Effects of Gas Layer Thickness on Capillary Interactions at Superhydrophobic Surfaces.

Strongly attractive forces act between superhydrophobic surfaces across water due to the formation of a bridging gas capillary. Upon separation, the attraction can range up to tens of micrometers as the gas capillary grows, while gas molecules accumulate in the capillary. We argue that most of these molecules come from the pre-existing gaseous layer found at and within the superhydrophobic coating. In this study, we investigate how the capillary size and the resulting capillary forces are affected by the thickness of the gaseous layer. To this end, we prepared superhydrophobic coatings with different thicknesses by utilizing different numbers of coating cycles of a liquid flame spraying technique. Laser scanning confocal microscopy confirmed an increase in gas layer thickness with an increasing number of coating cycles. Force measurements between such coatings and a hydrophobic colloidal probe revealed attractive forces caused by bridging gas capillaries, and both the capillary size and the range of attraction increased with increasing thickness of the pre-existing gas layer. Hence, our data suggest that the amount of available gas at and in the superhydrophobic coating determines the force range and capillary growth.

Structure Formation in Supraparticles Composed of Spherical and Elongated Particles.

We studied the evaporation-induced formation of supraparticles from dispersions of elongated colloidal particles using experiments and computer simulations. Aqueous droplets containing a dispersion of ellipsoidal and spherical polystyrene particles were dried on superamphiphobic surfaces at different humidity values that led to varying evaporation rates. Supraparticles made from only ellipsoidal particles showed short-range lateral ordering at the supraparticle surface and random orientations in the interior regardless of the evaporation rate. Particle-based simulations corroborated the experimental observations in the evaporation-limited regime and showed an increase in the local nematic ordering as the diffusion-limited regime was reached. A thin shell of ellipsoids was observed at the surface when supraparticles were made from binary mixtures of ellipsoids and spheres. Image analysis revealed that the supraparticle porosity increased with an increasing aspect ratio of the ellipsoids.

Durable, Ultrathin, and Antifouling Polymer Brush Coating for Efficient Condensation Heat Transfer.

Heat exchangers are made of metals because of their high heat conductivity and mechanical stability. Metal surfaces are inherently hydrophilic, leading to inefficient filmwise condensation. It is still a challenge to coat these metal surfaces with a durable, robust, and thin hydrophobic layer, which is required for efficient dropwise condensation. Here, we report the nonstructured and ultrathin (∼6 nm) polydimethylsiloxane (PDMS) brushes on copper that sustain high-performing dropwise condensation in high supersaturation. Due to the flexible hydrophobic siloxane polymer chains, the coating has low resistance to drop sliding and excellent chemical stability. The PDMS brushes can sustain dropwise condensation for up to ∼8 h during exposure to 111 °C saturated steam flowing at 3 m·s-1, with a 5-7 times higher heat transfer coefficient compared to filmwise condensation. The surface is self-cleaning and can reduce the level of bacterial attachment by 99%. This low-cost, facile, fluorine-free, and scalable method is suitable for a great variety of heat transfer applications.

Nanofilament-Coated Superhydrophobic Membranes Show Enhanced Flux and Fouling Resistance in Membrane Distillation.

Membrane distillation (MD) is an important technique for brine desalination and wastewater treatment that may utilize waste or solar heat. To increase the distillation rate and minimize membrane wetting and fouling, we deposit a layer of polysiloxane nanofilaments on microporous membranes. In this way, composite membranes with multiscale pore sizes are created. The performance of these membranes in the air gap and direct contact membrane distillation was investigated in the presence of salt solutions, solutions containing bovine serum albumin, and solutions containing the surfactant sodium dodecyl sulfate. In comparison to conventional hydrophobic membranes, our multiscale porous membranes exhibit superior fouling resistance while attaining a higher distillation flux without using fluorinated compounds. This study demonstrates a viable method for optimizing MD processes for wastewater and saltwater treatment.

A super liquid-repellent hierarchical porous membrane for enhanced membrane distillation.

Membrane distillation (MD) is an emerging desalination technology that exploits phase change to separate water vapor from saline based on low-grade energy. As MD membranes come into contact with saline for days or weeks during desalination, membrane pores have to be sufficiently small (typically <0.2 µm) to avoid saline wetting into the membrane. However, in order to achieve high distillation flux, the pore size should be large enough to maximize transmembrane vapor transfer. These conflicting requirements of pore geometry pose a challenge to membrane design and currently hinder broader applications of MD. To address this fundamental challenge, we developed a super liquid-repellent membrane with hierarchical porous structures by coating a polysiloxane nanofilament network on a commercial micro-porous polyethersulfone membrane matrix. The fluorine-free nanofilament coating effectively prevents membrane wetting under high hydrostatic pressure (>11.5 bar) without compromising vapor transport. With large inner micro-porous structures, the nanofilament-coated membrane improves the distillation flux by up to 60% over the widely used commercially available membranes, while showing excellent salt rejection and operating stability. Our approach will allow the fabrication of high-performance composite membranes with multi-scale porous structures that have wide-ranging applications beyond desalination, such as in cleaning wastewater.

Evaporation-driven Supraparticle Synthesis by Self-Lubricating Colloidal Dispersion Microdrops.

The surface-templated evaporation-driven (S-TED) method that uses liquid-repellent surfaces has attracted considerable attention for its use in fabricating supraparticles of defined shape, size, and porosity. However, challenges in achieving mass production have impeded the widespread adoption of the S-TED method. To overcome this limit, we introduce an evaporation-driven "multiple supraparticle" synthesis by drying arrays of self-lubricating colloidal dispersion microdrops. To facilitate this synthetic method, a hydrophilic micropattern is prepared on a hydrophobic substrate as a template. During the removal of the substrate out of a dispersion, liquid drops are trapped and generate a microdrop array. To produce supraparticles, the contact lines of the trapped drops must be able to recede freely during evaporation. However, hydrophilic micropatterns induce strong contact line pinning for microdrops that hinders supraparticle formation. Herein, we solve this contradiction by employing an Ouzo-like colloidal dispersion, where we can control the wettability of the drop trapping domain. The self-lubrication effect provided by the Ouzo-like solution enables smooth movement of the drops' contact lines during evaporation, thereby resulting in the successful fabrication of supraparticle arrays even within the trapping domain. This strategy offers a promising and scalable approach for large-scale evaporation-driven supraparticle synthesis with a potential for extension to various primary colloidal particles, further broadening its applicability.

Method to Measure Surface Tension of Microdroplets Using Standard AFM Cantilever Tips.

Surface tension is a physical property that is central to our understanding of wetting phenomena. One could easily measure liquid surface tension using commercially available tensiometers (e.g., Wilhelmy plate method) or by optical imaging (e.g., pendant drop method). However, such instruments are designed for bulk liquid volumes on the order of milliliters. In order to perform similar measurements on extremely small sample volumes in the range of femtoliters, atomic force microscope (AFM) is considered as a promising tool. It was previously reported that by fabricating a special "nanoneedle"-shaped cantilever probe, a Wilhelmy-like experiment can be performed with AFM. By measuring the capillary force between such special probes and a liquid surface, surface tension could be calculated. Here, we carried out measurements on microscopic droplets with AFM, but instead, using standard pyramidal cantilever tips. The cantilevers were coated with a hydrophilic polyethylene glycol-based polymer brush in a simple one-step process, which reduced its contact angle hysteresis for most liquids. Numerical simulations of a liquid drop interacting with a pyramidal or conical geometry were used to calculate surface tension from the experimentally measured force. The results on micrometer-sized drops agree well with bulk tensiometer measurement of three test liquids (mineral oil, ionic liquid, and glycerol), within a maximum error of 10%. Our method eliminates the need for specially fabricated "nanoneedle" tips, thus reducing the complexity and cost of measurement.

When crystals flow.

Semicrystalline polymers are solids that are supposed to flow only above their melting temperature. By using confinement within nanoscopic cylindrical pores, we show that a semicrystalline polymer can flow at temperatures below the melting point with a viscosity intermediate to the melt and crystal states. During this process, the capillary force is strong and drags the polymer chains in the pores without melting the crystal. The unexpected enhancement in flow, while preserving the polymer crystallites, is of importance in the design of polymer processing conditions applicable at low temperatures, e.g., cold drawn polymers such as polytetrafluoroethylene, self-healing, and in nanoconfined donor/acceptor polymers used in organic electronics.

Hierarchically Branched Siloxane Brushes for Efficient Harvesting of Atmospheric Water.

Atmospheric water harvesting is considered a viable source of freshwater to alleviate water scarcity in an arid climate. Water condensation tends to be more efficient on superhydrophobic surfaces as the spontaneous coalescence-induced droplet jumping on superhydrophobic surfaces enables faster condensate removal. However, poor water nucleation on these surfaces leads to meager water harvest. A conventional approach to the problem is to fabricate micro- and nanoscale biphilic structures. Nonetheless, the process is complex, expensive, and difficult to scale. Here, the authors present an inexpensive and scalable method based on manipulating the water-repellent coatings of superhydrophobic surfaces. Flexible siloxane can facilitate water nucleation, while a branched structure promotes efficient droplet jumping. Moreover, ToF-SIMS analysis indicated that branched siloxane provides a better water-repellent coating coverage than linear siloxane and the siloxanes comprise hydrophilic and hydrophobic molecular segments. Thus, the as-prepared superhydrophobic surface, TiO2 nanorods coated with branched siloxanes harvested eight times more water than a typical fluoroalkylsilane (FAS)-coated surface under a low 30% relative humidity and performed better than most reported biphasic materials.

Effects of liquid surface tension on gas capillaries and capillary forces at superamphiphobic surfaces.

The formation of a bridging gas capillary between superhydrophobic surfaces in water gives rise to strongly attractive interactions ranging up to several micrometers on separation. However, most liquids used in materials research are oil-based or contain surfactants. Superamphiphobic surfaces repel both water and low-surface-tension liquids. To control the interactions between a superamphiphobic surface and a particle, it needs to be resolved whether and how gas capillaries form in non-polar and low-surface-tension liquids. Such insight will aid advanced functional materials development. Here, we combine laser scanning confocal imaging and colloidal probe atomic force microscopy to elucidate the interaction between a superamphiphobic surface and a hydrophobic microparticle in three liquids with different surface tensions: water (73 mN m-1), ethylene glycol (48 mN m-1) and hexadecane (27 mN m-1). We show that bridging gas capillaries are formed in all three liquids. Force-distance curves between the superamphiphobic surface and the particle reveal strong attractive interactions, where the range and magnitude decrease with liquid surface tension. Comparison of free energy calculations based on the capillary menisci shapes and the force measurements suggest that under our dynamic measurements the gas pressure in the capillary is slightly below ambient.

Plasma-Induced Superhydrophobicity as a Green Technology for Enhanced Air Gap Membrane Distillation.

Superhydrophobicity has only recently become a requirement in membrane fabrication and modification. Superhydrophobic membranes have shown improved flux performance and scaling resistance in long-term membrane distillation (MD) operations compared to simply hydrophobic membranes. Here, we introduce plasma micro- and nanotexturing followed by plasma deposition as a novel, dry, and green method for superhydrophobic membrane fabrication. Using plasma micro- and nanotexturing, commercial membranes, both hydrophobic and hydrophilic, are transformed to superhydrophobic featuring water static contact angles (WSCA) greater than 150° and contact angle hysteresis lower than 10°. To this direction, hydrophobic polytetrafluoroethylene (PTFE) and hydrophilic cellulose acetate (CA) membranes are transformed to superhydrophobic. The superhydrophobic PTFE membranes showed enhanced water flux in standard air gap membrane distillation and more stable performance compared to the commercial ones for at least 48 h continuous operation, with salt rejection >99.99%. Additionally, their performance and high salt rejection remained stable, when low surface tension solutions containing sodium dodecyl sulfate (SDS) and NaCl (down to 35 mN/m) were used, showcasing their antiwetting properties. The improved performance is attributed to superhydrophobicity and increased pore size after plasma micro- and nanotexturing. More importantly, CA membranes, which are initially unsuitable for MD due to their hydrophilic nature (WSCA ≈ 40°), showed excellent performance with stable flux and salt rejection >99.2% again for at least 48 h, demonstrating the effectiveness of the proposed method for wetting control in membranes regardless of their initial wetting properties.

Drawing liquid bridges from a thin viscous film.

When a particle, such as dust, contacts a thin liquid film covering a surface it is trapped by the liquid. This effect is caused by the formation of a meniscus, resulting in a capillary force that makes the particle adhere to the surface. While capillary adhesion is well-characterised in static situations, the dynamic formation of the liquid bridge after the initial contact is highly intricate. Here, we experimentally study the evolution of a liquid bridge after a glass sphere is gently brought into contact with a thin viscous film. It is found that the contact creates a ripple on the thin film, which influences the growth of the meniscus. Initially, the ripple and the meniscus are coupled and exhibit similar dynamics. This initial regime is well accounted for by a scaling law derived from lubrication theory. At a later stage, the ripple is "detached" from the liquid bridge, leading to a second regime of bridge dynamics. As a result, capillary forces are time-dependent, highlighting the importance of dynamics on adhesion.

Mechanisms of detachment in fibrillar adhesive systems.

Several creatures can climb on smooth surfaces with the help of hairy adhesive pads on their legs. A rapid change from strong attachment to effortless detachment of the leg is enabled by the asymmetric geometry of the tarsal hairs. In this study, we propose mechanisms by which the hairy pad can be easily detached, even when the hairs possess no asymmetry. Here, we examine the possible function of the tibia-tarsus leg joint and the claws. Based on a spring-based model, we consider three modes of detachment: vertically pulling the pad while maintaining either a (1) fixed or a (2) free joint, or by (3) flexing the pad about the claw. We show that in all cases, the adhesion force can be significantly reduced due to elastic forces when the hairs deform non-uniformly across the array. Our proposed model illustrates the design advantage of such fibrillar adhesive systems, that not only provide strong adhesion, but also allow easy detachment, making them suitable as organs for fast locomotion and reliable hold. The presented approaches can be potentially used to switch the adhesion state in bio-inspired fibrillar adhesives, by incorporating artificial joints and claws into their design, without the need of asymmetric or stimuli-responsive fibrillar structures.

Squeezing Drops: Force Measurements of the Cassie-to-Wenzel Transition.

Superhydrophobic surfaces have long been the center of attention of many researchers due to their unique liquid repellency and self-cleaning properties. However, these unique properties rely on the stability of the so-called Cassie state, which is a metastable state with air-filled microstructures. This state tends to transit to the stable Wenzel state, where the inside of the microstructures eventually wets. For potential industrial applications, it is therefore critical to maintain the Cassie state. We investigate the Cassie-to-Wenzel transition on superhydrophobic micropillar surfaces by squeezing a water drop between the surface and a transparent superhydrophobic force probe. The probe's transparency allows the use of top-view optics to monitor the area of the drop as it is squeezed against a micropillared surface. The impalement, or Cassie-to-Wenzel transition, is identified as a sharp decrease in force accompanied by an abrupt change in the drop's contact area. We compare the force measured by the sensor with the capillary pressure force calculated from the observed drop shape and find a good agreement between both quantities. We also study the force and pressure at impalement as a function of the pillar's slenderness ratio. Finally, we compare the impalement pressure with three literature predictions and find that our experimental values are consistently lower than the theoretical values. We find that a possible cause of this earlier Cassie-to-Wenzel transition may be the coalescence of the squeezed drop with microdroplets that nucleate around the base of the micropillars.

Spontaneous Charging of Drops on Lubricant-Infused Surfaces.

When a drop of a polar liquid slides over a hydrophobic surface, it acquires a charge. As a result, the surface charges oppositely. For applications such as the generation of electric energy, lubricant-infused surfaces (LIS) may be important because they show a low friction for drops. However, slide electrification on LIS has not been studied yet. Here, slide electrification on lubricant-infused surfaces was studied by measuring the charge generated by series of water drops sliding down inclined surfaces. As LIS, we used PDMS-coated glass with micrometer-thick silicone oil films on top. For PDMS-coated glass without lubricant, the charge for the first drop is highest. Then it decreases and saturates at a steady state charge per drop. With lubricant, the drop charge starts from 0, then it increases and reaches a maximum charge per drop. Afterward, it decreases again before reaching its steady-state value. This dependency is not a unique phenomenon for lubricant-infused PDMS; it also occurs on lubricant-infused micropillar surfaces. We attribute this dependency of charge on drop numbers to a change in surface conductivity and depletion of lubricant. These findings are helpful for understanding the charge process and optimizing solid-liquid nanogenerator devices in applications.

Mechanically Robust and Flame-Retardant Superhydrophobic Textiles with Anti-Biofouling Performance.

The attachment of bio-fluids to surfaces promotes the transmission of diseases. Superhydrophobic textiles may offer significant advantages for reducing the adhesion of bio-fluids. However, they have not yet found widespread use because dried remnants adhere strongly and have poor mechanical or chemical robustness. In addition, with the massive use of polymer textiles, features such as fire and heat resistance can reduce the injuries and losses suffered by people in a fire accident. We developed a superhydrophobic textile covered with a hybrid coating of titanium dioxide and polydimethylsiloxane (TiO2/PDMS). Such a textile exhibits low adhesion to not only bio-fluids but also dry blood. Compared to a hydrophilic textile, the peeling force of the coated textile on dried blood is 20 times lower. The textile's superhydrophobicity survives severe treatment by sandpaper (400 mesh) at high pressure (8 kPa) even if some of its microstructures break. Furthermore, the textile shows excellent heat resistance (350 °C) and flame-retardant properties as compared to those of the untreated textile. These benefits can greatly inhibit the flame spread and reduce severe burns caused by polymer textiles adhering to the skin when melted at high temperatures.

Vapor Lubrication for Reducing Water and Ice Adhesion on Poly(dimethylsiloxane) Brushes.

Fast removal of small water drops from surfaces is a challenging issue in heat transfer, water collection, or anti-icing. Poly(dimethylsiloxane) (PDMS) brushes show good prospects to reach this goal because of their low adhesion to liquids. To further reduce adhesion of water drops, here, the surface to the vapor of organic solvents such as toluene or n-hexane is exposed. In the presence of such vapors, water drops slide at lower tilt angle and move faster. This is mainly caused by the physisorption of vapor and swelling of the PDMS brushes, which serves as a lubricating layer. Enhanced by the toluene vapor lubrication, the limit departure volume of water drop on PDMS brushes decreases by one order of magnitude compared to that in air. As a result, the water harvesting efficiency in toluene vapor increases by 65%. Benefits of vapor lubrication are further demonstrated for de-icing: driven by gravity, frozen water drops slide down the vertical PDMS brush surface in the presence of vapor.

High-throughput synthesis of CeO2 nanoparticles for transparent nanocomposites repelling Pseudomonas aeruginosa biofilms.

Preventing bacteria from adhering to material surfaces is an important technical problem and a major cause of infection. One of nature's defense strategies against bacterial colonization is based on the biohalogenation of signal substances that interfere with bacterial communication. Biohalogenation is catalyzed by haloperoxidases, a class of metal-dependent enzymes whose activity can be mimicked by ceria nanoparticles. Transparent CeO2/polycarbonate surfaces that prevent adhesion, proliferation, and spread of Pseudomonas aeruginosa PA14 were manufactured. Large amounts of monodisperse CeO2 nanoparticles were synthesized in segmented flow using a high-throughput microfluidic benchtop system using water/benzyl alcohol mixtures and oleylamine as capping agent. This reduced the reaction time for nanoceria by more than one order of magnitude compared to conventional batch methods. Ceria nanoparticles prepared by segmented flow showed high catalytic activity in halogenation reactions, which makes them highly efficient functional mimics of haloperoxidase enzymes. Haloperoxidases are used in nature by macroalgae to prevent formation of biofilms via halogenation of signaling compounds that interfere with bacterial cell-cell communication ("quorum sensing"). CeO2/polycarbonate nanocomposites were prepared by dip-coating plasma-treated polycarbonate panels in CeO2 dispersions. These showed a reduction in bacterial biofilm formation of up to 85% using P. aeruginosa PA14 as model organism. Besides biofilm formation, also the production of the virulence factor pyocyanin in is under control of the entire quorum sensing systems P. aeruginosa. CeO2/PC showed a decrease of up to 55% in pyocyanin production, whereas no effect on bacterial growth in liquid culture was observed. This indicates that CeO2 nanoparticles affect quorum sensing and inhibit biofilm formation in a non-biocidal manner.

Charging of drops impacting onto superhydrophobic surfaces.

When neutral water drops impact and rebound from superhydrophobic surfaces, they acquire a positive electrical charge. To measure the charge, we analyzed the trajectory of rebounding drops in an external electric field by high-speed video imaging. Although this charging phenomenon has been observed in the past, little is known about the controlling parameters for the amount of drop charging. Here we investigate the relative importance of five of these potential variables: impact speed, drop contact area, contact line retraction speed, drop size, and type of surface. We additionally apply our previously reported model for sliding drop electrification to the case of impacting drops, suggesting that the two cases contain the same charge separation mechanism at the contact line. Both our experimental results and our theoretical model indicate that maximum contact area is the dominant control parameter for charge separation.

Controlling supraparticle shape and structure by tuning colloidal interactions.

HYPOTHESIS: Assembly of colloids in drying colloidal suspensions on superhydrophobic surface is influenced by the colloidal interactions, which determine the shape and interior structure of the assembled supraparticle. The introduction of salt (electrolyte) into the assembly system is expected to influence the colloid interactions and packing during the evaporation process. Hence, both the outer shape and internal structure of supraparticles should be controlled by varying salt concentrations. EXPERIMENTS: Suspensions of electrostatically stabilized polystyrene particles with specified salt concentrations were chosen as model systems to conduct the evaporation on a superhydrophobic surface. A systematic study was performed by regulating the concentration and valency of salt. The morphology and interior of supraparticles were carefully characterized with electron scanning microscopy, while the colloidal interaction was established using colloidal probe atomic force microscopy. FINDINGS: Supraparticles displayed a spherical-to-nonspherical shape change due to the addition of salts. The extent of crystallization depended on salt concentration. These changes in shape and structure were correlated with salt-dependent single colloid interaction forces, which were not previously investigated in detail in radially symmetric evaporation geometry. Our findings are crucial for understanding assembly behavior during the drying process and offer guidance for preparing complex supraparticles to meet specific applications requirement.