directionality and scale observed in nature. Here, we report the directional self-organization of soft materials into three-dimensional geometries by the rapid propagation of a folding stimulus along a predetermined path. We engineered a unique Janus bilayer architecture with superior chemical and mechanical properties that enables the efficient transformation of surface energy into directional kinetic and elastic energies. This Janus bi-layer can respond to pinpoint water stimuli by a rapid, several centimeters long curl-up self-assembly that is reminiscent of the Mimosa pudica’s folding leaves. Application of the Mimosa effect to microfluidics demonstrate up to 100 times higher transport rate than traditional wicking-based devices, reaching velocities of 8 cm/s and flow rates of 4.7 µL/s. Unlimited by shape nor configuration, the Mimosa effect demonstrates efficient fabrication of curved, bent and split flexible channels with lengths in excess of 10 cm. This revolutionary phenomenon possesses immense potential in modular microfluidics, biosensors, water purification and collection.
WilliamSYWong © 2016
Wong, W. S. Y., Li, M., Nisbet, D. R., Craig, V. S. J., Wang, Z. and Tricoli, A. Mimosa Origami: a Nanostructure-enabled Directional Self-Organization Regime of Materials.
Science Advances 2016, 2, e1600417.
link
Reprinted (adapted) with permission from ACS. Copyright 2016 American Chemical Society.
Since the advent of biomimetics, natural superhydrophobic plants such as the native Australian mottlecah (Eucalyptus macrocapra) have received much industrial and research interest. Their durability stems from the strongly cohesive properties of multi-scale organic tissue. Today, although a myriad of synthetic replicas has been achieved in laboratories, severe practical drawbacks exist. Primarily, real-world impact of such materials continues to be diminished by the poor chemical-structural stability of nano-microstructures required for retaining highly dewetting superhydrophobic states. Here, we present a unique bi-layer composite design that biomimics the soft but tough nature of organic tissues in plants, enabling enhanced abrasion resistance. Scalable, transparent, and highly robust superhydrophobic coatings, were developed through a base-coat of sprayable interpenetrated PU-PMMA polymer network (IPN) integrated with a top-coat of superhydrophobic nanoparticles. The IPNs' PU component synergized with its partner PMMA component, providing superior elastic yielding properties while being simultaneously interlocked by the robust mechanical integrity of PMMA. The novel material demonstrated immense potential for nanoparticle-retention and thus functionality preservation. The durability exhibited was unprecedented on multiple scales, demonstrating enhanced cyclic abrasion, touch resilience, high intensity UVC, acid and oil resistance.
WilliamSYWong © 2016
Wong, W. S. Y., Stachurski Z. H., Nisbet D. R. and Tricoli, A. Ultra-Durable and Transparent Self-Cleaning Surfaces by Large-Scale Self-Assembly of Hierarchical Interpenetrated Polymer Networks.
ACS Applied Materials and Interfaces 2016, 8, 13615-13623
link (manuscript) link (patent)
Adapted with permission from The Royal Society of Chemistry.
The wax-based superhydrophobicity belonging to many superhydrophobic plants rely on spontaneously assembled cuticular patterns and epicuticular wax. Here, we design a synthetic version of such bioinspired superhydrophobic coatings based on the facile concept of van der Waals - attached "waxy layers" on self-assembled nanostructures. The one-step synthesis of ultraporous superhydrophobic nano-structures via a previously unknown route of on-the-fly functionalization is presented. Short exposure of surfaces to hot manganese, titania and zinc metalorganic aerosols resulted in ultraporous nanoparticle networks demonstrating fluorine-free Cassie-Baxter superhydrophobicity. The stability of these ultraporous morphologies was demonstrated by water immersion, revealing the Moses effect, capable of parting water up to 5 mm high. The scalable synthesis method presented here offers a flexible and rapid approach for the production of catalytic materials with moisture-resistant properties, including gas sensors, green catalysts and solar cells.
WilliamSYWong © 2016
Liu, G., Wong, W. S. Y., Nasiri, N. and Tricoli, A. Ultraporous Superhydrophobic Gas-Permeable Nano-Layers by Scalable Solvent-Free One-Step Self-Assembly.
Nanoscale 2016, 8, 6085-6093
link
Adapted with permission from Wiley.
Here, we biomimicked beyond nature's domains, where both the rose petal effect and lotus leaf effect are unified under a single material. We exploited a wrinkled wavy fiber texture that permitted reliable dynamic reversible tuning of wetting states from the superhydrophobic adhesive rose petal effect to the ultra-slippery lotus leaf effect. Optimal structures are capable of performing mechanical hand-like manipulation of water micro-droplets with pre-programmed actuated lift-off and controlled release. These surface textures are rapidly fabricated by a one-step self-assembly of wave-like stretchable nanofibers, offering a powerful and scalable approach for the synthesis of switchable wetting-dewetting surfaces with applications that include micro-fluidics, flexible electronics and water purification.
WilliamSYWong © 2015
Wong, W. S. Y., Gutruf, P., Sriram, S., Bhaskaran, M., Wang Z. and Tricoli, A. Strain Engineering of Wave-like Nanofibers for Dynamically Switchable Adhesive/Repulsive Surfaces.
Advanced Functional Materials 2016, 26, 399-407
link
Adapted with permission from Wiley.
Unlike the well-known superhydrophobic lotus leaf effect, the rose-petal effect remains scarcely investigated, and much less perfected. The perfect rose petal effect should ideally possess 1) Superhydrophobicity, 2) Lossless droplet capture and release and 3) High adhesive strength. Till date, no ideal synthetic rose petals have demonstrated all 3 of the above criteria. We demonstrate here, the ideal rose petal effect via highly adhesive super-hydrophobic bead-fiber nano-mesh interfaces. These coatings were synthesized through careful morphological tuning, enabling a hybridized blend of superhydrophobic lotus-like performing beads and superhydrophobic rose-like performing fibers. Surfaces demonstrated capability for macro- and micro-scopic lossless droplet transfer, with great ease of implementation within micro-reactor and micro-fluidic systems.
WilliamSYWong © 2015
Wong, W. S. Y., Nasiri, N., Liu, G., Rumsey-Hill, N., Craig, V. S. J., Nisbet, D. R. and Tricoli, A. Flexible Transparent Hierarchical Nanomesh for Rose Petal-Like Droplet Manipulation and Lossless Transfer.
Advanced Materials Interfaces 2015, 2, 1500071
link (manuscript) link (patent)
Adapted with permission from The Royal Society of Chemistry.
Superhydrophilicity is ubiquitious in nature, such as the wetting-driven nutrition of superhydrophilic Spanish moss (Tillandsia usneoides). Here we synthesized inorganic super-hydrophilic surfaces, induced through low-temperature calcination of polyvinyl pyrrolidone - titanium isopropoxide composite nanofibers, yielding an unprecedented amorphous-phased UV-independent, super-hydrophilic material with ultra-high mesoporous surface area. This facilely developed coating demonstrated immense potential for surface patterning, anti-fog coatings and fog harvesting water purification technologies.
WilliamSYWong © 2014
Wong, W. S. Y., Nasiri, N., Rodriguez, A. L., Nisbet, D. R. and Tricoli, A. Hierarchical amorphous nanofibers for transparent inherently super-hydrophilic coatings.
Journal of Materials Chemistry A 2014, 2, 15575-15581.
link
Fern-leaf like microstructural morphologies developed on composite polymer interfaces via self-assembly. These unique penetrated surfaces gave rise to toughened surface and sub-surface constructs with enhanced anti-scratch properties.
WilliamSYWong © 2013
US Patent 10,851,260
link
bottom of page