Superhydrophobic Fabrics Produced by Electrospinning and Chemical Vapor Deposition

Mimicking natural superhydrophobic surfaces and grasping the wetting process: a review on recent progress in preparing superhydrophobic surfaces

A typical superhydrophobic (ultrahydrophobic) surface can repel water droplets from wetting itself, and the contact angle of a water droplet resting on a superhydrophobic surface is greater than 150°, which means extremely low wettability is achievable on superhydrophobic surfaces. Many superhydrophobic surfaces (both manmade and natural) normally exhibit micro- or nanosized roughness as well as hierarchical structure, which somehow can influence the surface's water repellence. As the research into superhydrophobic surfaces goes deeper and wider, it is becoming more important to both academic fields and industrial applications. In this work, the most recent progress in preparing manmade superhydrophobic surfaces through a variety of methodologies, particularly within the past several years, and the fundamental theories of wetting phenomena related to superhydrophobic surfaces are reviewed. We also discuss the perspective of natural superhydrophobic surfaces utilized as mimicking models. The discussion focuses on how the superhydrophobic property is promoted on solid surfaces and emphasizes the effect of surface roughness and structure in particular. This review aims to enable researchers to perceive the inner principles of wetting phenomena and employ suitable methods for creation and modification of superhydrophobic surfaces.

Fabrication of superhydrophobic and antibacterial surface on cotton fabric by doped silica-based sols with nanoparticles of copper

The study discussed the synthesis of silica sol using the sol-gel method, doped with two different amounts of Cu nanoparticles. Cotton fabric samples were impregnated by the prepared sols and then dried and cured. To block hydroxyl groups, some samples were also treated with hexadecyltrimethoxysilane. The average particle size of colloidal silica nanoparticles were measured by the particle size analyzer. The morphology, roughness, and hydrophobic properties of the surface fabricated on cotton samples were analyzed and compared via the scanning electron microscopy, the transmission electron microscopy, the scanning probe microscopy, with static water contact angle (SWC), and water shedding angle measurements. Furthermore, the antibacterial efficiency of samples was quantitatively evaluated using AATCC 100 method. The addition of 0.5% (wt/wt) Cu into silica sol caused the silica nanoparticles to agglomerate in more grape-like clusters on cotton fabrics. Such fabricated surface revealed the highest value of SWC (155° for a 10-μl droplet) due to air trapping capability of its inclined structure. However, the presence of higher amounts of Cu nanoparticles (2% wt/wt) in silica sol resulted in the most slippery smooth surface on cotton fabrics. All fabricated surfaces containing Cu nanoparticles showed the perfect antibacterial activity against both of gram-negative and gram-positive bacteria.

Self-assembly of pillars modified with vapor deposited polymer coatings

In this paper, we demonstrate that thin layers of polymer coatings can be used to self-assemble pillars into stable microstructures. Polymer coatings are deposited onto elastomeric pillars using solventless initiated chemical vapor deposition and capillary forces are used to collapse the coated pillars into microstructures. The location of pillar collapse can be controlled by patterning regions of hydrophilicity and hydrophobicity. Poly(hydroxyethyl methacrylate) and poly(methacrylic acid) coatings stabilize the self-assembled microstructures by providing an adhesive force through solvent bonding. These solvent bonds allow the response of the microstructures to be tuned by varying the thickness of the polymer coating and the solubility parameter of the solvent. The coating process described in this paper is substrate-independent and therefore can be applied to pillars composed of any material.

Self-assembled biomimetic superhydrophobic CaCO3 coating inspired from fouling mineralization in geothermal water

Inspired from fouling self-mineralization in geothermal water, a novel biomimetic cactuslike CaCO(3) coating with superhydrophobic features is reported in this letter. The structure, morphologies, and phases of the CaCO(3) coating were characterized by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, and infrared spectrophotometry. After prenucleation treatment, a continuous cactuslike CaCO(3) coating with hierarchical nano- and microstructures was self-assembled on stainless steel surfaces after immersion in simulated geothermal water at 50 °C for 48 h. After being modified with a low-surface-energy monolayer of sodium stearate, the as-prepared coating exhibited superhydrophobic properties with a water contact angle of 158.9° and a sliding angle of 2°. Therefore, this work might open up a new application field of geothermal resources and provide insight into designing multidimensional structures with functional applications, including superhydrophobic surfaces.

Directed deposition of functional polymers onto porous substrates using metal salt inhibitors

This paper demonstrates the ability to control the location of polymer deposition onto porous substrates using vapor phase polymerization in combination with metal salt inhibitors. Functional polymers such as hydrophobic poly(1H,1H,2H,2H-perfluorodecyl acrylate), click-active poly(pentafluorophenyl methacrylate), and light-responsive poly(ortho-nitrobenzyl methacrylate) were patterned onto porous hydrophilic substrates using metal salts. A combinatorial screening approach was used to determine the effects of different transition metal salts and reaction parameters on the patterning process. It was found that CuCl(2) and Cu(NO(3))(2) were effective at uniformly inhibiting the deposition of all three polymers through the depth of the porous substrate and along the entire cross section. This study offers a new and convenient method to selectively deposit a wide variety of functional polymers onto porous materials and will enable the production of next-generation multifunctional paper-based microfluidic devices, polymeric photonic crystals, and filtration membranes.

Highly hydrophobic electrospun fiber mats from polyisobutylene-based thermoplastic elastomers

This paper is the first report of electrospinning neat polyisobutylene-based thermoplastic elastomers. Two generations of these materials are investigated: a linear poly(styrene-b-isobutylene-b-styrene) (L_SIBS) triblock copolymer and a dendritic poly(isobutylene-b-p-methylstyrene) (D_IB-MS), also a candidate for biomedical applications. Cross-polarized optical microscopy shows birefringence, indicating orientation in the electrospun fibers, which undergo large elongation and shear during electrospinning. In contrast to the circular cross section of L_SIBS fibers, D_IB-MS yields dumbbell-shaped fiber cross sections for the combination of processing conditions, molecular weight, and architecture. Hydrophobic surfaces with a water contact angle as high as 146 ± 3° were obtained with D_IB-MS that had the noncircular fiber cross section and a hierarchical arrangement of nano- to micrometer-sized fibers in the mat. These highly water repellent fiber mats were found to serve as an excellent scaffold for bovine chondrocytes to produce cartilage tissue.

Tuning hydrophobicity and water adhesion by electrospinning and silanization

Electrospinning and silanization were synergistically employed to fabricate poly(vinyl alcohol) (PVA) and PVA/silica mixtures into flexible and chemically modifiable nanostructured surfaces with varying degrees of hydrophobicity and water adhesion. Surfaces possessing the greatest advancing water contact angle yet exhibiting a high level of water adhesion (θ(A)/θ(R) ≈ 168°/0°) were achieved by the reaction of PVA fiber mats with multiple cycles of SiCl(4)/H(2)O treatment, followed by silanization with (1H,1H,2H,2H-perfluorooctyl)trichlorosilane. It is postulated that the strong pinning effect and hence the water adhesion originated from the collapse of the underlying fibrous structures and the removal of air pockets. The addition of silica to the PVA matrix improved the rigidity and thus prevented the fibers from collapsing, allowing air to remain trapped within the fibrous structure and giving the surface greater water repellency. Throughout the investigation, the three wetting models--Wenzel's, Cassie-Baxter's, and the Cassie-impregnating--were regularly referred to as a conceptual framework. The hydrophobic surface that exhibited strong water adhesion, or the so-called "Petal effect", was elucidated in correlation with the fibrous structure of the film, as reviewed by microscopic analysis. In summary, electrospinning as a facile and cost-effective method provides promising opportunities for investigating the mechanistic character of nanowetting, nanoprinting, and nanocoating where the precise control of the dynamical three-phase contact line is of paramount importance.

Study of superhydrophobic electrospun nanocomposite fibers for energy systems

Polystyrene (PS) and polyvinyl chloride (PVC) fibers incorporated into TiO(2) nanoparticles and graphene nanoflakes were fabricated by an electrospinning technique, and then the surface morphology and superhydrophobicity of these electrospun nanocomposite fibers were investigated. Results indicated that the water contact angle of the nanocomposite fiber surfaces increases to 178° on the basis of the fiber diameter, material type, nanoscale inclusion, heat treatment, and surface porosity/roughness. This is a result of the formation of the Cassie-Baxter state in the fibers via the nanoparticle decoration, bead formation, and surface energy of the nanofiber surface. Consequently, these superhydrophobic nanocomposite fibers can be utilized in designing photoelectrodes of dye-sensitized solar cells (DSSCs) as self-cleaning and anti-icing materials for the long-term efficiency of the cells.

Electrospinning of a functional perfluorinated block copolymer as a powerful route for imparting superhydrophobicity and corrosion resistance to aluminum substrates

Superhydrophobic aluminum surfaces with excellent corrosion resistance were successfully prepared by electrospinning of a novel fluorinated diblock copolymer solution. Micro- and nanostructuration of the diblock copolymer coating was obtained by electrospinning which proved to be an easy and cheap electrospinning technology to fabricate superhydrophobic coating. The diblock copolymer is made of poly(heptadecafluorodecylacrylate-co-acrylic acid) (PFDA-co-AA) random copolymer as the first block and polyacrylonitrile (PAN) as the second one. The fluorinated block promotes hydrophobicity to the surface by reducing the surface tension, while its carboxylic acid functions anchor the polymer film onto the aluminum surface after annealing at 130 °C. The PAN block of this copolymer insures the stability of the structuration of the surface during annealing, thanks to the infusible character of PAN. It is also demonstrated that the so-formed superhydrophobic coating shows good adhesion to aluminum surfaces, resulting in excellent corrosion resistance.

Superhydrophobic thermoplastic polyurethane films with transparent/fluorescent performance

In this paper, we report a simple and versatile route for the fabrication of superhydrophobic thermoplastic polyurethane (TPU) films. The approach is based on octadecanamide (ODAA)-directed assembly of nanosilica/TPU/ODAA hybrid with a well-defined sheetlike microstructure. The superhydrophobic hybrid film shows a transparent property, and its water contact angle reaches as high as 163.5° without any further low surface energy treatment. In addition, the superhydrophobic TPU hybrid film with fluorescent properties is achieved by smartly introducing CdTe quantum dots, which will extend potential application of the film to optoelectronic areas. The resulting fluorescent surface produced in this system is stable and has a water contact angle of 172.3°. This assembly method to control surface structures represents an intriguing and valuable route to tune the surface properties of organic-inorganic hybrid films.

Fluoroalkylated silicon-containing surfaces-estimation of solid-surface energy

The design of robust omniphobic surfaces, which are not wetted by low-surface-tension liquids such as octane (γlv=21.6 mN/m) and methanol (γlv=22.7 mN/m), requires an appropriately chosen surface micro/nanotexture in addition to a low solid-surface energy (γsv). 1H,1H,2H,2H-Heptadecafluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS) offers one of the lowest solid-surface energy values ever reported (γsv≈10 mN/m) and has become the molecule of choice for coating textured surfaces. In this work, we synthesize and evaluate a series of related molecules that either retain the POSS cage and differ in fluoroalkyl chain length or that retain the fluorodecyl chains surrounding a linear or cyclic molecular structure. The solid-surface energy (γsv) of these molecules was estimated using contact angle measurements on flat spin-coated silicon wafer surfaces. Zisman analysis was performed using a homologous series of n-alkanes (15.5≤γlv≤27.5 mN/m), whereas Girifalco-Good analysis was performed using a set of polar and nonpolar liquids with a wider range of liquid surface tension (15.5≤γlv≤72.1 mN/m). The hydrogen-bond-donating, hydrogen-bond-accepting, polar, and nonpolar (dispersion) contributions to the solid-surface energy of each compound were determined by probing the surfaces using a set of three liquid droplets of either acetone, chloroform, and dodecane or diiodomethane, dimethyl sulfoxide, and water.

Chemical vapor deposition of conformal, functional, and responsive polymer films

Chemical vapor deposition (CVD) polymerization utilizes the delivery of vapor-phase monomers to form chemically well-defined polymeric films directly on the surface of a substrate. CVD polymers are desirable as conformal surface modification layers exhibiting strong retention of organic functional groups, and, in some cases, are responsive to external stimuli. Traditional wet-chemical chain- and step-growth mechanisms guide the development of new heterogeneous CVD polymerization techniques. Commonality with inorganic CVD methods facilitates the fabrication of hybrid devices. CVD polymers bridge microfabrication technology with chemical, biological, and nanoparticle systems and assembly. Robust interfaces can be achieved through covalent grafting enabling high-resolution (60 nm) patterning, even on flexible substrates. Utilizing only low-energy input to drive selective chemistry, modest vacuum, and room-temperature substrates, CVD polymerization is compatible with thermally sensitive substrates, such as paper, textiles, and plastics. CVD methods are particularly valuable for insoluble and infusible films, including fluoropolymers, electrically conductive polymers, and controllably crosslinked networks and for the potential to reduce environmental, health, and safety impacts associated with solvents. Quantitative models aid the development of large-area and roll-to-roll CVD polymer reactors. Relevant background, fundamental principles, and selected applications are reviewed.

Systematic control of hydrophobic and superhydrophobic properties using double-rough structures based on mixtures of metal oxide nanoparticles

Double-rough surfaces were prepared by using mixtures of metal oxide nanoparticles with diameters on two different length scales. The degree of double-roughness of these surfaces was controlled using defined ratios of larger (approximately 300 nm) and smaller (approximately 30 nm) particles. If the preparation method was chosen such that the smaller particles decorate the larger ones, a roughness on two length scales was induced and an increase of contact angles from 130 degrees to 160 degrees with increasing double-roughness was observed. At highest double-roughness, a transition from the Wenzel to Cassie regime occurred. The results were quantitatively modeled on the basis of a modified Wenzel equation taking into account the two hierarchies of roughness.

Wetting behaviour of laser synthetic surface microtextures on Ti-6Al-4V for bioapplication

Wettability at the surface of an implant material plays a key role in its success as it modulates the protein adsorption and thereby influences cell attachment and tissue integration at the interface. Hence, surface engineering of implantable materials to enhance wettability to physiological fluid under in vivo conditions is an area of active research. In light of this, in the present work, laser-based optical interference and direct melting techniques were used to develop synthetic microtextures on Ti-6Al-4V alloys, and their effects on wettability were studied systematically. Improved wettability to simulated body fluid and distilled water was observed for Ca-P coatings obtained by direct melting technique. This superior wettability was attributed to both the appropriate surface chemistry and the three-dimensional surface features obtained using this technique. To assert a better control on surface texture and wettability, a three-dimensional thermal model based on COMSOL's multiphysics was employed to predict the features obtained by laser melting technique. The effect of physical texture and wetting on biocompatibility of laser-processed Ca-P coatings was evaluated in the preliminary efforts on culturing of mouse MC3T3-E1 osteoblast cells.

Scale dependence of omniphobic mesh surfaces

We provide a simple design chart framework to predict the apparent contact angle on a textured surface in terms of the equilibrium contact angle on a chemically identical smooth surface and details of the surface topography. For low surface tension liquids such as methanol (gamma(lv) = 22.7 mN/m) and octane (gamma(lv) = 21.6 mN/m), a solid-liquid-air composite interface on a textured surface is inherently metastable. Thus, on application of a sufficient pressure difference (e.g., an externally applied pressure or a sufficiently large Laplace pressure at small droplet size) the metastable composite interface transitions to a fully wetted interface. A dimensionless robustness factor is used to quantify the breakthrough pressure difference necessary to disrupt a metastable composite interface and to predict a priori the existence of a robust composite interface. The impact of the length scale (radius of the cylindrical features R varying from 18 to 114 microm) and the feature spacing ratio (D(*) = (R + D)/R varying from 2.2 to 5.1, where 2D is the spacing between the cylindrical features) on the robustness is illustrated by performing contact angle measurements on a set of dip-coated wire-mesh surfaces, which provide systematically quantifiable cylindrical texture. The design chart for a given feature size R shows how the two independent design parameters--surface chemistry as revealed in the equilibrium contact angle and texture spacing embodied in the dimensionless spacing ratio (D(*))--can be used to develop surfaces with desirably large values of the apparent contact angle and robustness of the metastable composite interface. Most revealing is the scaling of the robustness with the dimensionless parameter l(cap)/R (where l(cap = (gamma(lv)/rho g)(1/2) is the capillary length), which indicates clearly why, in the consideration of self-similar surfaces, smaller is better for producing omniphobic surfaces that resist wetting by liquids with low surface tension.

Robust superhydrophobic mats based on electrospun crystalline nanofibers combined with a silane precursor

We demonstrate the fabrication of solvent-resistant, mechanically robust, superhydrophobic nanofibrous mats by electrospinning of poly(vinylidene fluoride) (PVDF) in the presence of inorganic silane materials. The solvent resistance and mechanical strength of nanofibrous mats were dramatically increased through the crystallization of as-spun PVDF fibers or incorporation of a tetraethyl orthosilicate (TEOS) sol into the nanofibrous matrix. The electrospun nanofibrous mats yielded a water contact angle of 156 degrees that did not vary with TEOS content. The solvent resistance and mechanical robustness of the electrospun mats were significantly enhanced through extensive cross-linking of TEOS, even after short PVDF annealing times. The interpenetrating polymer network, which embeds polymer chains in a TEOS network, allows the fabrication of robust functional nanofibers by combining semicrystalline polymers with electrospinning techniques.

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

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.

Multiscale carbon structures fabricated by direct micropatterning of electrospun mats of SU-8 photoresist nanofibers

A novel method for the direct fabrication of arrays of micropatterned polymeric and carbon nanofiber structures on any substrate is developed. First SU-8, an epoxy-based negative photoresist, is electrospun under optimized conditions to produce a layer of polymeric nanofibers. Next, this nanofibrous mat is micropatterned using photolithography, and finally, pyrolysis produces ordered arrays of microdomains containing carbon nanofibers. The nanotextured surfaces of carbon nanofibers are shown to be very hydrophobic (water contact angle approximately 130 degrees). Micropatterning thus generates a substantial wettability contrast of nanofiber domains with intervening micropatches of very hydrophilic carbon (approximately 20 degrees) or silicon substrates.

Wrinkled, dual-scale structures of diamond-like carbon (DLC) for superhydrophobicity

We present a simple two-step method to fabricate dual-scale superhydrophobic surfaces by using replica molding of poly(dimethylsiloxane) (PDMS) micropillars, followed by deposition of a thin, hard coating layer of a SiO(x)-incorporated diamond-like carbon (DLC). The resulting surface consists of microscale PDMS pillars covered by nanoscale wrinkles that are induced by residual compressive stress of the DLC coating and a difference in elastic moduli between DLC and PDMS without any external stretching or thermal contraction on the PDMS substrate. We show that the surface exhibits superhydrophobic properties with a static contact angle over 160 degrees for micropillar spacing ratios (interpillar gap divided by diameter) less than 4. A transition of the wetting angle to approximately 130 degrees occurs for larger spacing ratios, changing the wetting from a Cassie-Cassie state (C(m)-C(n)) to a Wenzel-Cassie state (W(m)-C(n)), where m and n denote micro- and nanoscale roughness, respectively. The robust superhydrophobicity of the Cassie-Cassie state is attributed to stability of the Cassie state on the nanoscale wrinkle structures of the hydrophobic DLC coating, which is further explained by a simple mathematical theory on wetting states with decoupling of nano- and microscale roughness in dual scale structures.

Thermal annealing treatment to achieve switchable and reversible oleophobicity on fabrics

Surfaces that are strongly nonwetting to oil and other low surface tension liquids can be realized by trapping microscopic pockets of air within the asperities of a re-entrant texture and generating a solid-liquid-vapor composite interface. For low surface tension liquids such as hexadecane (gamma(lv) = 27.5 mN/m), this composite interface is metastable as a result of the low value of the equilibrium contact angle. Consequently, pressure perturbations can result in an irreversible transition of the metastable composite interface to the fully wetted interface. In this work, we use a simple dip-coating and thermal annealing procedure to tune the liquid wettability of commercially available polyester fabrics. A mixture of 10% 1H,1H,2H,2H-heptadecafluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS) and 90% polyethyl methacrylate (PEMA) is used to uniformly coat the fabric surface topography. Contact angle measurements show that a robust metastable composite interface with high apparent contact angles can be supported for hexadecane (gamma(lv) = 27.5 mN/m) and dodecane (gamma(lv) = 25.3 mN/m). To tune the solid surface energy of the coated surface, we also developed a reversible treatment using thermal annealing of the surface in contact with either dry air or water. The tunability of the solid surface energy along with the inherent re-entrant texture of the polyester fabric result in reversibly switchable oleophobicity between a highly nonwetting state and a fully wetted state for low surface tension liquids such as hexadecane and dodecane. This tunability can be explained within a design parameter framework, which provides a quantitative criterion for the transition between the two states, as well as accurate predictions of the measured values of the apparent contact angle (theta*) for the dip-coated polyester fabrics.

Solvent-assisted dewetting during chemical vapor deposition

This study examines the use of a nonreactive solvent vapor, tert-butanol, during initiated chemical vapor deposition (iCVD) to promote polymer film dewetting. iCVD is a solventless technique to grow polymer thin films directly from gas phase feeds. Using a custom-built axisymmetric hot-zone reactor, smooth poly(methyl methacrylate) films are grown from methyl methacrylate (MMA) and tert-butyl peroxide (TBPO). When solvent vapor is used, nonequilibrium dewetted structures comprising of randomly distributed polymer droplets are observed. The length scale of observed topographies, determined using power spectral density (PSD) analysis, ranges from 5 to 100 microm and is influenced by deposition conditions, especially the carrier gas and solvent vapor flow rates. The use of a carrier gas leads to faster deposition rates and suppresses thin film dewetting. The use of solvent vapor promotes dewetting and leads to larger length scales of the dewetted features. Control over lateral length scale is demonstrated by preparation of hierarchal "bump on bump" topographies. Vapor-induced dewetting is demonstrated on silicon wafer substrate with a native oxide layer and also on hydrophobically modified substrate prepared using silane coupling. Autophobic dewetting of PMMA from SiOx/Si during iCVD is attributed to a thin film instability driven by both long-range van der Waals forces and short-range polar interactions.

Superhydrophobic and oleophobic fibers by coaxial electrospinning

Control of surface wetting properties to produce strongly hydrophobic or hydrophilic effects is at the heart of many macro- and microfluidic applications. In this work, we have investigated coaxial electrospinning to produce core-sheath-structured nano/microfibers that combine different properties from individual core and sheath materials. Teflon AF is an amorphous fluoropolymer that is widely utilized as a hydrophobic material. Hydrophobic fluoropolymers are normally not electrospinnable because their low dielectric constant prevents sufficient charging for a solution to be electrospun. The first Teflon electrospun fibers are reported using coaxial electrospinning with Teflon AF sheath and poly(epsilon-caprolactone) (PCL) core materials. Using these core/sheath fibers, superhydrophobic and oleophobic membranes have been successfully produced. These coaxial fibers also preserve the core material properties as demonstrated with mechanical tensile tests. The fact that a normally nonelectrospinnable material such as Teflon AF has been successfully electrospun when combined with an electrospinnable core material indicates the potential of coaxial electrospinning to provide a new degree of freedom in terms of material combinations for many applications.