Large Contact Angle Hysteresis Research Articles

A Study of Cationic Contamination and Mitigation on Gdl/MPL Wettability and Metallic Plate Corrosion for PEFCs

Cationic impurities on performance loss are investigated by incorporation of foreign cation into the MEA in a form of salt solution. The hydrophobic property of the gas diffusion layer (GDL) acts as a barrier for transport of aqueous contaminant solution to reach the catalyst-coated membrane (CCM). In our previous work, the role of the GDL in cationic contamination and mitigation has been studied by an ex-situ soak method and an in-situ injection. Here, we present the effect of cationic solution to the water management in the cell and the metallic plate corrosion. Wetting property changes of the contaminated GDL and micro-porous layer (MPL) were tested using a force tensiometer, which is measured in the sample mass changes by contacting and penetrating the liquid. The three-phase boundary contact of solid-liquid-gas onto the GDL is moving during the fuel cell operation, therefore dynamic contact angles were measured in advancing and receding angles by Wilhelmy balance method. The measuring sample was prepared by a double-sided adhesive tape to get the GDL or the MPL on both sides. Figure 1 shows water submersion cycles of the wetted surface force acting on Freudenberg C4 MPL-coated GDL for the as-received and after soaking in DI water and CaSO4solution at 80 °C for 100hr. The contact angle can be calculated with the solid perimeter and the liquid surface tension because the measured force is related to the liquid-solid surface tension. The hydrophilic plain-paper GDL of Freudenberg C4 was resulted in larger contact angle hysteresis then the hydrophobic MPL. As shown in Table 1, the contaminated GDL shows the lowest contact angle in both immerging and emerging to the liquid, whereas there was no difference in the wetting property of MPL for all three cases due to its hydrophobicity. The GDL wettability after soaking in the DI water was also reduced by aging effects in the hot water for 100hr. Figure 2 shows wetting processes for the dynamic contact angle while measuring the force on the sample due to wetting. Cationic impurities are also originated from the corrosion of bipolar plates, current collecting plates, end plates and even tube fittings and piping systems. In addition, the corrosion rate accelerates when cell/system is tested with contaminated salt solution and acidic cleaning solution. Therefore, anodic corrosion measurements namely, polarization measurement (RP), corrosion current (Icorr) and corrosion rate (ICR) are performed for stainless tube fittings, aluminum endplates and gold coated copper current collecting plates in various concentrations of CaSO4solution and cleaning solution of 10mM sulfuric acid. Acknowledg e ments Financial support from the Department of Energy (DOE)-EERE, DE-EE00000467 (University of Hawaii, prime contractor, Jean St-Pierre, PI) is greatly acknowledged.

Droplets impact on textured surfaces: Mesoscopic simulation of spreading dynamics

Superhydrophobic surfaces have attracted much attention due to their excellent water-repellent property. In the present study, droplets in the ideal Cassie state were focused on, and a particle-based numerical method, many-body dissipative particle dynamics, was employed to explore the mechanism of droplets impact on textured surfaces. A solid–fluid interaction with three linear weight functions was used to generate different wettability and a simple but efficient method was introduced to compute the contact angle. The simulated results show that the static contact angle is in good agreement with the Cassie–Baxter formula for smaller ∅S and Fa, but more deviation will be produced for larger ∅S and Fa, and it is related to the fact that the Cassie–Baxter theory does not consider the contact angle hysteresis effect in their formula. Furthermore, high impact velocity can induce large contact angle hysteresis on textured surfaces with larger ∅S and Fa. The typical time-based evolutions of the spreading diameter were simulated, and they were analyzed from an energy transformation viewpoint. These results also show that the dynamical properties of droplet, such as rebounding or pinning, contact time and maximum spreading diameters, largely depend on the comprehensive effects of the material wettability, fraction of the pillars and impact velocities of the droplets.

Periodic parallel array of nanopillars and nanoholes resulting from colloidal stripes patterned by geometrically confined evaporative self-assembly for unique anisotropic wetting.

In this paper we present an economical process to create anisotropic microtextures based on periodic parallel stripes of monolayer silica nanoparticles (NPs) patterned by geometrically confined evaporative self-assembly (GCESA). In the GCESA process, a straight meniscus of a colloidal dispersion is initially formed in an opened enclosure, which is composed of two parallel plates bounded by a U-shaped spacer sidewall on three sides with an evaporating outlet on the fourth side. Lateral evaporation of the colloidal dispersion leads to periodic "stick-slip" receding of the meniscus (evaporative front), as triggered by the "coffee-ring" effect, promoting the assembly of silica NPs into periodic parallel stripes. The morphology of stripes can be well controlled by tailoring process variables such as substrate wettability, NP concentration, temperature, and gap height, etc. Furthermore, arrayed patterns of nanopillars or nanoholes are generated on a silicon wafer using the as-prepared colloidal stripes as an etching mask or template. Such arrayed patterns can reveal unique anisotropic wetting properties, which have a large contact angle hysteresis viewing from both the parallel and perpendicular directions in addition to a large wetting anisotropy.

Rings, igloos, and pebbles of salt formed by drying saline drops.

It is well-known that evaporation of sessile drops with suspended particles like colloids and coffee powders can yield a variety of two-dimensional patterns depending on the particle shapes and internal flow patterns. Here we show that ordered three-dimensional structures can be built via evaporation of saline drops on highly hydrophobic substrates like pristine PP (polypropylene) with micropores and nanostructured low-surface-energy PP. On pristine PP having a high contact angle but a large contact angle hysteresis (CAH) with water, either rings or igloos of salt are formed depending on the salt concentration and evaporation rate. On nanostructured low-surface-energy PP having extreme water repellency with a very low CAH, pebbles of salt are formed regardless of salt concentration and evaporation rate. These observations lead us to conclude that combined effects of solubility, evaporation rate, and mobility of the contact line determine the final three-dimensional shape of the salt precipitate.

Analysis of condensation heat transfer enhancement with dropwise-filmwise hybrid surface: Droplet sizes effect

The maximum droplet radius during dropwise condensation can significantly influence the heat transfer characteristics. On the dropwise-filmwise hybrid surface, droplets on the dropwise region can remove through filmwise region spontaneously driven by the surface free energy difference between the dropwise and filmwise regions, and thus the maximum droplet radius (equivalent to the departure radius for the complete dropwise condensation) can be adjusted by the configurations of hybrid surface. The condensation heat transfer enhancement of steam with hybrid surface is investigated in this paper by integrating the dropwise and filmwise condensation heat transfer models. The influence of maximum droplet radius on heat flux of dropwise region, the liquid film thickness and heat flux of filmwise region are investigated, respectively. Accordingly, the heat transfer contributions of two regions to the heat transfer characteristics of hybrid surface are analyzed. Finally, the influences of surface subcooling degree, contact angle and contact angle hysteresis on heat transfer characteristics of hybrid surface are discussed. The results indicate that an optimum value of the maximum droplet radius exists to enhance the condensation heat transfer of steam with the hybrid surface, which is dependent on surface subcooling degree, filmwise region width and contact angle while independent on contact angle hysteresis. With the surface subcooling degree increases ranging from 1.0K to 10K, the optimum maximum radius of droplet on dropwise region increases from about 0.2mm to 0.375mm. The optimum maximum radius of droplet increases from about 0.25mm to 0.5mm when the filmwise region width increases from 0.5mm to 1.0mm. The optimum maximum radius of droplet decreases with the increase of the contact angle. Therefore, steam condensation heat transfer performance can be regulated and optimized by designing the hybrid surface appropriately. The results also indicate that heat transfer enhancement factor for the hybrid surface is larger than 1.0 for the low surface subcooling or the condensing surface with small contact angle or large contact angle hysteresis. Therefore, the hybrid surface can effectively enhance the condensation heat transfer for the special cases, such as those with low surface subcooling degree, or with small contact angle and large contact angle hysteresis. For the case discussed in this paper, the maximum enhancement factor can approach up to about 1.18.

Fabrication of superhydrophobic and highly oleophobic silicon-based surfaces via electroless etching method

This study reports on a simple method for the preparation of superhydrophobic and highly oleophobic nanostructured silicon surfaces. The technique relies on metal-assisted electroless etching of silicon in sodium tetrafluoroborate (NaBF4) aqueous solution. Then, silver particles were deposited on the obtained surfaces, changing their overall physical morphology. Finally, the surfaces were coated by either C4F8, a fluoropolymer deposited by plasma, or by SiOx overlayers chemically modified with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS) through silanization reaction. All these surfaces exhibit a superhydrophobic character (large apparent contact angle and low hysteresis with respect to water). In addition, they present high oleophobic properties, i.e. a high repellency to low surface energy liquids with various contact angle hysteresis, both depending on the morphology and type of coating.

Structural analysis and surface wettability of a novel alternated vinylidene cyanide with fluorinated vinyl ether copolymer

The structure, thermal properties and surface wettability of a novel alternating poly(VCN-alt-FAVE8) copolymer based on vinylidene cyanide (VCN) and perfluorooctyl ethyl vinyl ether (FAVE8) were investigated via both temperature-variable wide-angle X-ray powder diffraction (WAXD) and thin-film WAXD using a small angle incidence synchrotron radiation source, differential scanning calorimetry and contact angle measurements, respectively. The results of the WAXD and thin-film WAXD indicated that the alternating copolymer possessed a smectic liquid-crystalline phase at ambient temperature with a couple of fluoroalkyl side chains forming a bilayer with a 3.24 nm spacing and that the fluoroalkyl side chains in a thin-film on a Si wafer align perpendicularly to the surface. The isotropization temperature of the alternating copolymer was 150 °C, which was higher than the 29.5 °C for the poly(FAVE8) homopolymer. The spin-coated poly(VCN-alt-FAVE8) copolymer thin-film yielded high static contact angles for both water and oil similar to those for poly(perfluorooctyl ethyl acrylate), which is known as a superhydrophobic polymer. Dynamic contact angle and X-ray photoelectron spectroscopy measurements in both the dry and hydrated states suggested that the polar cyano and ether groups are enriched at the surface in the wet state, and this reorganization of the surface polar groups caused a large hysteresis of the contact angle. High ordered structure, thermal property and surface wettability of a novel alternated copolymer of vinylidene cyanide and perfluorooctyl ethyl vinyl ether were investigated with wide-angle X-ray diffraction, differential scanning calorimetry, X-ray photoelectron spectroscopy and contact angle measurement. The alternated copolymer showed a smectic liquid crystalline structure and extremely high isotropization temperature of 150 °C, compared with 29.5 °C for the perfluorooctyl ethyl vinyl ether homopolymer. The alternated copolymer also showed large contact angle hysteresis, indicating that surface reorganization of the polar cyano and ether groups occur.

Stability of flowing open fluidic channels

Open fluidic systems have a distinct advantage over enclosed channels in that the fluids exposed nature makes for easy external interaction, this finds uses in introduction of samples by adding liquid droplets or from the surrounding gaseous medium. This work investigates flowing open channels and films, which can potentially make use of the open section of the system as an external interface, before bringing the sample into an enclosed channel. Clearly, in this scenario a key factor is the stability of the flowing open fluid. The open channels investigated include a straight open channel defined by a narrow strip of solid surface, the edges of which allow large contact angle hysteresis, and a wider structure allowing for multiple inputs and outputs. A model is developed for fluid flow, and the findings used to describe the process of failure in both cases.

Applicability of Washburn capillary rise for determining contact angles of powders/porous materials

The Washburn capillary rise (WCR) technique has been widely utilized for determining contact angles of powders or porous materials; however, there are concerns regarding powder size and powder packing, especially for materials that exhibit large contact angle hysteresis. In this paper, some of these concerns were addressed. Due to the large water contact angle hysteresis on flat nylon 6/6 films, these films were ground into powders of different sizes and then used as model packing materials. The powders were packed in glass tubes to result in various packing structures that affected the penetration (i.e. advancing) rate of the test liquids. While all advancing contact angles obtained from WCR were found to be overestimated, more reasonable values were resulted when relatively large powders (e.g. 500–2000μm) were used to pack the tubes. With larger powders, the packing contained bigger voids and consequently lead to slower penetration rates of the liquids, hence a relatively smaller advancing contact angle. The smaller advancing contact angle obtained from the slower advancing rate was also observed by using the sessile drop method. To verify the applicability of using large powders (500–2000μm) for contact angle determination by using WCR, the advancing water contact angles of a bacterial cellulose/alginate composite sponge (BCA) with and without UV/ozone treatment were measured. The results showed that by using relatively large powders, WCR could be applied to obtain a reasonable advancing contact angle and assess the wettability change of complex porous materials.

From micro to nano reentrant structures: hysteresis on superomniphobic surfaces

The paper reports on the wetting characterization of two surfaces presenting reentrant shapes at micro- and nanoscale using low surface tension liquids (down to 28 mN/m). On the one hand, mushroom-like microstructures are fabricated by molding poly(dimethylsiloxane) (PDMS) onto a patterned sacrificial photoresist bilayer. On the other hand, zinc oxide nanostructures (ZnO NS) are synthesized by easy and fast chemical bath deposition technique. The PDMS and ZnO NS surfaces are then chemically modified with 1H,1H,2H,2H-perfluorodecyltrichlorosilane in vapor phase. Both PDMS and ZnO NS surfaces exhibit a large apparent contact angle (>150°) and contact angle hysteresis varying from 50° to a quasi-null value. This large discrepancy can be ascribed to the length scale and topography of the structures, promoting either a vertical imbibition or a lateral spreading within the roughness.

Drastically Lowered Protein Adsorption on Microbicidal Hydrophobic/Hydrophilic Polyelectrolyte Multilayers

Polyelectrolyte multilayer films assembled from a hydrophobic N-alkylated polyethylenimine and a hydrophilic polyacrylate were discovered to exhibit strong antifouling, as well as antimicrobial, activities. Surfaces coated with these layer-by-layer (LbL) films, which range from 6 to 10 bilayers (up to 45 nm in thickness), adsorbed up to 20 times less protein from blood plasma than the uncoated controls. The dependence of the antifouling activity on the nature of the polycation, as well as on assembly conditions and the number of layers in the LbL films, was investigated. Changing the hydrophobicity of the polycation altered the surface composition and the resistance to protein adsorption of the LbL films. Importantly, this resistance was greater for coated surfaces with the polyanion on top; for these films, the average zeta potential pointed to a near neutral surface charge, thus, presumably minimizing their electrostatic interactions with the protein. The film surface exhibited a large contact angle hysteresis, indicating a heterogeneous topology likely due to the existence of hydrophobic-hydrophilic regions on the surface. Scanning electron micrographs of the film surface revealed the existence of nanoscale domains. We hypothesize that the existence of hydrophobic/hydrophilic nanodomains, as well as surface charge neutrality, contributes to the LbL film's resistance to protein adsorption.

Single-Step Direct Fabrication of Pillar-on-Pore Hybrid Nanostructures in Anodizing Aluminum for Superior Superhydrophobic Efficiency

Conventional electrochemical anodizing processes of metals such as aluminum typically produce planar and homogeneous nanopore structures. If hydrophobically treated, such 2D planar and interconnected pore structures typically result in lower contact angle and larger contact angle hysteresis than 3D disconnected pillar structures and, hence, exhibit inferior superhydrophobic efficiency. In this study, we demonstrate for the first time that the anodizing parameters can be engineered to design novel pillar-on-pore (POP) hybrid nanostructures directly in a simple one-step fabrication process so that superior surface superhydrophobicity can also be realized effectively from the electrochemical anodization process. On the basis of the characteristic of forming a self-ordered porous morphology in a hexagonal array, the modulation of anodizing voltage and duration enabled the formulation of the hybrid-type nanostructures having controlled pillar morphology on top of a porous layer in both mild and hard anodization modes. The hybrid nanostructures of the anodized metal oxide layer initially enhanced the surface hydrophilicity significantly (i.e., superhydrophilic). However, after a hydrophobic monolayer coating, such hybrid nanostructures then showed superior superhydrophobic nonwetting properties not attainable by the plain nanoporous surfaces produced by conventional anodization conditions. The well-regulated anodization process suggests that electrochemical anodizing can expand its usefulness and efficacy to render various metallic substrates with great superhydrophilicity or -hydrophobicity by directly realizing pillar-like structures on top of a self-ordered nanoporous array through a simple one-step fabrication procedure.

Fabrication of raspberry SiO2/polystyrene particles and superhydrophobic particulate film with high adhesive force

We report a facile and novel strategy on the fabrication of well-defined raspberry SiO2/polystyrene (SiO2/PS) particles via radiation miniemulsion polymerization. Starting from methacryloxypropyltrimethoxysilane (MPS)-functionalized SiO2 particles (176 nm), raspberry SiO2/PS particles (257 nm) with a submicron SiO2 core decorated by nano-sized PS latex particles (58 nm) are obtained after γ-ray induced miniemulsion polymerization of styrene (St). It is found that MPS grafted density on the surface of submicron SiO2 particles, the weight ratio of St to SiO2 particles (WSt/o-SiO2), as well as the surfactant concentration will affect the morphology and wettability of the resultant SiO2/PS hybrid particles. When the well-defined raspberry SiO2/PS particles are deposited on a blank glass substrate, a dual-size roughness surface topology was observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). This film has a static water contact angle as high as 151°. However, this film exhibits a large contact angle hysteresis (∼116°) and strong adhesion to water. Furthermore, this kind of superhydrophobic particulate film can be used as a “mechanical hand” for transportation of small water droplets without loss, thus it may have potential applications in industrial fields.

Micro to nano: Surface size scale and superhydrophobicity

This work looks at the fundamental question of how the surface mobility of drops in the composite state is related to the size scale of the roughness features of the surface. To this end, relevant literature is first reviewed and the important terms are clarified. We then describe and discuss contact and roll-off angle measurements on a set of hydrophobicized silicon post surfaces for which all parameters except for the surface size scale were held constant. It was found that a critical transition from “sticky superhydrophobic” (composite state with large contact angle hysteresis) to “truly superhydrophobic” (composite state with low hysteresis) takes place as the size of the surface features reaches 1 μm.

Adhesion Properties of Self-Polymerized Dopamine Thin Film

We report an experimental study of the adhesion properties of polydopamine thin films and their bonding behavior on polymer, glass and metal surfaces. Dopamine is able to adsorb onto all surfaces and self-polymerize into a thin hydrophilic film. Dynamic contact angle measurements revealed a large contact angle hysteresis between advancing and receding angles and a possible hydration layer when exposed to water. Polydopamine-coated surfaces in air are relatively inert having a low self-adhesion compared with polydimethylsiloxane elastomer surfaces, reflecting the non- conformal, glassy nature of polydopamine thin films. The dopamine aqueous solution was found able to bond two rigid surfaces (e.g. aluminum and glass) but it might not be suitable for joining soft or flexible polymer surfaces as polydopamine films are glassy and subject to internal cracks induced by the mismatch of elastic modulus. The research findings provide insights into the potential application of dopamine self-polymerization for adhesive bonding or joining of dissimilar materials.

Pressure-driven flow in open fluidic channels

At the boundary between a hydrophilic and hydrophobic surface coating a large contact angle hysteresis exists which can be used to retain fluid on a flat surface, a similar effect exists at the edge of a solid surface. In this work, pressure-driven flow is used to create fluid flow through a fluid volume confined along a 1mm wide strip of glass. Very high flow rates are shown to be achievable, reaching a value of 500μL/min over a 30mm length; at such values the maximum flow velocity is found through modeling to be 0.13m/s. By consideration of the minimum energy state the shape a certain fluid volume will adopt on a strip of material are well known for static fluids, we demonstrate flow through the two key types, the case resembling a section of a cylinder and the case of a pronounced bulge. This combination of fluid constrained though locations of high contact angle hysteresis combined with induced flow allows applications in detection of air-borne contaminants, the detection of changing fluid composition, and easy interfacing between microfluidic system and external tools.

Slip-stick wetting and large contact angle hysteresis on wrinkled surfaces

Wetting on a corrugated surface that is formed via wrinkling of a hard skin layer formed by UV oxidation (UVO) of a poly(dimethylsiloxane) (PDMS) slab is studied using advancing and receding water contact angle measurements. The amplitude of the wrinkled pattern can be tuned through the pre-strain of the PDMS prior to surface oxidation. These valleys and peaks in the surface topography lead to anisotropic wetting by water droplets. As the droplet advances, the fluid is free to move along the direction parallel to the wrinkles, but the droplet moving orthogonal to the wrinkles encounters energy barriers due to the topography and slip-stick behavior is observed. As the wrinkle amplitude increases, anisotropy in the sessile droplet increases between parallel and perpendicular directions. For the drops receding perpendicular to the wrinkles formed at high strains, the contact angle tends to decrease steadily towards zero as the drop volume decreases, which can result in apparent hysteresis in the contact angle of over 100°. The wrinkled surfaces can exhibit high sessile and advancing contact angles (>115°), but the receding angle in these cases is generally vanishing as the drop is removed. This effect results in micrometer sized drops remaining in the grooves for these highly wrinkled surfaces, while the flat analogous UVO-treated PDMS shows complete removal of all macroscopic water drops under similar conditions. These wetting characteristics should be considered if these wrinkled surfaces are to be utilized in or as microfluidic devices.

Engineering Sticky Superomniphobic Surfaces on Transparent and Flexible PDMS Substrate

Following the achievement of superhydrophobicity which prevents water adhesion on a surface, superomniphobicity extends this high repellency property to a wide range of liquids, including oils, solvents, and other low surface energy liquids. Recent theoretical approaches have yield to specific microstructures design criterion to achieve such surfaces, leading to superomniphobic structured silicon substrate. To transfer this technology on a flexible substrate, we use a polydimethylsiloxane (PDMS) molding process followed by surface chemical modification. It results in so-called sticky superomniphobic surfaces, exhibiting large apparent contact angles (>150°) along with large contact angle hysteresis (>10°). We then focus on the modified Cassie equation, considering the 1D aspect of wetting, to explain the behavior of droplets on these surfaces and compare experimental data to previous works to confirm the validity of this model.

Investigation on hydrophobicity of lotus leaf: Experiment and theory

A detail experiment investigation on hydrophobicity of lotus leaf had been given, including contact angle, entrapped gas, micrometer and nanometer surface topographies. Four kinds of lotus leaf surfaces were involved: fresh front (FF), dried front (DF), fresh back (FB) and dried back (DB) surfaces. The basic mechanisms of the liquid spreading on a rough surface and the entrapped gas under a drop had been discussed. Based on the mechanisms and experiment, hydrophobicity of the four lotus leaf surfaces was analyzed. The investigation results showed that the hydrophobicity of the front surfaces was dominated by the entrapped gas under the drop and the absolute stable position of the contact line while the back surfaces by metastable positions of the contact line, resulting in the ultrahydrophobicity on the front surfaces and large contact angle hysteresis on the back surfaces. The effects of dual-size surface structures on the hydrophobicity were discussed in detail. Because the nano-pillars were firmly close together, there was no entrapped gas in the nano-structure on the front surfaces. The structure of micro-pillars was a main cause for the ultrahydrophobicity of the front surfaces. However, on the FB surface, the structure of nano-pillars was a main cause to induce entrapped gas.

Formation of superhydrophobic poly(dimethysiloxane) by ultrafast laser-induced surface modification

The formation of hemispherical nanostructures and microscaled papilla by ultrafast laser irradiation was found to be a potential method to generate superhydrophbic surface of synthetic polymers. Irradiation of femtosecond laser creates roughened poly(dimethylsiloxane) (PDMS) surface in nano- and microscales, of which topography fairly well imitate a Lotus leaf in nature. The modified surface showed superhydrophobicity with a contact angle higher than 170 degrees as well as sliding angle less than 3 degrees. We further demonstrated that negative replica of the processed PDMS surface exhibit large contact angle hysteresis with a sliding angle of 90 degrees while the positive replica maintains superhydrophobicity.