Superwetting Surfaces under Different Media: Effects of Surface Topography on Wettability

Superwetting surfaces in air, such as superhydrophobic and superoleophobic surfaces that are governed by surface chemical compositions and surface topographies, are one of the most extensively studied topics in this field. However, it is not well‐understood how surface topographies affect the behaviors of immiscible liquids and gases under other kinds of media, although it is significant in diverse fields. The main aim of this work is to systematically investigate the wetting behaviors of liquids (water and oil) and gas (air) on silicon surfaces with different topographies (i.e., smooth, micro, nano, and micro‐/nanostructures) under various media (i.e., air, water, and oil). The contact angles, as well as contact‐angle hysteresis, sliding angles, and adhesive forces, were utilized to evaluate the wettability of these surfaces. As a result, the microstructured surfaces typically exhibit high contact‐angle hysteresis, high sliding angles, and high adhesive forces, whereas the micro‐/nanostructured surfaces display low contact‐angle hysteresis, low sliding angles, and low adhesive forces, even if they have high (>150°) and similar contact angles. Furthermore, when transferring the same surface from one kind of medium to another, different superwetting states can be reversibly switched.     

Wetting ability of biological liquids in presence of metallic nanoparticles

The wetting ability of water and of some biological liquids was measured on different biocompatible surfaces with and without different colloidal metals. Insoluble nanoparticles disperse in biological tissues enhance some properties, such as the interface adhesion between two surfaces, the X-ray contrast of medical images and the absorbed dose during radiotherapy treatments. The introduction of nanoparticles in the liquids generally improves the wetting ability and changes other properties of the solution, due to the different distribution of the adhesion forces, to the nature, morphology and concentration of the added nanoparticles. An investigation on the contact angle of the liquid drops, physiological liquids, including the human blood, placed on different substrates (polymers, ceramics and metals) with and without the use of metallic nanoparticles is presented, evaluated and discussed.     

Wetting Properties of Steel Surfaces Modified by Laser Interference Metallurgy

The wetting properties of 100Cr6 bearing steel surfaces modified using laser interference metallurgy (LIMET) are analyzed. The steel surfaces are structured with line‐like patterns with line‐spacing. The topography of the ridged surface is analyzed by means of white light interferometry and scanning electron microscopy and surface chemistry of the different topographic regions by Raman spectroscopy. Contact angle (CA) measurements are performed on modified and non‐irradiated surfaces, using bi‐distilled water and FVA2 industrial oil. The angles are measured parallel and perpendicular to the line‐pattern orientation. The topographical analysis shows steep line‐pattern produced by laser. Raman analysis indicates that the laser irradiation does not significantly change the chemical species of the modified surfaces. The CA measurements elucidates that the parallel orientation provides a better wetting of the surface, because the laser line‐pattern acts as capillary flow channels, whereas the perpendicular orientation imposes energy barrier thus preventing wetting. As expected, the wetting coverage is more effective for larger than for smaller periodic structures, due to the larger area of flat contact. These novel results highlight the relevant use of LIMET to tailor the wetting properties of steel surfaces.     

Concentration‐Controlled Reversible Phase Transitions in Self‐Assembled Monolayers on HOPG Surfaces

Rational control of molecular ordering on surfaces and interfaces is vital in supramolecular chemistry and nanoscience. Here, a systematic scanning tunneling microscopy (STM) study for controlling the self‐assembly behavior of alkoxylated benzene (B‐OC_n) molecules on a HOPG surface is presented. Three different phases have been observed and, of great importance, they can transform to each other by modifying the solute concentration. Further studies, particularly in situ diluting and concentrating experiments, demonstrate that the transitions among the three phases are highly controllable and reversible, and are driven thermodynamically. In addition, it is found that concentration‐controlled reversible phase transitions are general for different chain lengths of B‐OC_n molecules. Such controllable and reversible phase transitions may have potential applications in the building of desirable functional organic thin films and provide a new understanding in thermodynamically driven self‐assembly of organic molecules on surfaces and interfaces.     

Polymer Brush Gradients by Adjusting the Functional Density Through Temperature Gradient

The group of silanes is one of the most abundant classes of molecules used for surface modification. In most studies, silanization is made from the vapor phase or solution. Here, an easy, robust, and fast way not only to modify, but also to map, control, and predict the wetting profiles on silicon surfaces after silanization and the final characteristics of a brush layer polymerized from this silane template profile are presented. The initiator molecule, 2‐bromo‐2‐methyl‐N‐3‐(triethoxysilyl) propyl propanamide (BTPAm), is spin‐casted on a silicon substrate and a thermal gradient is applied using a combinatorial approach for studying the influence of temperature on the spin‐casted silanes. Subsequently, polyacrylamide (PAAm) brushes are grown from the initiating end group of the BTPAm molecules through atom transfer radical polymerization (ATRP). Simulations of the heat distribution inside the silicon wafer allow both confirming the mapping of surface properties and designing desired profiles by predicting thermal distributions. An analytical expression for quantification is also provided. Thus, the wetting properties, surface roughness, and morphology of the brush layer after polymerization are mapped and correlated with the initial BTPAm gradient profile. The studies presented are envisioned to be of interest for designing surface profiles with different wetting properties, facilitating polymer brush growth, and to be used as predictive tools.     

Surface Energy and Surface Stability of Ag Nanocrystals at Elevated Temperatures and Their Dominance in Sublimation‐Induced Shape Evolution

The surface energy and surface stability of Ag nanocrystals (NCs) are under debate because the measurable values of the surface energy are very inconsistent, and the indices of the observed thermally stable surfaces are apparently in conflict. To clarify this issue, a transmission electron microscope is used to investigate these problems in situ with elaborately designed carbon‐shell‐capsulated Ag NCs. It is demonstrated that the {111} surfaces are still thermally stable at elevated temperatures, and the victory of the formation of {110} surfaces over {111} surfaces on the Ag NCs during sublimation is due to the special crystal geometry. It is found that the Ag NCs behave as quasiliquids during sublimation, and the cubic NCs represent a featured shape evolution, which is codetermined by both the wetting equilibrium at the Ag–C interface and the relaxation of the system surface energy. Small Ag NCs (≈10 nm) no longer maintain the wetting equilibrium observed in larger Ag NCs, and the crystal orientations of ultrafine Ag NCs (≈6 nm) can rotate to achieve further shape relaxation. Using sublimation kinetics, the mean surface energy of Ag NCs at 1073 K is calculated to be 1.1–1.3 J m^?2.     

Measurement of wetting properties of individual boron nitride nanotubes with the wilhelmy method using a nanotube-based force sensor

The wetting properties of individual boron nitride nanotubes (BNNTs) were studied with the Wilhelmy method in ambient conditions. A nanotube-based force sensor having a force resolution of 0.1 nN, calibrated with the wetting force method, was used to study the interactions between BNNTs and liquids in situ. The static contact angles of the liquids on BNNTs were evaluated, and the surface tension of the BNNT along with its surface tension components was determined based on the Owens and Wendt method and the van Oss-Chaudhury-Good acid-base theory.     

Bubble Meets Droplet: Particle‐Assisted Reconfiguration of Wetting Morphologies in Colloidal Multiphase Systems

Wetting phenomena are ubiquitous in nature and play key functions in various industrial processes and products. When a gas bubble encounters an oil droplet in an aqueous medium, it can experience either partial wetting or complete engulfment by the oil. Each of these morphologies can have practical benefits, and controlling the morphology is desirable for applications ranging from particle synthesis to oil recovery and gas flotation. It is known that the wetting of two fluids within a fluid medium depends on the balance of interfacial tensions and can thus be modified with surfactant additives. It is reported that colloidal particles, too, can be used to promote both wetting and dewetting in multifluid systems. This study demonstrates the surfactant‐free tuning and dynamic reconfiguration of bubble‐droplet morphologies with the help of cellulosic particles. It further shows that the effect can be attributed to particle adsorption at the fluid interfaces, which can be probed by interfacial tensiometry, making particle‐induced transitions in the wetting morphology predictable. Finally, particle adsorption at different rates to air–water and oil–water interfaces can even lead to slow, reentrant wetting behavior not familiar from particle‐free systems.     

Delaying Ice and Frost Formation Using Phase‐Switching Liquids

Preventing water droplets from transitioning to ice is advantageous for numerous applications. It is demonstrated that the use of certain phase‐change materials, which are in liquid state under ambient conditions and have melting point higher than the freezing point of water, referred herein as phase‐switching liquids (PSLs), can impede condensation–frosting lasting up to 300 and 15 times longer in bulk and surface infused state, respectively, compared to conventional surfaces under identical environmental conditions. The freezing delay is primarily a consequence of the release of trapped latent heat due to condensation, but is also affected by the solidified PSL surface morphology and its miscibility in water. Regardless of surface chemistry, PSL‐infused textured surfaces exhibit low droplet adhesion when operated below the corresponding melting point of the solidified PSLs, engendering ice and frost repellency even on hydrophilic substrates. Additionally, solidified PSL surfaces display varying degrees of optical transparency, can repel a variety of liquids, and self‐heal upon physical damage.     

Strongly anisotropic wetting on one-dimensional nanopatterned surfaces

This communication reports strongly anisotropic wetting behavior on one-dimensional nanopatterned surfaces. Contact angles, degree of anisotropy, and droplet distortion are measured on micro- and nanopatterned surfaces fabricated with interference lithography. Both the degree of anisotropy and the droplet distortion are extremely high as compared with previous reports because of the well-defined nanostructural morphology. The surface is manipulated to tune with the wetting from hydrophobic to hydrophilic while retaining the structural wetting anisotropy with a simple silica nanoparticle overcoat. The wetting mechanisms are discussed. Potential applications in microfluidic devices and evaporation-induced pattern formation are demonstrated.     

Liquid spreading on rough metal surfaces

The influence of surface roughness on the equilibrium spreading of liquids on aluminium and stainless steel surfaces with well-characterized rough machine finishes and a well-defined technique of attaining liquid drop equilibrium has been experimentally studied. The surfaces were prepared under practical conditions, i.e. without rigorous purification or attempting to eliminate anisotropy or microheterogeneities in surface-free energy. Depending on the type of roughness, i.e. spiral-grooved, radial-grooved and porous, the advancing contact angle was in approximate agreement with one of the classical contact angle/surface roughness equations. Capillary channelling along machine grooves profoundly affected the spreading and wetting behaviour and was highly dependent on the orientation and texture of roughness. Although the observed spreading was generally smooth on all surfaces it was probable that microscopic surface asperities produce small-scale non-equilibrium contact line movements and are responsible for the extensive wetting hysteresis during drop retraction.     

To Wet or Not to Wet: That Is the Question

Wetting transitions have been predicted and observed to occur for various combinations of fluids and surfaces. This paper describes the origin of such transitions, for liquid films on solid surfaces, in terms of the gas-surface interaction potentials V(r), which depend on the specific adsorption system. The transitions of light inert gases and H₂ molecules on alkali metal surfaces have been explored extensively and are relatively well understood in terms of the least attractive adsorption interactions in nature. Much less thoroughly investigated are wetting transitions of Hg, H₂O, heavy inert gases and other molecular films. The basic idea is that nonwetting occurs, for energetic reasons, if the adsorption potential’s well-depth D is smaller than, or comparable to, the well-depth ε of the adsorbate-adsorbate mutual interaction. At the wetting temperature, T _w, the transition to wetting occurs, for entropic reasons, when the liquid’s surface tension is sufficiently small that the free energy cost in forming a thick film is sufficiently compensated by the fluid-surface interaction energy. Guidelines useful for exploring wetting transitions of other systems are analyzed, in terms of generic criteria involving the “simple model”, which yields results in terms of gas-surface interaction parameters and thermodynamic properties of the bulk adsorbate.     

Enhancement of Magneto‐Mechanical Actuation of Micropillar Arrays by Anisotropic Stress Distribution

Magnetically active shape‐reconfigurable microarrays undergo programmed actuation according to the arrangement of magnetic dipoles within the structures, achieving complex twisting and bending deformations. Cylindrical micropillars have been widely used to date, whose circular cross‐sections lead to identical actuation regardless of the actuating direction. In this study, micropillars with triangular or rectangular cross‐sections are designed and fabricated to introduce preferential actuation directions and explore the limits of their actuation. Using such structures, controlled liquid wetting is demonstrated on micropillar surfaces. Liquid droplets pinned on magnetic micropillar arrays undergo directional spreading when the pillars are actuated as depinning of the droplets is enabled only in certain directions. The enhanced deformation due to direction dependent magneto‐mechanical actuation suggests that micropillar arrays can be fundamentally tailored to possess application specific responses and opens up opportunities to exploit more complex designs such as micropillars with polygonal cross sections. Such tunable wetting of liquids on microarray surfaces has potential to improve printing technologies via contactless reconfiguration of stamp geometry by magnetic field manipulation.     

Laser‐Induced Graphene in Controlled Atmospheres: From Superhydrophilic to Superhydrophobic Surfaces

The modification of graphene‐based materials is an important topic in the field of materials research. This study aims to expand the range of properties for laser‐induced graphene (LIG), specifically to tune the hydrophobicity and hydrophilicity of the LIG surfaces. While LIG is normally prepared in the air, here, using selected gas atmospheres, a large change in the water contact angle on the as‐prepared LIG surfaces has been observed, from 0° (superhydrophilic) when using O₂ or air, to >150° (superhydrophobic) when using Ar or H₂. Characterization of the newly derived surfaces shows that the different wetting properties are due to the surface morphology and chemical composition of the LIG. Applications of the superhydrophobic LIG are shown in oil/water separation as well as anti‐icing surfaces, while the versatility of the controlled atmosphere chamber fabrication method is demonstrated through the improved microsupercapacitor performance generated from LIG films prepared in an O₂ atmosphere.     

NIR‐Triggered Photothermal Responsive Coatings with Remote and Localized Tunable Underwater Oil Adhesion

Tunable underwater oil adhesion is a critical issue in interfacial science and industrial applications. Although much progress has been made to date, development of novel smart coating materials that can selectively change the wetting property at different areas is considerably scarce. Here, a simple strategy is proposed to fabricate photothermal responsive coatings, which can change the oil adhesion behavior from low‐adhesive rolling state to high‐adhesive pinning state for a variety of oily liquids in a remote, local, and reversible manner. Owing to this unique controllability, the adhesion and no‐adhesion of oil droplets on the coated surfaces can be easily manipulated by remote and local near‐infrared radiation.     

The relevance of the surface structure and surface chemistry of carbon fibres in their adhesion to high temperature thermoplastics

The paper is concerned with the surface chemistry of several different carbon fibres subjected to various surface treatments. The microstructure and nanostructures of these fibres were investigated in the Part I of this series of papers. For analysis of the surface chemistry of the fibres, X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) were employed; the first method was used for identification and semi-quantitative determination of functional surface groups, while the second method was used for a quantitative determination of these groups. The possible interactions of the various carbon-fibre surfaces due to different surface treatments (and therefore to different functional groups) were analysed by wetting studies using the Wilhelmy technique and aqueous solutions of different pH values as test liquids. By variation of the pH value of the test liquids, the distinct acid-base complexes that formed with the functional groups were identified. The same test liquids were used for characterization of the surface chemistry of the high-temperature thermoplastics (polycarbonate and polyethersulphone) used as matrix materials in the fabrication of the composites in this study. Acid-base interactions at the carbon-fibre surfaces are mainly determined by carboxylic groups of different acidity. The concentration of these groups as determined by desorption of carbon dioxide up to 500 °C is shown to be directly proportional to the measured work of adhesion of each group.     

Antibioadhesive Superhydrophobic Syringe Needles Inspired by Mussels and Lotus Leafs

Fouling of surfaces due to bioadhesion is one of the hurdles in terms of their practical applications. Inspired by mussel and lotus leaf, antibioadhesive superhydrophobic syringe needles are fabricated by sequential bonding of polydopamine, Ag nanoparticles, and 1H,1H,2H,2H‐perfluorodecanethiol (PFDT). The morphology and surface chemical composition of the needles are characterized. The wetting properties and antibioadhesive properties of the needles are evaluated by contact angle (CA) and sliding angle (SA) of water and various aqueous solutions, and their residues on the needles after repeatedly used for liquid handling. The superhydrophobic needles show a rough surface with a layer of PFDT, which endow them with very high CA and low SA for water and various aqueous solutions. In addition, the aqueous solutions are in the Cassie–Baxter state on the surface of the superhydrophobic needles. Thus, the adhesion force between the superhydrophobic needles and aqueous solutions is very low. This endues the needles with excellent antibioadhesive properties and can be repeatedly used for withdrawing and dispensing various aqueous solutions. Moreover, the superhydrophobic needles can be used for the transport of aqueous solutions at high accuracy. The antibioadhesive superhydrophobic needles may find applications in various fields such as liquids transport and inkjet printing devices.     

Potenziale von ionischen Flüssigkeiten als Elektrolyte in der Galvanotechnik

Application potentials of ionic liquids based electrolytes in electroplating Electroplating is essential for a variety of industries including, electronics, sensors, optics, automotive and aerospace. The electroplating industry, which dates back well over 100 years, is based solely on aqueous electrolytes. The advancement of new materials and increasing requirements to functionalized metallic surfaces however constricts the application potentials of classical electrolyte systems. New application fields for the electroplating industry could arise by further developments and optimisation of aprotic electrolytes based on ionic liquids. In this article a short insight will be presented to possibly applications of ionic liquids as metal electrolytes for surface technology and electroplating. After a short introduction to ionic liquids an overview will be given to relevant physical‐chemical properties of ionic liquids, like metal solubility, viscosity and electrical conductivity. In addition current experimental results from our laboratory to the metal deposition of aluminium, cobalt and palladium will be presented and discussed. The main focus was set in the choice of different electrolyte systems, deposition parameters and achieved surface morphology of the produced metal deposits.     

Wetting behavior of polymer coated nanoporous anodic alumina films: transition from super-hydrophilicity to super-hydrophobicity

We show that nanoporous anodic alumina films, with pore diameters in the range 10-80 nm, can be transformed from being very hydrophilic (or super-hydrophilic) to very hydrophobic (or super-hydrophobic) by coating the surface with a thin (2-3 nm) layer of a hydrophobic polymer. This dramatic transformation happens as a result of the interplay between surface morphology and surface chemistry. The coated surfaces exhibit 'sticky' hydrophobicity as a result of ingress of water into the pores by capillary action. The wetting parameters (contact angle and contact angle hysteresis) exhibit qualitatively different dependences on pore diameters in coated and uncoated films, which are explained by invoking appropriate models for wetting.     

Hyperhydrophilic rough surfaces and imaginary contact angles

The complete wetting of rough surfaces is only poorly understood, since the underlying phenomena can neither be described by the Cassie‐Baxter nor the Wenzel equation. An experimental accessiblility by the sessile drop method is also very limited. The term “superhydrophilicity” was an attempt to understand the wetting of rough surfaces, but a clear definition is still forthcoming, mainly because non‐superhydrophilic surfaces can also display a contact angle of zero. Since the Wilhelmy balance is based on force measurements, it offers a technology for obtaining signals during the whole wetting process. We have obtained evidence that additional forces occur during the complete wetting of rough surfaces and that mathematically contact angles for a hydrophilicity beyond the contact angle of zero can be defined by imaginary numbers. A hydrophilized TPS‐surface obtained by chemical wettability switching from a superhydrophobic surface has been previously characterized by dynamic imaginary contact angles of 20i°–21i° and near‐zero hysteresis. Here an extremely high wetting rate is demonstrated reaching a virtual imaginary contact angle of Θ_V,Adv > 3.5i° in less than 210 ms. For a rough surface displaying imaginary contact angles and extremely high wetting rates we suggest the term hyperhydrophilicity. Although, as will be shown, the physical basis of imaginary contact angles is still unclear, they significantly expand our methodology, the range of wettability measurements and the tools for analyzing rough hydrophilic surfaces. They may also form the basis for a new generation of rationally constructed medicinal surfaces.