Superhydrophobic Regime Research Articles

Self-regenerative superhydrophobic metallic coatings with enhanced durability

Lotus-inspired superhydrophobic surfaces are highly sought owing to exquisite properties such as de-wetting, self-cleaning, anti-corrosion, anti-fouling. However, their use is limited due to low durability, high processing cost, and scalability issues. In the present work, we used low cost and simple flame spraying for developing self-regenerative superhydrophobic metallic coatings with high durability and corrosion resistance. The aluminum coatings were developed on low-carbon steel at optimized parameters, followed by annealing at different temperatures. The developed coating showed hierarchical structure with nanoscale features superimposed on microscale lamellae. The nano-sponge like morphology obtained through flame spraying after annealing transformed into nano-spheroid with reduced microscale porosity. The fabricated aluminum coatings inherently exhibited superhydrophilicity, which transformed to superhydrophobicity after silanization. The coatings exhibited advancing angles >150° and receding angles >148° with a 5° sliding angle. The change in morphology with annealing showed a significant influence on wettability, adhesion, and electrochemical corrosion. The contact angles decreased slightly with annealing temperature while still in the superhydrophobic regime. The adhesion of the coatings with the water droplet increased from ~2 μN to 7.3 μN with the increase in annealing temperatures while still being less than that of the lotus leaf (18 μN). The electrochemical corrosion resistance of the coatings showed significant improvement after annealing, which is related to the change in microporosity and formation of stable oxides as verified by x-ray photoelectron spectroscopy (XPS). The current density decreased from 0.48 μA/cm2 to 0.06 μA/cm2 due to stable passive layer formation. The coatings showed increased durability against abrasion, and impacting droplets due to improved inter-lamellar cohesion and increased adhesion with the substrate. The present work provide a low cost and facile solution for the development of superhydrophobic coatings for various applications.

Integrated molecular dynamics and experimental approach to characterize low-free-energy perfluoro-decyl-acrylate (PFDA) coated silicon

Low-free-energy surfaces have attracted an intense academic and industrial interest over the last decade. A reduction of the surface free energy (SFE) has been found to enhance self-cleaning, hydrophobic, and non-fouling properties of surfaces, which are highly desirable in many industrial applications. However, tuning the surface chemistry and topography to achieve tailored low free energy surfaces has been found extremely challenging. In this work, we first show that an accurate refinement of the atmospheric plasma technique guarantees a polymeric coating near to the super-hydrophobic regime. Second, by coupling modelling and experimental measurements we suggest a reliable workflow for the surface characterization and smart design. Specifically, the case study proposed in this contest is capable of quantitatively distinguishing the contribution of a Perfluoro Decyl Acrylate (PFDA) coating, and hence decoupling the role of surface chemistry and roughness, in the reduction of the surface free energy of a bare silicon sample. Beyond the specific case study, our results also emphasize that a synergistic combination of models and experiments can unveil the optimal pathway for designing low-free-energy surfaces.

The Role of the Surface Nano-Roughness on the Wettability Performance of Microstructured Metallic Surface Using Direct Laser Interference Patterning.

Superhydrophobic natural surfaces usually have multiple levels of structure hierarchy, particularly microstructures covered with nano-roughness. The multi-scale nature of such a surface reduces the wetting of water and oils, and supports self-cleaning properties. In this work, in order to broaden our understanding of the wetting properties of technical surfaces, biomimetic surface patterns were fabricated on stainless steel with single and multi-scale periodic structures using direct laser interference patterning (DLIP). Micropillars with a spatial period of 5.5 µm and a structural depth of 4.2 µm were fabricated and covered by a sub-micro roughness by using ultrashort laser pulses, thus obtaining a hierarchical geometry. In order to distinguish the influence of the different features on the wettability behavior, a nanosecond laser source was used to melt the nano-roughness, and thus to obtain single-scale patterns. Then, a systematic comparison between the single- and multi-scale structures was performed. Although, the treated surfaces showed hydrophilic behavior directly after the laser treatment, over time they reached a steady-state hydrophobic condition. However, the multi-scale structured metal showed a contact angle 31° higher than the single-scale geometry when the steady-state conditions were reached. Furthermore, the impact of the surface chemistry was investigated by energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) analyses. Finally, a hydrophobizing agent was applied to the laser treated samples in order to further enhance the water contact angles and to determine the pure contribution of the surface topography. In the latter case, the multi-scale periodic microstructures reached static contact angles of 152° ± 2° and a contact angle hysteresis of only 4° ± 2°, while the single-scale structures did not show superhydrophobic behavior. These results definitely suggest that multi-scale DLIP structures in conjunction with a surface chemistry modification can promote a superhydrophobic regime.

TiO2/SnO2 and SnO2/TiO2 heterostructures as photocatalysts for degradation of stearic acid and methylene blue under UV irradiation

The effect TiO2/SnO2 (TiO2 is on the top and SnO2 at the bottom) and SnO2/TiO2 (SnO2 is on the top and TiO2 at the bottom) heterostructures towards photocatalytic degradation of stearic acid and methylene blue is reported. The two heterostructures, wherein SnO2 (or TiO2) is the top or bottom layer, are prepared by the sol-gel technique. TiO2/SnO2 film exhibit better photocatalytic activity as compared to SnO2/TiO2, TiO2 and SnO2 films. Wettability studies reveal that TiO2/SnO2 films modified with stearic acid (8 mM concentration of stearic acid in ethyl alcohol) tend towards superhydrophobic regime with contact angle as high as 137°. Further, TiO2/SnO2 films are efficient in degrading the stearic acid under UV irradiation. Interestingly, TiO2/SnO2 films that display flake like structures, with increased charge separation, are responsible for better degradation of pollutants under UV irradiation because of coupling heterostructures.

How Micropatterning and Surface Functionalization Affect the Wetting Behavior of ZnO Nanostructured Surfaces

Smart surfaces with a tunable wetting behavior between superhydrophobic and superhydrophilic states are of great interest for different applications, such as novel self‐cleaning/antifogging layers, or in view of the development of microfluidic devices and chemical/biochemical sensors of new generation. This work shows the tuning of the wetting behavior of hydrothermally grown zinc oxide nanowires (NWs) by combining the dual effect resulting from either surface chemical functionalization of the NWs and their growth on substrates prepatterned with different micro‐sized geometries. The characterization results highlight that vertically aligned ZnO NWs successfully cover all the microfeatures of the patterned substrates and can be successfully functionalized with aminopropyl groups. This chemical functionalization drives the initial transition of as‐prepared, superhydrophilic ZnO NWs toward a superhydrophobic regime. Then, protonation/deprotonation of amine groups, induced by their interaction with acid or basic media, changes the wetting state of ZnO NWs between hydrophilic and hydrophobic regimes. Reversible and successful switching from hydrophobic to hydrophilic states can be achieved under ad hoc conditions, in particular using appropriate micropatterns that do not favor the formation and entrapment of air bubbles.

Brushes of semiflexible polymers in equilibrium and under flow in a super-hydrophobic regime.

We performed molecular dynamics simulations to study the equilibrium and flow properties of a liquid in a nano-channel with confining surfaces coated with a layer of grafted semiflexible polymers. The coverage spans a wide range of grafting densities from essentially isolated chains to dense brushes. The end-grafted polymers were described by a bead spring model with a harmonic potential to include the bond stiffness of the chains. We varied the rigidity of the chains, from fully flexible polymers to rigid rods, in which the configurational entropy of the chains is negligible. The brush-liquid interaction was tuned to obtain a super-hydrophobic channel, in which the liquid did not penetrate the polymer brush, giving rise to a Cassie-Baxter state. Equilibrium properties such as brush height and bending energy were measured, varying the grafting density and the stiffness of the polymers. We also studied the characteristics of the brush-liquid interface and the morphology of the polymer chains supporting the liquid for different bending rigidities. Non-equilibrium simulations were performed, moving the walls of the channel in opposite directions at constant speed, obtaining a Couette velocity profile in the bulk liquid. The molecular degrees of freedom of the polymers were studied as a function of the Weissenberg number. Also, the violation of the no-slip boundary condition and the slip properties were analyzed as a function of the shear rate, grafting density and bending stiffness. At high grafting densities, a finite slip length independent of the shear rate or bending constant was found, while at low grafting densities a very interesting non-monotonic dependence on the bending constant is observed.

Partial Wetting of Aqueous Solutions on High Aspect Ratio Nanopillars with Hydrophilic Surface Finish

In semiconductor manufacturing, potential wetting issues with aqueous chemistries are becoming a concern as feature dimensions are continuously scaled down and novel materials with different wetting properties are introduced in new technology nodes. The wetting behavior of silicon nanopillars with different dimensions and surface modifications has been studied using static contact angle, decoration by etching, and attenuated total reflection infra-red spectroscopy (ATR-FTIR). The contact angle measurements showed a consistent deviation from the classic wetting models for patterned substrates with an hydrophilic surface termination. Under these conditions the decoration and ATR-FTIR studies gave evidence for partial wetting, with residual gas lasting for more than 30 min. It is proposed that this was resulting from the formation of long-lasting surface nanobubbles localized in-between or at the bottom of nanopillars. On the other hand the residual gas volume estimated by ATR-FTIR seemed too small to explain the contact angle deviations. It is proposed that the apparent extension of the superhydrophobic regime to lower contact angles resulted from modifications of the wettability of the surface of nanopillars caused by the manufacturing process. Both the formation of nanobubbles and the extension of the superhydrophobic regime may present challenges for aqueous cleaning in semiconductor manufacturing.

(Invited) Wetting Behavior of Aqueous Solutions on High Aspect Ratio Nanopillars with Hydrophilic Surface Finish

In semiconductor manufacturing, potential wetting issues are becoming a concern as feature dimensions are continuously scaled down and novel materials with different wetting properties are introduced in new technology nodes. The wetting behavior of silicon nanopillars with different dimensions and surface modifications has been studied using static contact angle (θ), attenuated total reflection infra-red spectroscopy (ATR-FTIR), and decoration by etching. The contact angle measurements showed a consistent deviation from the classic wetting models for patterned substrates with an hydrophilic surface termination (60°<θ < 80°). The ATR-FTIR and decoration studies gave evidence for partial wetting under these conditions, resulting from the formation of long-lasting nanobubbles. However the residual gas volumes estimated by ATR-FTIR were too small to explain the contact angle deviations. It is proposed that the apparent extension of the superhydrophobic regime to lower contact angles resulted from modifications caused by the manufacturing process to the wettability of the surface of nanopillars.

Wetting and hydrophobic modification of cellulose surfaces for paper applications

The use of papers in packaging and development of novel technological applications for paper substrates largely depends on the control of the hydrophilic properties of the cellulose fibres and improvement of the water-repellent properties. This review provides an actual summary of available literature on theoretical concepts and practical methods to improve the hydrophobicity of cellulose fibres and paper webs. In the first part, the interaction of water with cellulose fibres and paper webs is described at different levels ranging from the molecular scale over the micro- to macroscale fibre properties towards the interactions with porous substrates. The concepts for hydrophobicity and superhydrophobicity applied to cellulose fibres are reviewed, considering the surface chemistry and topographical features. In the second part, current techniques for hydrophobization based on sizing or direct fibre surface functionalization are described. Besides traditional sizing procedures, novelties in nanoparticle applications as hydrophobic sizing agent are reviewed. Novel trends in physical, chemical and nanotechnological fibre surface modifications or surface coatings are reviewed to turn the wetting properties into the superhydrophobic regime. The main future concern in controlling cellulose wettability lies in the development of sustainable modification techniques based on renewable resources such as biopolymers and green chemistry.

Biologically inspired tunable hydrophilic/hydrophobic surfaces: a copper oxide self-assembly multitier approach

In this study, a fabrication method for biologically inspired superhydrophobic micro- and nano-structured tier surfaces, each made of a self-assembled copper oxide, is presented. The method is controllable and applicable to bulk production when compared to existing high-end fabrication techniques. By modulating wet chemistry, different shapes and scales of tier structures were created. We demonstrated that their wetting behaviors are closely related to morphological information such as pitch, height and shape. To characterize their wetting behaviors, several experiments were designed and executed. In static water contact angle (WCA) measurements, morphological modulation led to wide WCA range (17°–95°). After hydrophobic self-assembly monolayer of 1-dodecanethiol, their WCA was escalated into superhydrophobic regime. In dynamic WCA, the contact angle hysteresis is greatly reduced by hybridizing the micro- and nano-tier (multiple tiers) when compared to utilizing a single tier. Also, the modification of the surface structure influences the rate of evaporation. In an analytical approach, the multiple tiers show a lower surface free energy compared to that of the single tier. By hybridizing different scales and shapes of tiers–such as hemispheric and conic shapes–the multiple tiers can efficiently reduce the surface energy barrier. Eventually, these manipulations lead to a subtle WCA hysteresis during the liquid motion testing. The analytical results are consistent with the dynamic WCA measurements. The multiple tiers also stabilize the Cassie regime and result in an increased hydrophobicity, which is more than when a single tier is employed.

Guided Transport of Water Droplets on Superhydrophobic–Hydrophilic Patterned Si Nanowires

We present a facile method to fabricate hydrophilic patterns in superhydrophobic Si nanowire (NW) arrays for guiding water droplets. The superhydrophobic Si NW arrays were obtained by simple dip-coating of dodecyltrichlorosilane (DTS). The water contact angles (CAs) of DTS-coated Si NW arrays drastically increased and saturated at the superhydrophobic regime (water CA ≥ 150°) as the lengths of NWs increased. The demonstrated superhydrophobic surfaces show an extreme water repellent property and small CA hysteresis of less than 7°, which enable the water droplets to easily roll off. The wettability of the DTS-coated Si NW arrays can be converted from superhydrophobic to hydrophilic via UV-enhanced photodecomposition of the DTS, and such wettability conversion was reproducible on the same surfaces by repeating the DTS coating and photodecomposition processes. The resulting water guiding tracks were successfully demonstrated via selective patterning of the hydrophilic region on superhydrophobic Si NW arrays, which could enable water droplets to move along defined trajectories.

Simulations of microfluidic droplet formation using the two-phase level set method

Microdroplet formation is an emerging area of research due to its wide-ranging applications within microfluidic based lab-on-a-chip devices. Our goal is to understand the dynamics of droplet formation in a microfluidic T-junction in order to optimize the operation of the microfluidic device. Understanding of this process forms the basis of many potential applications: synthesis of new materials, formulation of products in pharmaceutical, cosmetics and food industries. The two-phase level set method, which is ideally suited for tracking the interfaces between two immiscible fluids, has been used to perform numerical simulations of droplet formation in a T-junction. Numerical predictions compare well with experimental observations. The influence of parameters such as flow rate ratio, capillary number, viscosity ratio and the interfacial tension between the two immiscible fluids is known to affect the physical processes of droplet generation. In this study the effects of surface wettability, which can be controlled by altering the contact angle, are investigated systematically. As competitive wetting between liquids in a two-phase flow can give rise to erratic flow patterns, it is often desirable to minimize this phenomenon as it can lead to a disruption of the regular production of uniform droplets. The numerical simulations predicted that wettability effects on droplet length are more prominent when the viscosity ratio λ (the quotient of the viscosity of the dispersed phase with the viscosity of the continuous phase) is O(1), compared to the situation when λ is O(0.1). The droplet size becomes independent of contact angle in the superhydrophobic regime for all capillary numbers. At a given value of interfacial tension, the droplet length is greater when λ is O(1) compared to the case when λ is O(0.1). The increase in droplet length with interfacial tension, σ , is a function of ln σ with the coefficients of the regression curves depending on the viscosity ratio.

Wetting properties of silicon films from alkyl-passivated particles produced by mechanochemical synthesis

A facile and efficient method using high energy ball milling (HEBM) to produce surfaces with a static and advancing contact angle in the superhydrophobic regime consisting of alkyl-passivated crystalline silicon particles is described. Deposition of the functionalized silicon material forms stable films on a variety of surfaces due to strong hydrophobic interactions between the individual particles. The process offers the ability to control the particle size from a micro-scale to a nano-scale region and thus to tune the surface roughness. Because of changing surface morphology and the decreasing surface roughness of the films due to the increasing milling times the static and dynamic contact angles follow a polynomial function with a maximum dynamic advancing contact angle of 171°. This trend is correlated to the commonly used Wenzel and Cassie-Baxter models.

Macroscopically flat and smooth superhydrophobic surfaces: Heating induced wetting transitions up to the Leidenfrost temperature

We present an investigation of the change in wettability of water droplets on 3 different flat, smooth substrates with an elevation in temperature. Two methods were employed. In the first method the droplet was placed on the substrate before it was heated and in the second method the droplets were induced to fall onto a preheated substrate. We find that the intrinsic wettability of the surface is important and that fundamentally different behavior is observed on a hydrophobic surface relative to hydrophilic surfaces. For the hydrophobic surface and employing the first method, we have observed three different regimes over the temperature range of 65 degrees C to 270 degrees C. In regime I (65 degrees C to 110 degrees C), the contact angle of water droplets exhibit a slight decrease from 108 degrees to 105 degrees and an accompanying significant decrease in droplet lifetime (tau) from approximately 111 s to approximately 30 s is observed. In regime II (120 degrees C to 190 degrees C), tau remains constant at approximately 20 s however the contact angle significantly increases from 127 degrees to 158 degrees--that is we enter a superhydrophobic regime on a flat surface. In this regime the droplet remains stationary on the surface. Regime III (210 degrees C to 270 degrees C), is the Leidenfrost regime in which the water droplet exhibits a rapid motion on the solid surface with a contact angle higher than 160 degrees. In comparison, the wetting behavior of a water droplet on two relatively hydrophilic surfaces (Au and GaAs) have also been investigated as a function of temperature. Here no wetting transition is observed from 65 degrees C up to 365 degrees C. In the second method, the wetting behavior on the hydrophobic surface is similar to that observed in the first method for temperatures below the Leidenfrost temperature and the water droplet rebounds from the solid surface at higher temperatures. Additionally, the Leidenfrost phenomenon can be observed above 280 degrees C for the hydrophilic surfaces.