Water Contact Angle Hysteresis Research Articles

Fabrication and Characterization of Superhydrophobic Poly(vinylidene fluoride-co- hexafluoropropylene)/TiO2 Nanocomposite Coating for Corrosion Protection Applications

Superhydrophobic surfaces have a strong potential for implementation in variety of fields, including self-cleaning surfaces, biomedical and coatings for corrosion protection. Preparation of superhydrophobic surfaces requires both an optimum roughness and low surface energy; therefore, these surfaces are conventionally prepared employing two steps: roughening a surface and lowering its surface energy. In this study, the superhydrophobic Poly(vinylidene fluoride-co-hexafluoropropylene)/TiO2 nanocomposite coatings were fabricated using a simple one-step coating method on a carbon steel substrate to study the corrosion behavior in harsh environments. The surface wettability and morphology of the prepared coatings were characterized using the sessile droplet method and scanning electron microscopy, respectively. X-ray photoelectron spectroscopy and X-ray diffractometer were used to identify the elemental composition and chemical oxidation states of the prepared surface. The surface roughness of the coatings was measured using atomic force microscope. The chemical stability of the prepared coatings was inspected by immersion the specimens in different pH solutions. The results showed that stable superhydrophobic surfaces with high water contact angle and low contact angle hysteresis can be fabricated on the carbon steel surface. As well as, the corrosion resistant performance of the produced superhydrophobic surface on carbon steel was carried out by electrochemical impedance spectroscopy (EIS) measurements and potentiodynamic polarization in 3.5 wt % NaCl solution. The electrochemical corrosion tests revealed that the as-prepared superhydrophobic surface significantly improved the corrosion resistant of the C-steel substrate.

Robust superhydrophobic silicon without a low surface-energy hydrophobic coating.

Superhydrophobic surfaces without low surface-energy (hydrophobic) modification such as silanization or (fluoro)polymer coatings are crucial for water-repellent applications that need to survive under harsh UV or IR exposures and mechanical abrasion. In this work, robust low-hysteresis superhydrophobic surfaces are demonstrated using a novel hierarchical silicon structure without a low surface-energy coating. The proposed geometry produces superhydrophobicity out of silicon that is naturally hydrophilic. The structure is composed of collapsed silicon nanowires on top and bottom of T-shaped micropillars. Collapsed silicon nanowires cause superhydrophobicity due to nanoscale air pockets trapped below them. T-shaped micropillars significantly decrease the water contact angle hysteresis because microscale air pockets are trapped between them and can not easily escape. Robustness is studied under mechanical polishing, high-energy photoexposure, high temperature, high-pressure water shower, and different acidic and solvent environments. Mechanical abrasion damages the nanowires on top of micropillars, but those at the bottom survive. Small increase of hysteresis is seen, but the surface is still superhydrophobic after abrasion.

Polydimethylsiloxane-polymethacrylate block copolymers tethering quaternary ammonium salt groups for antimicrobial coating

Block copolymers PDMS-b-PDMAEMA were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization involving N,N-dimethylaminoethyl methacrylate (DMAEMA) by using poly(dimethylsiloxane) (PDMS) macro-chain transfer agent. And, the tertiary amino groups in PDMAEMA were quaternized with n-octyliodide to provide quaternary ammonium salts (QPDMAEMA). The well-defined copolymers generated composition variation and morphology evolvement on film surfaces, which were characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle measurements. The results indicated that the enrichment of QPDMAEMA brought about lower elemental ratios of Si/N on the film surfaces. The surface morphologies evolved with the variations of QPDMAEMA content, and the variation trend of film roughness was exactly opposite to that of water contact angle hysteresis. With regard to structure-antimicrobial relationships, the copolymer films had more evident antimicrobial activity against Gram-positive, Bacillus subtilis, and the surfaces with heterogeneous morphology and higher N+ content presented better antimicrobial activity. The functionalized copolymers based PDMS and quaternary ammonium salts materials have the potential applications as antimicrobial coatings.

Fabrication of hierarchical structures with ZnO nanowires on micropillars by UV soft imprinting and hydrothermal growth for a controlled morphology and wettability

Hierarchical structures of ZnO nanowires (NWs) with different lengths on well-defined micropillars are fabricated by UV soft imprinting and hydrothermal growth. X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray photoelectron spectroscope (XPS) are employed to characterize the crystal structure, morphology and chemical composition of the hierarchical structures of ZnO nanowires on micropillars. By varying the growth time, ZnO NWs of different lengths and morphology were obtained, showing distinct wettability which can be further modified by surface coating with octadecyltrichlorosilane (OTS). Our results show that under optimal growth conditions, the water contact angle (CA) and contact angle hysteresis (CAH) of the fabricated hierarchical structures after OTS surface treatment can reach 159±1° and 6°, respectively.

Facile immobilization of ag nanocluster on nanofibrous membrane for oil/water separation.

Superhydrophobic and superoleophilic electrospun nanofibrous membranes exhibiting excellent oil/water separation performance were green fabricated by a facile route combining the amination of electrospun polyacrylonitrile (APAN) nanofibers and immobilization of a Ag nanocluster with an electroless plating technique, followed by n-hexadecyl mercaptan (RSH) surface modification. By introducing the hierarchically rough structures and low surface energy, the pristine superhydrophilic APAN nanofibrous membranes could be endowed with a superhydrophobicity with water contact angle of 171.1 ± 2.3°, a superoleophilicity with oil contact angle of 0° and a self-cleaning surface arising from the extremely low water contact angle hysteresis (3.0 ± 0.6°) and a low water-adhesion property. Surface morphology studies have indicated that the selective wettability of the resultant membranes could be manipulated by tuning the electroless plating time as well as the hierarchical structures. More importantly, the extremely high liquid entry pressure of water (LEPw, 175 ± 3 kPa) and the robust fiber morphology of the APAN immobilized Ag nanocluster endowed the as-prepared membranes with excellent separation capability and stability for oil/water separation by a solely gravity-driven process. The resultant membranes exhibited remarkable separation efficiency in both hyper-saline environment and broad pH range conditions, as well as excellent recyclability, which would make them a promising candidate for industrial oil-contaminated water treatments and marine spilt oil cleanup, and provided a new prospect to achieve functional nanofibrous membranes for oil/water separation.

Fabrication of superhydrophobic surfaces on flexible fluorinated foils by using dual-scale patterning

This paper investigates the interest of combining NanoImprint Lithography with plasma treatment in order to easily create dual-scale superhydrophobic surfaces on flexible fluorinated foils. The studies were led on FEP and PCTFE materials with conditions compatible with standard NIL equipments. Different pattern geometries, densities and aspect ratio have been investigated and we show that patterning at a nanometer scale improves hydrophobic behaviour compared to microstructuration. Water-contact angle (WCA) of 154° (and water contact angle hysteresis of 11 ± 2°) were measured, which corresponds to a superhydrophobic surface. However, patterning large surfaces at nanoscale with a high aspect ratio is more difficult to achieve and limits the use of such a process for industrial applications. So, we have decided to induce a nanopatterning on microstructures previously printed using plasma etching. This plasma roughening leads to a highly superhydrophobic surface and WCA values as high as 170°.

Preparation and characterization of looped polydimethylsiloxane brushes

We have synthesized and characterized a looped polymer that is covalently bonded to a surface at both chain termini. We used the ‘grafting to’ method to anchor polydimethylsiloxane (PDMS) onto a Si substrate by making use of triethoxysilyl terminal units. Atomic force microscopy revealed that a linear polymer brush provides a rough surface in water. As water is a poor solvent for PDMS, linear polymer brushes tend to be dewetted, forming aggregates. In contrast, a looped polymer brush surface is smooth in air and water. Aggregation was suppressed in the looped polymer brushes because both ends were tethered to the surface. As surface characteristics, the looped polymer brushes shows low water contact angle hysteresis and low sliding angle. Technologically useful surface properties are provided on the surface by the PDMS coatings. Both looped and linear polydimethylsiloxane (PDMS) polymer brushes were fabricated onto a Si substrate using the ‘grafting to’ method. As water is a poor solvent for PDMS, linear polymer brushes tended to be dewetted in water, forming aggregates. Therefore, a linear polymer brush showed a rough surface in water. In contrast, a looped polymer brush surface was smooth even in water. Aggregation was suppressed because both ends were tethered to the surface.

The ice repellency of plasma polymerized hexamethyldisiloxane coating

A superhydrophobic thin film was deposited on an aluminium oxide substrate by low pressure plasma polymerization of hexamethyldisiloxane (HMDSO). The coating was determined to be superhydrophobic due to its high water contact angle (∼158°) and low contact angle hysteresis. The aim of this work is to verify the icephobic properties of the HMDSO thin films obtained under simulated atmospheric conditions in a wind tunnel at sub-zero temperature and subsequent centrifugal adhesion test. The results obtained showed that the ice adhesion strength of the superhydrophobic HMDSO coating was 3.5 times lower than that on an aluminium substrate as reference tests. Although ice adhesion strength of the surface increased after 15 times icing/de-icing cycles, the surface kept its icephobic properties (ice adhesion strength 1.4 times lower than untreated aluminium).

Preparation and evaluation of hydrophobic surfaces of polyacrylate-polydimethylsiloxane copolymers for anti-icing

Two series of polyacrylate-polydimethylsiloxane (PDMS) copolymers, namely, polyacrylate-b-PDMS and polyacrylate-g-PDMS with three different molecular weights of PDMS blocks or side chains, were synthesized for formation of hydrophobic surfaces for anti-icing. The main purpose of this paper was to investigate the relationship between ice adhesion strength and the surface structure of the copolymers, and to find out how the prepared PDMS-containing polyacrylate copolymers are potentially used for anti-icing. The microphase-separated structure and the surface chemical composition were analyzed by transmission electron microscopy, atomic force microscopy and X-ray photoelectron spectroscopy, and ice adhesion strength was measured using a universal testing machine in a pull off mode. Results suggested that microphase separation appeared clearly in all the copolymers, especially for the block ones. The PDMS chains aggregated on the top of the polymer surfaces caused by microphase separation could weaken the interaction between the polymer surface and water, mainly hydrogen bond, which was demonstrated because of decrease of water contact angle hysteresis. Then, ice adhesion strength was decreased by the contribution of PDMS in the block copolymers or the graft copolymer with longer PDMS side chains. It is suggested that the polyacrylate-b-PDMS or polyacrylate-g-PDMS copolymers would have practical applications in preparation of anti-icing coatings.

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.

Controlling surface properties of polyelectrolyte multilayers by assembly pH

The control of surface properties such as the morphology, roughness, stiffness, and wettability of polyelectrolyte multilayers was carried out using poly(diallyldimethylammonium chloride) (PDADMAC) and poly(4-styrenesulfonic acid-co-maleic acid, 1 : 1 SS : MA) sodium salt (PSSMA 1 : 1) as the building blocks via adjusting the assembly pH value. We found that the surface roughness of the multilayers increased with increasing assembly pH value, and that the morphology was quite different at various assembly pH values when PSSMA was assembled as the final layer. However, the surface roughness and morphology of the multilayers (PDADMAC as the final layer) showed no apparent change at various assembly pH values. Interestingly, the stiffness of the PSSMA/PDADMAC multilayers could be tuned. Nanoindentation measurements by SFM showed that the hardness of the multilayers was very different at various pH values when PSSMA was the outermost layer. Conversely, the hardness of the multilayers remained unchanged regardless of the pH when PDADMAC was the outermost layer. The water contact angle hysteresis (the difference between the advancing and receding contact angles) of the multilayers was largely affected by both surface roughness and surface hydrophilic groups which could also be tuned by assembly pH.

Single-step UV diffraction lithography to define a hydrophobic SU-8 interconnected hoodoo structure

Micro-hoodoo (inverse trapezoidal) structures fabricated by silicon etching, soft-lithography, or photolithography have attracted great interest due to their improved hydrophobicity and self-cleaning properties. We present here a simple single-step UV diffraction lithography technique for fabricating micro-hoodoo structures with or without adjustable interconnecting bridges using a negative SU-8 photoresist. The theoretical calculations and the fabrication results revealed that the sizes and sidewall profiles of the micro-hoodoo structures and interconnecting bridges could be precisely controlled by the fabrication conditions, including the pattern-to-pattern spacing, exposure dose, and gap between the mask and the SU-8 surfaces. The theoretical calculations were conducted using an integrated model based on a Fresnel diffraction model for estimating the hoodoo size and an exponent decay model for estimating the sidewall profile. The integrated model agreed well with the fabricated hoodoo sizes and sidewall profiles, and the model provided an explanation for the structural instabilities observed during formation of the interconnecting bridges between hoodoos. The interconnecting bridges made the hydrophobic hoodoo structures sticky toward water, with a water contact angle hysteresis of up to 86.6°. The directional bridge interconnections produced directionally sticky hydrophobic surfaces that successfully mimicked function in butterfly wings to enable directional water removal.

Novel hybrid fluoro-carboxylated copolymers deposited by initiated chemical vapor deposition as protonic membranes

Copolymers of 1H, 1H, 2H, 2H,-perfluorodecyl acrylate and methacrylic acid are synthesized by initiated chemical vapor deposition (iCVD) to serve as proton exchange membranes for potential application in miniaturized fuel cells. The copolymerization of hydrophobic and hydrophilic monomers is difficult by conventional liquid-phase technique. We demonstrate that the use of iCVD is very promising for the synthesis of these membranes. Changing the monomer feed ratio we have a systematical control over the copolymer composition. The calculation of the copolymer ratios shows a moderate alternate composition of the copolymer which is advantageous to obtain highly conductive (acid functionalities) and stable (hydrophobic functionalities) membrane in the operating conditions of the fuel cells. The copolymers show a high degree of chain mobility when passing from dry to hydrated state as demonstrated by XPS analysis and by the high water contact angle hysteresis measured (>30°). An ionic conductivity of 70 mS/cm is obtained in conditions of highest percentage of hydrophobic functionalities. This conductivity value is comparable with the ionic conductivity of commercial Nafion®.

Smooth, Aggregate-Free Self-Assembled Monolayer Deposition of Silane Coupling Agents on Silicon Dioxide

Silane coupling agents (SCAs) are notorious for aggregating during deposition on oxide substrates, leading to nonuniform surface morphologies. To ameliorate this problem, we describe a vapor-phase deposition technique for silane coupling agents employing a spin-coated perfluoropolyether (PFPE) diffusion barrier that facilitates the formation of smooth, aggregate-free self-assembled monolayers (SAMs). Samples fabricated using PFPE barrier layers yielded SAMs exhibiting similar water contact angles, reduced water contact angle hysteresis, and a 2-fold reduction in rms roughness relative to those without a barrier. X-ray photoelectron spectroscopy confirms that the barrier layer can be completely removed after deposition, leaving behind a smooth monolayer. A basic analysis of the agglomerate separation ability of the barrier layers is discussed to understand the critical parameters involved. Generalized guidelines for selecting barrier materials are presented.

Origin of the Contact Angle Hysteresis of Water on Chemisorbed and Physisorbed Self-Assembled Monolayers

Self-assembled monolayers (SAMs) are known to form on a variety of substrates either via chemisorption (i.e., through chemical interactions such as a covalent bond) or physisorption (i.e., through physical interactions such as van der Waals forces or "ionic" bonds). We have studied the behavior and effects of water on the structures and surface energies of both chemisorbed octadecanethiol and physisorbed octadecylamine SAMs on GaAs using a number of complementary techniques including "dynamic" contact angle measurements (with important time and rate-dependent effects), AFM, and electron microscopy. We conclude that both molecular overturning and submolecular structural changes occur over different time scales when such SAMs are exposed to water. These results provide new insights into the time-dependent interactions between surfaces and colloids functionalized with SAMs when synthesized in or exposed to high humidity or bulk water or wetted by water. The study has implications for a wide array of phenomena and applications such as adhesion, friction/lubrication and wear (tribology), surfactant-solid surface interactions, the organization of surfactant-coated nanoparticles, etc.

Modification of wetting properties of laser-textured surfaces by depositing triboelectrically charged Teflon particles

Hydrophilic laser-textured silicon wafers with natural oxide surfaces were rendered hydrophobic by depositing electrostatically charged submicrometer Teflon particles, a process termed as triboelectric Teflon adhesion. Silicon surfaces were micro-textured (∼5 μm) by laser ablation using a nanosecond pulsed UV laser. By varying laser fluence, micro-texture morphology of the wafers could be reproduced and well controlled. Wetting properties of the triboelectrically charged Teflon-deposited surfaces were studied by measuring apparent static water contact angles and water contact angle hysteresis as a function of substrate roughness and the amount of Teflon deposited. A similar study was also performed on various micro-textured silicon carbide surfaces (sandpapers). If the average substrate roughness is between 15 and 60 μm, superhydrophobic surfaces can be easily formed by Teflon deposition with water contact angle hysteresis less than 8°. This environmentally benign solvent-free process is a highly efficient, rapid, and inexpensive way to render contact-charged rough surfaces hydrophobic or superhydrophobic.

Condensation-induced wetting state and contact angle hysteresis on superhydrophobic lotus leaves

In this paper, we demonstrate how condensed moisture droplets wet classical superhydrophobic lotus leaf surfaces and analyze the mechanism that causes the increase of contact angle hysteresis. Superhydrophobic lotus leaves in nature show amazing self-cleaning property with high water contact angle (>150°) and low contact angle hysteresis (usually 145°), suggesting that the wetting state deviates from a fully wetted Wenzel state. When the condensed water droplets are subjected to evaporation at room conditions, a thin water film is observed bridging over the micropillar structures of the lotus leaves. This causes the dew to stick to the surface. This result suggests that the condensed moisture does not uniformly wet the superhydrophobic lotus leaf surfaces. Instead, there occurs a mixed wetting state, between classical Cassie–Baxter and Wenzel states that causes a distinct increase of contact angle hysteresis. It is also observed that the mixed Cassie–Baxter/Wenzel state can be restored to the original Cassie–Baxter state by applying ultrasonic vibration which supplies energy to overcome the energy barrier for the wetting transition. In contrast, when the surface is fully wetted (classical Wenzel state), such restoration is not observed with ultrasonic vibration. The results reveal that although the superhydrophobic lotus leaves are susceptible to being wetted by condensing moisture, the configured wetting state is intermediate between the classical Cassie–Baxter and Wenzel states.

Dropwise condensation rate of water breath figures on polymer surfaces having similar surface free energies

This study investigates the effect of surface roughness, wettability, water contact angle hysteresis (CAH) and wetting hysteresis (WH) of polymeric substrates to the water drop condensation rate. We used five polyolefin coatings whose surface free energies were in a close range of 30–37mJ/m2 but having different surface roughness and CAH. The formation of water breath figures was monitored at a temperature just below the dew point. The initial number of the condensed droplets per unit area (N0) and droplet surface coverage were determined during the early stage of drop condensation where the droplet coalescence was negligible. It was found that the mean drop diameter of condensed droplets on these polymer surfaces grow according to a power law with exponent 1/3 of time, similar to the previous reports given in the literature. It was determined that surface roughness and corresponding CAH and WH properties of polymers have important effects on the number of nucleation sites and growth rate of the condensed water droplets. N0 values and the surface coverage increased with the increase in surface roughness, CAH and WH of the polymer surfaces. The total condensed water drop volume also increased with the increase in surface roughness in accordance with the increase of the number of nucleated droplets.

Use of diffusion controlled drop evaporation equations for dropwise condensation during dew formation and effect of neighboring droplets

In this study, we determined that the equations derived for the diffusion controlled drop evaporation processes can be successfully applied to the condensation rate of water droplets on polymer surfaces having a surface temperature just below the dew point. The differences between the growth rates of condensed isolated droplets on five different polyolefin surfaces, whose surface free energies were in a close range of 30–37mJ/m2, were attributed to the difference in surface roughness which alters the droplet nucleation rate, drop pinning effect and initial contact angles. The condensation rate of isolated droplets decreased with the increase of surface roughness, water contact angle and contact angle hysteresis in the order of ethylene–vinyl acetate copolymer (EVA)>low density polyethylene (LDPE)>high density polyethylene (HDPE)>polypropylene–polyethylene copolymer (PPPE)>polypropylene (PP). The drop radius of the individual isolated droplets was found to grow according a power law with exponent 1/3 except PP similar to previous reports. We also compared the volume increase of an isolated single droplet with the volume increase of another similar sized single droplet which was surrounded by neighboring droplets and found that when neighboring droplets are present in close proximity, the condensation rate was 14–40% lower than that of a single isolated droplet due to blocking of lateral water vapor diffusion. This effect was more pronounced on substrates having high surface roughness.

Fabrication of polymeric surfaces with similar contact angles but dissimilar contact angle hysteresis

This note provides methods to fabricate polymer surfaces which have similar advancing or receding contact angles with distilled water but dissimilar contact angle hysteresis. Poly(methyl methacrylate) (PMMA), Polystyrene, Polypropylene and Teflon AF were chosen, and three different methods namely spin coating, spray coating, and heat press were used in the fabrication. Contact angle measurements were performed using the sessile drop method, and atomic force microscopy was used to measure the roughness of the fabricated surfaces. Surfaces obtained from spin coating, spray coating and heat press showed very different wettability, which can be explained by their different surface roughness or heterogeneity. For spin coating, by changing the spinning rate and concentration of the solution, contact angle hysteresis of the PMMA surface can be changed in the range of 13–25°. For heat press, contact angle hysteresis can be varied from 21° to 29° by changing the plate and/or the method of separating the plate from the surface. For Teflon AF surfaces, the contact angle hysteresis can be varied from 10° to 23° by using different surface fabrication methods. For surfaces made of mixed solution of PMMA and Polystyrene, the ratio of PMMA:Polystyrene was shown to affect the surface wettability, and the results were explained by changes in the pattern and size of the heterogeneous domains on the surface.