Wetting on silicone surfaces.

Silicone is frequently used as a model system to investigate and tune wetting on soft materials. Silicone is biocompatible and shows excellent thermal, chemical, and UV stability. Moreover, the mechanical properties of the surface can be easily varied by several orders of magnitude in a controlled manner. Polydimethylsiloxane (PDMS) is a popular choice for coating applications such as lubrication, self-cleaning, and drag reduction, facilitated by low surface energy. Aiming to understand the underlying interactions and forces, motivated numerous and detailed investigations of the static and dynamic wetting behavior of drops on PDMS-based surfaces. Here, we recognize the three most prevalent PDMS surface variants, namely liquid-infused (SLIPS/LIS), elastomeric, and liquid-like (SOCAL) surfaces. To understand, optimize, and tune the wetting properties of these PDMS surfaces, we review and compare their similarities and differences by discussing (i) the chemical and molecular structure, and (ii) the static and dynamic wetting behavior. We also provide (iii) an overview of methods and techniques to characterize PDMS-based surfaces and their wetting behavior. The static and dynamic wetting ridge is given particular attention, as it dominates energy dissipation, adhesion, and friction of sliding drops and influences the durability of the surfaces. We also discuss special features such as cloaking and wetting-induced phase separation. Key challenges and opportunities of these three surface variants are outlined.

Dynamics of Droplets Moving on Lubricated Polymer Brushes.

Understanding the dynamics of drops on polymer-coated surfaces is crucial for optimizing applications such as self-cleaning materials or microfluidic devices. While the static and dynamic properties of deposited drops have been well characterized, a microscopic understanding of the underlying dynamics is missing. In particular, it is unclear how drop dynamics depends on the amount of uncross-linked chains in the brush, because experimental techniques fail to quantify those. Here we use coarse-grained simulations to study droplets moving on a lubricated polymer brush substrate under the influence of an external body force. The simulation model is based on the many body dissipative particle dynamics (MDPD) method and designed to mimic a system of water droplets on poly(dimethylsiloxane) (PDMS) brushes with chemically identical PDMS lubricant. In agreement with experiments, we find a sublinear power law dependence between the external force F and the droplet velocity v, F ∝ vα with α < 1; however, the exponents differ (α ∼ 0.6-0.7 in simulations versus α ∼ 0.25 in experiments). With increasing velocity, the droplets elongate and the receding contact angle decreases, whereas the advancing contact angle remains roughly constant. Analyzing the flow profiles inside the droplet reveals that the droplets do not slide but roll, with vanishing slip at the substrate surface. Surprisingly, adding lubricant has very little effect on the effective friction force between the droplet and the substrate, even though it has a pronounced effect on the size and structure of the wetting ridge, especially above the cloaking transition.

Control of spontaneous charging of sliding water drops by plasma-surface treatment.

Slide electrification is the spontaneous separation of electric charges at the rear of water drops sliding over solid surfaces. This study delves into how surfaces treated with a low-pressure plasma impact water slide electrification. Ar, O2, and N2 plasma treatment reduced the drop charge and contact angles on glass, quartz, and SU-8 coated with 1H,1H,2H,2H-perfluoroctyltrichlorosilane (PFOTS), and polystyrene. Conversely, 64% higher drop charge was achieved using electrode-facing treatment in plasma chamber. Based on the zeta potential, Kelvin potential, and XPS measurements, the plasma effects were attributed to alterations of the topmost layer's chemistry, such as oxidation and etching, and superficially charge deposition. The surface top layer charges were less negative after electrode-facing and more negative after bulk plasma treatment. As a result, the zeta potential was less negative after electrode-facing and more negative after bulk plasma treatment. Although the fluorinated layer was applied after plasma activation, we observed a discernible impact of plasma-glass treatment on drop charging. Plasma surface modification offers a means to adjust drop charges: electrode-facing treatment of the fluorinated layer leads to an enhanced drop charge, while plasma treatment on the substrate prior to fluorination diminishes drop charges, all without affecting contact angles or surface roughness.

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.

Fluorine-Free Super-Liquid-Repellent Surfaces: Pushing the Limits of PDMS.

Methods for fabricating super-liquid-repellent surfaces have typically relied on perfluoroalkyl substances. However, growing concerns about the environmental and health effects of perfluorinated compounds have caused increased interest in fluorine-free alternatives. Polydimethylsiloxane (PDMS) is most promising. In contrast to fluorinated surfaces, PDMS-coated surfaces showed only superhydrophobicity. This raises the question whether the poor liquid repellency is caused by PDMS interacting with the probe liquid or whether it results from inappropriate surface morphology. Here, we demonstrate that a well-designed two-tier structure consisting of silicon dioxide nanoparticles combined with surface-tethered PDMS chains allows super-liquid-repellency toward a range of low surface tension liquids. Drops of water-ethanol solutions with surface tensions as low as 31.0 mN m-1 easily roll and bounce off optimized surface structures. Friction force measurements demonstrate excellent surface homogeneity and easy mobility of drops. Our work shows that fluorine-free super-liquid-repellent surfaces can be achieved using scalable fabrication methods and environmentally friendly surface functionalization.

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.

Phase Separation in Wetting Ridges of Sliding Drops on Soft and Swollen Surfaces.

Drops in contact with swollen, elastomeric substrates can induce a capillary mediated phase separation in wetting ridges. Using confocal microscopy, we visualize phase separation of oligomeric silicone oil from a cross-linked silicone network during steady-state sliding of water drops. We find an inverse relationship between the oil tip height and the drop sliding speed, which is rationalized by competing transport timescales of the oil molecules: separation rate versus drop-advection speed. Separation rates in highly swollen networks are as fast as diffusion in pure melts.

Scanning Drop Friction Force Microscopy.

Wetting imperfections are omnipresent on surfaces. They cause contact angle hysteresis and determine the wetting dynamics. Still, existing techniques (e.g., contact angle goniometry) are not sufficient to localize inhomogeneities and image wetting variations. We overcome these limitations through scanning drop friction force microscopy (sDoFFI). In sDoFFI, a 15 µL drop of Milli-Q water is raster-scanned over a surface. The friction force (lateral adhesion force) acting on the moving contact line is plotted against the drop position. Using sDoFFI, we obtained 2D wetting maps of the samples having sizes in the order of several square centimeters. We mapped areas with distinct wetting properties such as those present on a natural surface (e.g., a rose petal), a technically relevant superhydrophobic surface (e.g., Glaco paint), and an in-house prepared model of inhomogeneous surfaces featuring defined areas with low and high contact angle hysteresis. sDoFFI detects features that are smaller than 0.5 mm in size. Furthermore, we quantified the sliding behavior of drops across the boundary separating areas with different contact angles on the model sample. The sliding of a drop across this transition line follows a characteristic stick-slip motion. We use the variation in force signals, advancing and receding contact line velocities, and advancing and receding contact angles to identify zones of stick and slip. When scanning the drop from low to high contact angle hysteresis, the drop undergoes a stick-slip-stick-slip motion at the interline. Sliding from high to low contact angle hysteresis is characterized by the slip-stick-slip motion. The sDoFFI is a new tool for 2D characterization of wetting properties, which is applicable to laboratory-based samples but also characterizes biological and commercial surfaces.

Cloaking Transition of Droplets on Lubricated Brushes.

We study the equilibrium properties and the wetting behavior of a simple liquid on a polymer brush, with and without the presence of lubricant by multibody Dissipative Particle Dynamics simulations. The lubricant is modeled as a polymeric liquid consisting of short chains that are chemically identical with the brush polymers. We investigate the behavior of the brush in terms of the grafting density and the amount of lubricant present. Regarding the wetting behavior, we study a sessile droplet on top of the brush. The droplet consists of nonbonded particles that form a dense phase. Our model and choice of parameters result in the formation of a wetting ridge and in the cloaking of the droplet by the lubricant; i.e., the lubricant chains creep up onto the droplet and eventually cover its surface completely. Cloaking is a phenomenon that is observed experimentally and is of integral importance to the dynamics of sliding droplets. We quantify the cloaking in terms of its thickness, which increases with the amount of lubricant present. The analysis reveals a well-defined transition point where the cloaking sets in. We propose a thermodynamic theory to explain this behavior. In addition, we investigate the dependence of the contact angles on the size of the droplet and the possible effect of line tension. We quantify the variation of the contact angle with the curvature of the contact line on a lubricant free brush and find a negative value for the line tension. Finally we investigate the effect of cloaking/lubrication on the contact angles and the wetting ridge. We find that lubrication and cloaking reduce the contact angles by a couple of degrees. The effect on the wetting ridge is a reduction in the extension of the brush chains near the three phase contact line, an effect that was also observed in experiments of droplets on cross-linked gels.

Linear shrinkage of hydrogel coatings exposed to flow: interplay between dissolution of water and advective transport.

We investigate the shrinkage of a surface-grafted water-swollen hydrogel under shear flows of oils by laser scanning confocal microscopy. Interestingly, external shear flows of oil lead to linear dehydration and shrinkage of the hydrogel for all investigated flow conditions irrespective of the chemical nature of the hydrogel. The reason is that the finite solubility of water in oil removes water from the hydrogel continuously by diffusion. The flow advects the water-rich oil, as demonstrated by numerical solutions of the underlying convection-diffusion equation. In line with this hypothesis, shear does not cause gel shrinkage for water-saturated oils or non-solvents. The solubility of water in the oil will tune the dehydration dynamics.

Frost spreading and pattern formation on microstructured surfaces.

Frost is found in nature as a symphony of nucleation and heat and mass transport, cascading from angstroms to several meters. Here, we use laser-induced fluorescence microscopy to investigate the pattern formation of frost growth in experiments which tune the mesoscopic length scale by using microstructured pillar arrays as a frost condenser surface. By controlling the degree of surface supercooling and the amount of condensate, different modes of frost patterning are uncovered, ranging from complete surface coverage to fractal-looking and limited-coverage structures of spiky appearance.

The Force Required to Detach a Rotating Particle from a Liquid-Fluid Interface.

The force required to detach a particle from a liquid-fluid interface is a direct measure of the capillary adhesion between the particle and the interface. Analytical expressions for the detachment force are available but are limited to nonrotating particles. In this work, we derive analytical expressions for the force required to detach a rotating spherical particle from a liquid-fluid interface. Our theory predicts that the rotation reduces the detachment force when there is a finite contact angle hysteresis between the particle and the liquid. For example, the force required to detach a particle with an advancing contact angle of 120° and a receding contact angle of 80° (e.g., polydimethylsiloxane particle at a water-air interface) is expected to be 25% lower when the particle rotates while it is detached.

Super liquid repellent surfaces for anti-foaming and froth management.

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.

Wetting-regulated gas-involving (photo)electrocatalysis: biomimetics in energy conversion.

(Photo)electrolysis of water or gases with water to species serving as industrial feedstocks and energy carriers, such as hydrogen, ammonia, ethylene, propanol, etc., has drawn tremendous attention. Moreover, these processes can often be driven by renewable energy under ambient conditions as a sustainable alternative to traditional high-temperature and high-pressure synthesis methods. In addition to the extensive studies on catalyst development, increasing attention has been paid to the regulation of gas transport/diffusion behaviors during gas-involving (photo)electrocatalytic reactions towards the goal of creating industrially viable catalytic systems with high reaction rates, excellent long-term stabilities and near-unity selectivities. Biomimetic surfaces and systems with special wetting capabilities and structural advantages can shed light on the future design of (photo)electrodes and address long-standing challenges. This article is dedicated to bridging the fields of wetting and catalysis by reviewing the cutting-edge design methodologies of both gas-evolving and gas-consuming (photo)electrocatalytic systems. We first introduce the fundamentals of various in-air/underwater wetting states and their corresponding bioinspired structural properties. The relationship amongst the bubble transport behavior, wettability, and porosity/tortuosity is also discussed. Next, the latest implementations of wetting-related design principles for gas-evolving reactions (i.e. the hydrogen evolution reaction and oxygen evolution reaction) and gas-consuming reactions (i.e. the oxygen reduction reaction and CO2 reduction reaction) are summarized. For photoelectrode designs, additional factors are taken into account, such as light absorption and the separation, transport and recombination of photoinduced electrons and holes. The influences of wettability and 3D structuring of (photo)electrodes on the catalytic activity, stability and selectivity are analyzed to reveal the underlying mechanisms. Finally, remaining questions and related future perspectives are outlined.

Ultrafast Bubble Bursting by Superamphiphobic Coatings.

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 important for any application. In this work, the hierarchical structure of superamphiphobic surfaces is tuned in order to rapidly rupture contacting bubbles. Surfaces prepared using liquid flame spray 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 demonstrated by preventing the accumulation of air in the plastron via a connection of the plastron to the environment. The times recorded for bubble rupture and complete reorganization of air are reduced by approximately a factor of 3 compared to previously reported values. The concept is utilized to passively control surfactant-rich foam in froth flotation. Material collection efficiency increased by more than 60 times compared to controls.

Capillary Torque on a Particle Rotating at an Interface.

Small particles attach to liquid-fluid interfaces due to capillary forces. The influence of rotation on the capillary force is largely unexplored, despite being relevant whenever particles roll at a liquid-fluid interface or on a moist solid. Here, we demonstrate that due to contact angle hysteresis, a particle needs to overcome a resistive capillary torque to rotate at an interface. We derive a general model for the capillary torque on a spherical particle. The capillary torque is given by M = γRLk(cos ΘR - cos ΘA), where γ is the interfacial tension, R is the radius of the particle, L is the diameter of the contact line, k = 24/π3 is a geometrical constant, and ΘR and ΘA are the receding and advancing contact angles, respectively. The expression for the capillary torque (normalized by the radius of the particle) is equivalent to the expression for the friction force that a drop experiences when moving on a flat surface. Our theory predicts that capillary torque reduces the mobility of wet granular matter and prevents small (nano/micro) particles from rotating when they are in Brownian motion at an interface.

Super liquid repellent coatings against the everyday life wear: Heating, freezing, scratching.

Super liquid repellent coatings are among the most promising candidates for self-cleaning surfaces for indoor and outdoor applications. However, the characteristic nano- and micro-scale protrusions can easily be damaged. Improving the durability of these coatings belongs to the most important challenges to increase the coating's application potential. Here, we show that commercial polyester fabrics coated with silicone nanofilaments maintain their self-cleaning properties throughout repeated freezing-unfreezing cycles, ironing, and mechanical stress. The coating improves the heat resistance of the fabric. The surface keeps its water repellency until the fabric is almost destroyed by scratching with sandpaper or a metal sponge. The excellent performance results from the synergetic effects of i) the interwoven structure of the fabric and ii) the intrinsic hydrophobic and flexible nature of the fabric and of the nanofilaments coating. The combination of these factors generates a product which overcomes the most claimed drawbacks of super liquid repellent coatings.

How Frost Forms and Grows on Lubricated Micro- and Nanostructured Surfaces.

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.

How a water drop removes a particle from a hydrophobic surface.

To understand the removal of particles from surfaces by water drops, we used an inverted laser scanning confocal microscope to image the collision between a water drop and a particle on a flat polydimethylsiloxane (PDMS) surface. The dynamic drop-particle contact line was monitored by fixing the drop directly above the objective lens while moving the sample stage at well-defined speeds (10-500 µm s-1). The lateral force acting on the drop during the collision was measured as a function of speed, using a force sensor mounted on the microscope. Depending on the collision speed, the particle either stays attached at the rear of the drop or detaches from it. We propose a criterion to determine whether the particle remains attached to the drop based on the capillary and resistive forces acting on the particle during the collision. The forces measured when the particle crosses the air-water interface are compared to existing models. We adapted these to account for rolling of the particle. By comparing our experimental measurements with an analytical model for the capillary torque acting on a particle rolling at an interface, we provide detailed insights on the origins of the resistive force acting on the particle when it is pushed or pulled by the drop. A low friction force between the surface and the particle increases the likelihood of particle removal.