Showing papers on "Wetting transition published in 2010"

The rose petal effect and the modes of superhydrophobicity.

Abstract: The wetting of rough surfaces remains a subject of active investigation by scientists. The contact angle (CA) is a traditional parameter used to characterize the hydrophobicity/philicity of a solid surface. However, it was found recently that high CAs can coexist with strong adhesion between water and a solid surface in the case of the so-called 'rose petal effect'. Several additional parameters have been proposed to characterize the interaction of water with a rough solid surface, including the CA hysteresis, the ability of water droplets to bounce off a solid surface, the tilt angle needed to initiate the flow of a droplet, and the normal and shear adhesion. It is clear now that wetting is not characterized by a single parameter, since several modes or regimes of wetting of a rough surface can exist, including the Wenzel, Cassie, lotus and petal. Understanding the wetting of rough surfaces is important in order to design non-adhesive surfaces for various applications.

Evaporation-triggered wetting transition for water droplets upon hydrophobic microstructures.

Abstract: When placed on rough hydrophobic surfaces, water droplets of diameter larger than a few millimeters can easily form pearls, as they are in the Cassie-Baxter state with air pockets trapped underneath the droplet. Intriguingly, a natural evaporating process can drive such a Fakir drop into a completely wetting (Wenzel) state. Our microscopic observations with simultaneous side and bottom views of evaporating droplets upon transparent hydrophobic microstructures elucidate the water-filling dynamics and suggest the mechanism of this evaporation-triggered transition. For the present material the wetting transition occurs when the water droplet size decreases to a few hundreds of micrometers in radius. We present a general global energy argument which estimates the interfacial energies depending on the drop size and can account for the critical radius for the transition.

Atomic layer deposition and abrupt wetting transitions on nonwoven polypropylene and woven cotton fabrics.

Abstract: Atomic layer deposition (ALD) of aluminum oxide on nonwoven polypropylene and woven cotton fabric materials can be used to transform and control fiber surface wetting properties. Infrared analysis shows that ALD can produce a uniform coating throughout the nonwoven polypropylene fiber matrix, and the amount of coating can be controlled by the number of ALD cycles. Upon coating by ALD aluminum oxide, nonwetting hydrophobic polypropylene fibers transition to either a metastable hydrophobic or a fully wetting hydrophilic state, consistent with well-known Cassie-Baxter and Wenzel models of surface wetting of roughened surfaces. The observed nonwetting/wetting transition depends on ALD process variables such as the number of ALD coating cycles and deposition temperature. Cotton fabrics coated with ALD aluminum oxide at moderate temperatures were also observed to transition from a natural wetting state to a metastable hydrophobic state and back to wetting depending on the number of ALD cycles. The transitions on cotton appear to be less sensitive to deposition temperature. The results provide insight into the effect of ALD film growth mechanisms on hydrophobic and hydrophilic polymers and fibrous structures. The ability to adjust and control surface energy, surface reactivity, and wettability of polymer and natural fiber systems using atomic layer deposition may enable a wide range of new applications for functional fiber-based systems.

Fully Reversible Transition from Wenzel to Cassie−Baxter States on Corrugated Superhydrophobic Surfaces

Abstract: Liquid drops on textured surfaces show different dynamical behaviors depending on their wetting states. They are extremely mobile when they are supported by composite solid−liquid−air interfaces (Cassie−Baxter state) and immobile when they fully wet the textured surfaces (Wenzel state). By reversibly switching between these two states, it will be possible to achieve control over the fluid dynamics. Unfortunately, these wetting transitions are usually prevented by surface energy barriers. We demonstrate here a new, simple design paradigm consisting of parallel grooves with an appropriate aspect ratio that allows for the controlled, barrierless, reversible switching of the wetting states upon application of electrowetting. We report a direct observation of the barrierless dynamical pathway for the reversible transitions between the Wenzel (collapsed) and Cassie−Baxter (suspended) states and present a theory that accounts for these transitions, including detailed lattice Boltzmann simulations.

Tailoring Anisotropic Wetting Properties on Submicrometer-Scale Periodic Grooved Surfaces

Abstract: The use of simple plasma treatments and polymer deposition to tailor the anisotropic wetting properties of one-dimensional (1D) submicrometer-scale grooved surfaces, fabricated using interferometric lithography in photoresist polymer films, is reported. Strongly anisotropic wetting phenomena are observed for as-prepared 1D grooved surfaces for both positive and negative photoresists. Low-pressure plasma treatments with different gas compositions (e.g., CHF(3), CF(4), O(2)) are employed to tailor the anisotropic wetting properties from strongly anisotropic and hydrophobic to hydrophobic with very high contact angle and superhydrophilic with a smaller degree of wetting anisotropy and without changing the structural anisotropy. The change of the surface wetting properties for these 1D patterned surfaces is attributed to a change in surface chemical composition, monitored using XPS. In addition, the initial anisotropic wetting properties on 1D patterned samples could be modified by coating plasma treated samples with a thin layer of polymer. We also demonstrated that the wetting properties of 1D grooved surfaces in a Si substrate could be tuned with similar plasma treatments. The ability to tailor anisotropic wetting on 1D patterned surfaces will find many applications in microfluidic devices, lab-on-a-chip systems, microreactors, and self-cleaning surfaces.

Silicon surface structure-controlled oleophobicity.

Abstract: Superoleophobic surfaces display contact angles >150° with liquids that have lower surface energies than does water. The design of superoleophobic surfaces requires an understanding of the effect of the geometrical shape of etched silicon surfaces on the contact angle and hysteresis observed when different liquids are brought into contact with these surfaces. This study used liquid-based metal-assisted etching and various silane treatments to create superoleophobic surfaces on a Si(111) surface. Etch conditions such as the etch time and etch solution concentration played critical roles in establishing the oleophobicity of Si(111). When compared to Young’s contact angle, the apparent contact angle showed a transition from a Cassie to a Wenzel state for low-surface-energy liquids as different silane treatments were applied to the silicon surface. These results demonstrated the relationship between the re-entrant angle of etched surface structures and the contact angle transition between Cassie and Wenzel be...

Molecular dynamics simulation of wetting behavior at CO2/water/solid interfaces

Abstract: We used molecular dynamics simulation to demonstrate the microscopic wetting behavior of two solid model surfaces for the first time. Hydrophilic and hydrophobic features were modeled in a dense CO2 fluid environment under various densities. The water droplet loses contact with the surface under the influence of higher density CO2 fluids on the hydrophobic surface. For the hydrophilic surface, no separation between the water droplet and the surface was observed. However, the contact angle of the water droplet on the hydrophilic surface was found to increase with the fluid density. The effect of dense CO2 fluid on the surface wettability can be interpreted in terms of enhanced interactions from the surrounding CO2 molecules.

Effect of polyethylene glycol on hydrophilic TiO2 films: Porosity-driven superhydrophilicity

Abstract: Polyethylene glycol (PEG) 2000 was used as a templating reagent to synthesize porous TiO2 thin film by sol–gel process. The nanopores resulting from the presence of growing cracks were ultimately formed on the surface when PEG content was higher than the critical value. Surprisingly, stable pore structure disappeared and surface became fluctuating and dehiscent after PEG amount increased to 0.02 M. Besides, two main hypotheses were proposed in order to explain this superhydrophilic behavior, namely the Wenzel and Cassie wetting impregnating models. Furthermore, the transition between these two wetting regimes was investigated and the criteria for the design and construction of Cassie impregnating wetting surface was also discussed. It was found that Cassie state shifting from Wenzel state could be easily achieved with increasing hole depth on TiO2 surface. The study of transition between Wenzel and Cassie impregnating wetting regimes on porous films provides valuable wetting mechanism of porosity-driven wettability for the design of superhydrophilic surfaces.

Cassie-state wetting investigated by means of a hole-to-pillar density gradient.

Abstract: The superhydrophobicity of rough surfaces owes its existence to heterogeneous wetting. To investigate this phenomenon, density gradients of randomly placed holes and pillars have been fabricated by means of photolithography. On such surfaces, drops can be observed in the Cassie state over the full range of f1 (fraction of the drop’s footprint area in contact with the solid). The gradient was produced with four different surface chemistries: native PDMS (polydimethylsiloxane), perfluorosilanized PDMS, epoxy, and CH3-terminated thiols on gold. It was found that f1 is the key parameter influencing the static water contact angle. Advancing and receding contact angles at any given position on the gradient are sensitive to the type of surface feature—hole or pillar—that is prevalent. In addition, roll-off angles have been measured and found to be influenced not only by the drop weight but also by suction events, edge pinning, and f1.

A simple strategy to realize biomimetic surfaces with controlled anisotropic wetting

Abstract: The study of anisotropic wetting has become one of the most important research areas in biomimicry. However, realization of controlled anisotropic surfaces remains challenging. Here we investigated anisotropic wetting on grooves with different linewidth, period, and height fabricated by laser interference lithography and found that the anisotropy strongly depended on the height. The anisotropy significantly increased from 9° to 48° when the height was changed from 100 nm to 1.3 μm. This was interpreted by a thermodynamic model as a consequence of the increase of free energy barriers versus the height increase. According to the relationship, controlled anisotropic surfaces were rapidly realized by adjusting the grooves’ height that was simply accomplished by changing the resin thickness. Finally, the perpendicular contact angle was further enhanced to 131°±2° by surface modification, which was very close to 135°±3° of a common grass leaf.

Influence of surface charge on wetting kinetics.

Abstract: The wettability of a titania surface, partially covered with octadecyltrihydrosilane, has been investigated as a function of solution pH. The results show that surface charge affects both static wettability and wetting kinetics. The static contact angle decreases above and below the point of zero charge of the titania surface in a Lippman-like manner as the pH is altered. The dependence of dynamic contact angle on velocity is also affected by pH. The molecular-kinetic theory (MKT) is used to interpret the dynamic contact angle data. The frequency of molecular displacement κ(0) strongly varies with surface charge, whereas the mean molecular displacement length λ is essentially unaffected. There is an exponential dependence of contact-line friction upon work of adhesion, which is varied simply by altering the pH.

Wetting of nanopatterned grooved surfaces

Abstract: The wetting by perfluoromethylcyclohexane of a well-defined silicon grating with a channel width of 16 nm has been studied using transmission small angle x-ray scattering. Prefilling, capillary filling, and postfilling wetting regimes have been identified. A detailed comparison of the data with theory reveals the importance of long-ranged substrate-fluid and fluid-fluid interactions for determining the wetting behavior on these length scales, especially at the onset of capillary condensation and in the prefilling regime.

Superoleophobic behavior induced by nanofeatures on oleophilic surfaces.

Abstract: The control of surface wetting properties to produce robust and strong hydrophobic and oleophobic effects on intrinsically oleophilic surfaces is at the heart of many technological applications. In this paper, we explore the conditions to observe such effects when the roughness of the substrate is of fractal nature and consists of nanofeatures obtained by the ion track etching technique. The wetting properties were investigated using eight different liquids with surface tensions γ varying from 18 to 72 mN m-1.While it is observed that all the tested oils readily wet the flat substrates, it is found that the contact angles are systematically exalted on the rough surfaces even for the liquids with very low surface tension. For liquids with γ ≥ 25 mN m-1 an oleophobic behavior is clearly induced by the nanostructuration. For liquids with γ < 25 mN m-1, although the contact angle is enhanced on the nanorough surfaces, it conserves its oleophilic character (θ* lower than 90°). Moreover, our experiments show that even in the case of hexane, liquid having the lowest surface tension, the homogeneous wetting (Wenzel state) is never reached. This high resistance to liquid impregnation is discussed within the framework of recent approaches explaining the wetting properties of superoleophobic surfaces.

Wetting characteristics on hierarchical structures patterned by a femtosecond laser

Abstract: Micro-scale hierarchical structures consisting of parallel grooves decorated by embossed triangle patterns are prepared by femtosecond laser irradiation on silicon (Si) wafers. The effects of surface morphology on wetting properties are investigated, and the results show that increasing the vertex angle of the triangle and groove spacing will lead to the enhancement of wettability and anisotropy, respectively. The structured surfaces also exhibit high adhesive force with droplets remaining attached to the surface even when the sample is turned upside down. Furthermore, the evaporation process of a water droplet on such an anisotropic surface is characterized to study its dynamic wetting behavior.

Wetting Invasion and Retreat across a Corner Boundary

Abstract: The wetting phenomenon in the vicinity of a corner boundary is ubiquitous. The daily-life examples include the meniscus of water near the mouth of a container and the halt of the movement of a slid...

Does Young's equation hold on the nanoscale? A Monte Carlo test for the binary Lennard-Jones fluid

Abstract: When a phase-separated binary (A+B) mixture is exposed to a wall, that preferentially attracts one of the components, interfaces between A-rich and B-rich domains in general meet the wall making a contact angle ?. Young's equation describes this angle in terms of a balance between the A-B interfacial tension ?AB and the surface tensions ?wA, ?wB between, respectively, the A- and B-rich phases and the wall, ?ABcos??=?wA??wB. By Monte Carlo simulations of bridges, formed by one of the components in a binary Lennard-Jones liquid, connecting the two walls of a nanoscopic slit pore, ? is estimated from the inclination of the interfaces, as a function of the wall-fluid interaction strength. The information on the surface tension difference ?wA??wB are obtained independently from a new thermodynamic integration method, while ?AB is found from the finite-size scaling analysis of the concentration distribution function. We show that Young's equation describes the contact angles of the actual nanoscale interfaces for this model rather accurately and the location of the (first-order) wetting transition is estimated.

Wetting Transition Characteristics on Microstructured Hydrophobic Surfaces

Abstract: Hydrophobicity and wetting transition behavior of water droplets were investigated on microstructured hydrophobic rough surfaces with pillar arrays, fabricated by self-replication with hydrophobic polydimethylsiloxane(PDMS) together with the use of CNC machine. The surfaces consist of microscale pillars(diameter: 105 μm, height: 150 μm) with varying spacing-to-diameter ratio (s⁄d) ranging from ∼1.0 to ∼3.3. A de-ionized(DI) water droplet of 4.3 μl was placed on hydrophobic surfaces and contact angles(CA) were measured by the digital image processing algorithm. A wetting transition from the Cassie state to the Wenzel state was demonstrated depending on the values of s⁄d, from ∼1.81 to ∼2.95. In the transition regime, a partial penetration of liquid meniscus which moves downward in the groove formed by four pillar posts was observed. It was also found that the contact angle prediction using the Cassie-Baxter equation showed fairly good agreement with experimental data, whereas in the transition regime, the rapid decrease in CA was found.

Design of superhydrophobic ultraoleophobic NyCo

Abstract: The apparent contact angles of dodecane droplets deposited on a 50:50 nylon:cotton blended woven fabric (NyCo) were measured, and the characteristics required for an ultraoleophobic surface were described. The metastable Cassie–Baxter model, a transition status from the original Cassie–Baxter model to the Wenzel model, was investigated to design ultraoleophobic surfaces and to understand the wetting behavior of such surfaces. Using chemical and geometrical modifications of NyCo, a surface having contact angles to dodecane of greater than 150° and water contact angles of greater than 165° has been prepared. Good agreement between the predicted and measured contact angles was obtained. Developing a superhydrophobic ultraoleophobic material has been achieved by two criteria: a low-surface-energy and a properly designed surface morphology.

Droplets wetting on filament rails: Surface energy and morphology transition

Abstract: The morphology of liquid droplets wetting on filaments depends on the filament configuration, droplet volume, and contact angle. A stable morphology is the one that minimizes the potential energy of the droplet–filament system, while morphology transition may happen when an intermediate state exists which corresponds to a higher potential energy. This paper aims to explore such morphology transition of droplet wetting on filament rails made of two parallel identical microfilaments. Detailed numerical simulations were performed to extract the surface energy of the droplet–filament system at varying filament spacings, droplet volumes, and contact angles. Critical conditions of the morphology transition between two symmetrical wetting morphologies (i.e., liquid droplet bridge and barrel-shaped droplet) were determined. A family of characteristic curves in terms of the dimensionless droplet volume vs the filament spacing at varying contact angles was obtained, which can be used as a universal law to govern the morphology transition for such droplet–filament rail systems. The results and concepts presented in this work can be extended to broad wetting systems and utilized for the analysis and design of microfluidic devices and testers based on droplet–filament systems.

Effect of self-affine fractal characteristics of surfaces on wetting

Abstract: In this letter, we show experimentally that the wetting property of a solid surface crucially depends on the surface morphological parameters such as; (1) root mean square (rms) roughness σ, (2) in-plane roughness correlation length ξ, and (3) roughness exponent α of the self-affine surface. We have shown that the contact angle monotonically decreases with the increase in the rms local surface slope ρ (∝σ/ξα). We have shown that the same solid surface can be made hydrophobic or hydrophilic by merely tuning these self-affine surface morphological parameters.

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

Abstract: 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 °C to 270 °C. In regime I (65 °C to 110 °C), the contact angle of water droplets exhibit a slight decrease from 108° to 105° and an accompanying significant decrease in droplet lifetime (τ) from ∼111 s to ∼30 s is observed. In regime II (120 °C to 190 °C), τ remains constant at ∼20 s however the contact angle significantly increases from 127° to 158° - that is we enter a superhydrophobic regime on a flat surface. In this regime the droplet remains stationary on the surface. Regime III (210 °C to 270 °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°. 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 °C up to 365 °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 °C for the hydrophilic surfaces.

Wetting Properties of Molten Silicon with Graphite Materials

Abstract: The wetting behavior of refractory materials by molten silicon is important in the refining and casting of silicon with respect to production of low-cost solar cells. Here we studied the wetting properties of several graphite materials by molten silicon. These materials are used in the photovoltaic industry. The sessile drop method is employed to measure the contact angles. Initially, molten silicon does not wet graphite materials. The initial contact angles measured are approximately 120 deg. Molten silicon will react with C to form β-SiC and to infiltrate the refractory. Because losses of Si should be minimized, infiltration of Si into the refractory also is a problem. Surface roughness increases the contact area between Si and refractory and thus the loss of Si. Equilibrium wetting angles of 0 deg to 31 deg are measured. With increasing surface roughness, the equilibrium wetting angles decrease. The results show that the infiltration depth of molten silicon increases with the average pore size of graphite materials.

Molecular Simulation Study of Anisotropic Wetting

Abstract: We study anisotropic wetting in systems governed by Lennard−Jones interactions. Molecular simulation is used to obtain the macroscopic contact angle a fluid adopts on face-centered-, body-centered-, and simple-cubic lattices with the (100), (110), or (111) face in contact with the fluid. Several amorphous substrates are also examined. Substrates are modeled as a static collection of particles. For a given set of calculations, the atomistic density of the substrate and the particle−particle interactions (surface−fluid and fluid−fluid) remain fixed. These constraints enable us to focus on the extent to which substrate structure influences the contact angle. Three substrate−fluid interaction strengths are considered, which provide wetting conditions that span from near-dry to near-wet. Our results indicate that the manner in which particles are organized within the substrate significantly influences the contact angle. For strong substrates (near-wet case), a change in the substrate structure can change the c...

Calculation of interfacial properties via free-energy-based molecular simulation: The influence of system size.

Abstract: We examine several issues related to the calculation of interfacial properties via analysis of an interface potential obtained from grand canonical Monte Carlo simulation. Two model systems are examined. One includes a monatomic Lennard-Jones fluid that interacts with a structureless substrate via a long-ranged substrate potential. The second model contains a monatomic Lennard-Jones fluid that interacts with an atomistically detailed substrate via a short-ranged potential. Our results are presented within the context of locating the wetting point. Two methods are used to compute the wetting temperature. In both cases we examine the system size dependence of the key property used to deduce the wetting temperature as well as the robustness of the scaling relationship employed to describe the evolution of this property with temperature near the wetting point. In the first approach we identify the wetting transition as the point at which the prewetting and bulk saturation curves meet. In this case, the prewetting saturation chemical potential is the key quantity of interest. In the second approach we find the point at which the spreading coefficient evaluates to zero. We find that the effect of system size is adequately described by simple scaling functions. Moreover, estimates of the wetting temperature for finite-sized systems characterized by a linear dimension greater than 12 fluid diameters differ by less than 1% from an otherwise equivalent macroscopic system. Modification of the details regarding the use of simulation data to compute the wetting temperature can also produce a shift in this quantity of up to 1%. As part of this study, we also examine techniques for describing the shape of the interface potential at a relatively high surface density. This analysis is particularly relevant for systems with long-ranged substrate potentials for which the interface potential approaches a limiting value asymptotically.

Vapor stabilizing substrates for superhydrophobicity and superslip.

Abstract: The success of rough substrates designed for superhydrophobicity and superslip relies crucially on the presence of air pockets in the roughness grooves. This air is supplied by the surrounding environment. However, if the rough substrates are used in enclosed configurations, such as in fluidic networks, the air pockets may not be sustained in the roughness grooves and may diffuse out. In this work, a design approach based on sustaining a vapor phase of the liquid in the roughness grooves, instead of relying on the presence of air, is explored. The resulting surfaces, referred to as vapor stabilizing substrates, are deemed to be more robust against wetting transition even if no air is present.

Hydrophobic forces in the wetting films of water formed on xanthate-coated gold surfaces

Abstract: The kinetics of thinning of water films on hydrophobic gold substrates has been studied using the thin film pressure balance (TFPB) technique. The changes in the thickness of the wetting films have been monitored by recording the profiles of the dimpled films as a function of time using a high-speed video camera. It was found that the kinetics, measured at the barrier rim of a wetting film formed on a hydrophilic silica surface, could be predicted using the Reynolds lubrication approximation with the no-slip boundary condition. However, the wetting films formed on hydrophobized gold substrates thinned much faster, and the kinetics increased with increasing hydrophobicity. The data obtained with gold surfaces of different hydrophobicities have been fitted to the Reynolds approximation to determine the hydrophobic force constants (K132) of a power law. K132 increased with increasing contact angle and decreased with electrolyte (NaCl) concentration. It was also found that the K132 values can be predicted from the hydrophobic force constants (K131) for the interaction between hydrophobic surfaces and the same (K232) for the foam films using the geometric mean combining rule that is frequently used to predict asymmetric molecular forces from symmetric ones.