The possibility of using supercritical carbon dioxide (scCO2) as the primary solvent in a spray process for producing superhydrophobic surfaces have been examined in this work. Using scCO2 as solvent will have considerably lower environmental impact compared to an organic solvent since scCO2 is considered a green solvent as it is non-toxic, non-flammable and recyclable. To be able to work at the pressures needed to reach the supercritical state of carbon dioxide, a high-pressure technique called rapid expansion of supercritical solutions (RESS) has been used to produce the coatings. Fluorinated compounds are often used when producing superhydrophobic coatings due to their intrinsic water repellent properties, but generally these compound do not degrade in nature. Due to this, a wax and a biodegradable polymer have been used as the coating materials in this work. Two RESS set-ups were used to spray a polymer from solutions of scCO2 and acetone. The first system was based on a continuous flow of the solvent mixture and the polymer particles were collected on silica surfaces. Some of the coatings had superhydrophobic properties and the limitation with this technique was the loss of particles between the nozzle and the surface. In the second set-up, RESS was combined with electrostatic deposition (ED) to improve the particle collection. Different processing parameters were examined and most of the RESS-ED sprayed surfaces were superhydrophobic. This was demonstrated by high contact angles against water, low contact angle hysteresis and low tilt angles at which a water droplet rolls off the surface. It was also shown that the surface structures created when spraying using RESS-ED induced the important two-level roughness that was needed to achieve superhydrophobicity. A semi-continuous process for scaling-up the RESS system when spraying the wax has been developed. Temperature and pressure was investigated to find the highest solubility of the wax in scCO2, and 250 bar and 67 °C resulted in the largest amount of sprayed wax. It was also shown that the system is suitable for spray-coating the wax on different substrates such as glass, paper, aluminium etc. since all of these surfaces showed superhydrophobic properties. The wear resistance of the coatings were examined by different methods. Scratch resistance, vertical compression and the friction between the surface and a finger were analysed. The polymer coated surfaces showed a larger robustness compared with the wax surfaces in the scratch tests. The superhydrophobicity was lost for the wax coatings exposed to compression loads above 59 kPa and in the frictions test, one finger stroke over the coating destroyed the surface roughness. Finally, the wax surfaces were investigated as coating barriers to protect steel from corrosion. The superhydrophobic coating was stable up to 10 days before corrosion of the steel started. / <p>QC 20140922</p>

Capillarity is a physical phenomenon that acts as a driving force in the displacement of one fluid by another within a porous medium. This mechanism operates on the micro and nanoscale, and is responsible for countless observable events. This can include applications such as absorption in various hygiene products, self-cleaning surfaces such as water beading up and rolling off a specially-coated windshield, anti-icing, and water management in fuel cells, among many others. The most significant research into capillarity has occurred within the last century or so. Traditional formulations for fluid absorption include the Lucas–Washburn model for porous media, which is a 1-D model that reduces a porous medium to a series of capillary tubes of some educated equivalent radius. The Richards equation allows for modeling fluid saturation as a function of time and space, but requires additional information on capillary pressure as a function of saturation (pc(S)) in order to solve for absorption. In both approaches, the surface can only possess one fluid affinity. This thesis focuses on developing capillary models necessary for predicting fluid absorption and repulsion in fibrous media. Some of the work entails utilizing approximations based on pore space available to the fluid, which allows for capillary pressure simulation in media with arbitrary fiber orientation. This thesis also presents models for tracking the fluid interface in fibrous media and coatings with simpler geometries such as horizontally and vertically aligned fibers and orthogonal fiber layers. This method hinges on solving for the true fluid interface shape between the fibers based on the balance of forces across it, ensuring the accurate location and total content of fluid in the medium, and therefore accurate pc(S). Using this approach also allows, for the first time, fibers of different fluid affinities to exist in the same structure, to examine their combined influence on fluid behavior. The models in this thesis focus mainly on absorbent fabrics and superhydrophobic coatings, but can be easily expanded for use in other applications such as water filtration from fuel, fluid transport and storage in microchannels, polymer impregnation in fiber-reinforced composite materials, among countless others.

Polystyrene (PS) fiber mats incorporating iron (Fe) particles were fabricated by electrospinning and the hydrophobicity of the resulting magnetic fabrics was investigated with and without an applied magnetic field. The results show that the hydrophobicity (as measured using water droplet contact angle) increases in the presence of a magnetic field and the hysterisis in the advancing/receding contact angle (a measure of the stickiness of the surface) decreases in the presence of a magnetic field. It is also shown that the contact angle and hysterises increase with decreasing fiber diameter and mat thickness.

Master of Science / Department of Apparel, Textiles, and Interior Design / Jooyoun Kim / Seong-O Choi / Superhydrophobicity (water contact angle, WCA >150˚) can be achieved by introducing surface roughness and decreasing surface energy. Polystyrene (PS) electrospun web can be used as an excellent substrate for superhydrophobic surface due to its low surface energy (~33 mN/m) and processibility to form various roughness. As the Cassie-Baxter model explains, the presence of roughness amplifies anti-wettability of materials whose surface energy is low (hydrophobic, WCA >90˚). This study aims to fabricate superhydrophobic PS nonwoven webs by electrospinning process and vapor deposition of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFDTS) and to investigate the influence of fiber morphology and surface energy on wettability. To this end, PS webs with various fiber morphologies were electrospun under different polymer concentrations and solvent mixtures. PS substrates were treated by air plasma to attach –OH groups before the vapor deposition of PFDTS. Air plasma treatment itself increased the surface energy of PS; however, with PFDTS coating, the surface energy was decreased. The wettability was characterized by WCA and sliding angle measurement. WCAs on the electrospun webs were greater than that of flat PS film (WCA=95˚) due to the increased roughness of the web. The web with beads or grooved fibers achieved superhydrophobicity (WCA>150˚). PFDTS deposition lowered the surface energy of PS surface to about 15.8 mN/m. PS web with PFDTS deposition presented high water contact angle up to 169˚ and low sliding angle about 3˚. Also it was attempted to characterize the interfacial area between water and a solid surface on irregular fibrous webs. The fraction of solid surface area wet by the liquid (solid fraction) was observed by staining the rough electrospun web with a hydrophobic fluorescent dye, coumarin. The actual solid fraction corresponded fairly well with the theoretical solid fraction calculated by the Cassie-Baxter equation, demonstrating that the treated superhydrophobic surface follows the Cassie-Baxter wetting state.

The interest in superhydrophobic coatings have increased exponentially in the recentdecades due to their potential and versatility in their applications. The use forsuperhydrophobic surfaces range from water repellent fabric, to self cleaning surfacesand numerous applications in industry. In this project the durability of 6 differentsuperhydrophobic coatings have been examined. The durability was tested bydropping sand on the surfaces from a set hight of 10 cm and a flow of 40 g/min. Thesurfaces were mounted on a 45° angle. The surfaces were abraded for 30 seconds ata time and the static, receding, and advancing contact angles along with the roll-ofangle was measured. Five of the surfaces were built up with nano particles and onewas sand blasted and anodized to create a superhydrophobic structure. The surfacesthat withstood the most abrasion was the surface that had been calcined to improveadhesion and the surface that had been sand blasted and anodized. Measurementsshowed that the roll-off angle and the receding contact angle were the two bestindicators of the deterioration of a surface, while the static contact angle and theadvancing contact angle varied little with abrasion. The project was done at thecompany Technical Research Institute of Sweden (SP) at their chemistry, surfaces andmaterials department in Stockholm. All coatings and equipment was supplied by SP.

Interaction of a liquid droplet and a fiber or layer of fibers is ubiquitous in nature and in a variety of industrial applications. It plays a crucial role in fog harvesting, coalescence filtration, membrane desalination, self-cleaning and fiber based microfluidics, among many others. This work presents a quantitative investigation on the interactions of a droplet with a fiber or layers of fibers. More precisely, the present work is focused on 1) predicting the effects of fiber’s size and material on its ability to withhold a droplet against external forces and on the liquid residue left on the fiber after the droplet detachment, 2) predicting the outcome of two fibers competing to attract the same droplet, and 3) predicting the wetting stability of a droplet deposited on a layer of electrospun fibers. This work is comprised of series of computational and experimental studies for mutual validation and/or calibration. The simulations were conducted using the Surface Evolver code and the experiments were devised using a ferrofluid and a magnet. We also investigated the drag reduction performance of fibrous coatings because of its close connection with droplet-fiber interaction. We started by studying the drag reduction performance of a superhydrophobic granular coating because of its geometrical simplicity. We modeled the flow of water over the granular coating and studied the effects of hydrostatic pressure and microstructural properties on the drag reduction performance of the coating. We then examined the drag reduction performance of a lubricant infused surface with trapped air made of layers of parallel fibers (FLISTA). A mathematical model was developed to predict the shape of the water-lubricant interface and lubricant-air interface under a given hydrostatic pressure. This information was used to solve the flow field over the coating in a Couette configuration to find the effects of hydrostatic pressure and microstructural properties of the coating on its drag reduction performance.

The effect of superhydrophobicity on liquid water impingement on a flat horizontal surface was explored. The surfaces combined a hydrophobic surface chemistry with a patterned microstucture in order to produce high contact angles with water. Three sets of experiments were performed, one for jet impingement and two for droplet impingement, which advance previous work in characterizing the interaction of water and superhydrophobic surfaces.Jet impingement experiments were performed to characterize a transitional regime between an unsubmerged and a completely submerged superhydrophobic surface by varying an imposed downstream depth. For low downstream depths, the surface remained unsubmerged and displayed only break up of the thin film, while at high downstream depths, the surface was completely submerged and only a hydraulic jump occurred. Within the transition, the surface was partially submerged and both thin film breakup and a hydraulic jump were observed. Experiments were performed for three Reynolds numbers, Re, ranging from 1.9 x 104 to 2.2 x 104 (based on the volume flow rate). For all Re, the transition was characterized by a reduction in the hydraulic jump radius as downstream depth increased. Also, as Re increased, the downstream depths over which the transition occurred was greater. When a droplet impinges on a surface covered with a liquid film, a thin liquid wall, or crown, forms and propagates outward. Here a comparison of this crown dynamic was made for smooth hydrophilic surfaces and superhydrophobic (SH) surfaces patterned with post or rib microfeatures. Due to the high contact angle of the SH surfaces, a relatively thick film (h ≈ 5 mm) of water was required to maintain a film. This resulted in negligible differences between the surfaces utilized. Droplet train impingement on the same post and rib SH surfaces was also investigated. When each individual droplet impinged on the surface, a crown formed which spread out radially until reaching a semi-stable or regularly oscillating breakup diameter. At this point, the water would either build up or breakup into droplets or filaments and then continue radially outward. In some cases the crown would break up, causing splashing. A comparison to previous experiments on hydrophilic surfaces shows a distinct difference in splashing at low frequency. The breakup diameter was measured over a Weber number range of 72-2800. The data was collapsed as a function of a combination of the Reynolds number (Re), Capillary number (Ca), and Strouhal number (St), resulting in Re0.7CaSt. The rib SH surface displayed an elongated breakup due to the anisotropic surface features. The breakup diameter for the droplet train was compared to the breakup diameter which has been shown to occur with a jet impinging on a SH surface.

Flame retardant (FR) intumescent additives and montmorillonite (MMT) organoclay incorporated nylon-6 nanocomposite (FR-NC-PA6) fibers with a diameter of about 200 nm were fabricated by electrospinning. Before electrospinning, dispersion and exfoliation of the FR additive and MMT in nylon-6 were achieved by twin-screw extrusion. Tensile, TGA and UL-94 flammability tests were first performed using injection-molded bulk samples. The tensile modulus of FR-NC-PA6 was 45% higher than that of neat PA6, but tensile strength and elongation at break decreased by 23% and 98.7%, respectively. It is worth noting that although the TGA results show that FR-NC-PA6 has a slightly earlier decomposition temperature than neat PA6, it did not drip under fire and had the best rating (V-0) in UL 94 test, while neat PA6 is only rated as V-2. SEM and EDX of char residues after the UL 94 test clearly show the oxygen-rich protective char layer on the surface. These results indicate the advantage of using clay and FR additive in bulk-form PA6. Flammability of electrospun nanocomposite fibers was characterized by Micro-combustion calorimeter (MCC), a small-scale test to screen flammability of polymer materials. The MCC results show that the nano-fillers in both bulk and fiber form could effectively improve flame retardant properties of the material. Electrospun fibers had similar combustion properties as bulk materials. In addition to FR applications, superhydrophobic surface was another area that was explored using the electrospun nanocomposite fibers. Static water contact angle (WCA) test showed that samples with 5wt% clay even without plasma treatment greatly improved the WCA to 140°, probably due to the barrier effect of nanoclay platelets. Plasma treatment was used to modify the surface energy, further improving WCA to as high as 160°. However, fiber structure was partially etched away when overexposed to the plasma. This etching effect increased the surface roughness. Clay incorporated samples had higher level of surface roughness and better resistance to plasma etching compared to neat nylon 6. / text

This paper investigates the role of end-effects in superhydrophobic microchannels for frictional reduction through COMSOL based modeling. Two precursor derivations, the Kim & Hidrovo and Enright model are discussed and expanded upon through analytical and numerical simulations. The author performed numerical models on superhydrophobic microchannels with planar, stationary and finite separation distance of surface roughness element with perfect Cassie-Baxter air-layers. The simulations indicate an asymptotic limit for the flow-rate, indicating an optimum air-layer thickness. Numerical post processing reveals that this phenomenon is due to the recirculation end-effects that are relevant when the surface roughness separation distance is on order of magnitude of the channel width. These results are the first that identify end-effects as inducing a plateauing flow-rate and can serve as a benchmark for future studies. / text

In this study, we developed a facile method for preparing a superhydrophobic paper surface using a layer-by-layer deposition of polydiallyldimethylammonium chloride (polyDADMAC) and silica particles, followed by a fluorination surface treatment with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS, CF3(CF2)5CH2CH2Si(OC2H5)3). The wood fiber products prepared in this study had contact angles of water greater than 150 degree and sliding angles less than 5 degree. Besides their high water repelling property, the superhydrophobic paper products kept a high tensile strength at high relative humidity condition. The superhydrophobic paper products also showed high resistance to bacterial contamination.