Superhydrophobic surfaces are able to entrap gas pockets in-between surface roughness elements when submerged in water. These entrapped gas pockets give these surfaces the potential to reduce drag due to the overlying flow being able to locally slip over the gas pockets, resulting in a mean slip at the surface. This thesis investigates the different effects that slip and the texturing of the surface have on turbulence over superhydrophobic surfaces. It is shown that, after filtering out the texture-induced flow, the background, overlying turbulence experiences the surface as a homogeneous slip boundary condition. For texture sizes, expressed in wall units, up to $L^+ \lesssim 20$ the only effect of the surface texture on the overlying flow is through this surface slip. The direct effect of slip does not modify the dynamics of the overlying turbulence, which remains canonical and smooth-wall-like. In these cases the flow is governed by the difference between two virtual origins, the virtual origin of the mean flow and the virtual origin experienced by the overlying turbulence. Streamwise slip deepens the virtual origin of the mean flow, while spanwise slip acts to deepen the virtual origin perceived by the overlying turbulence. The drag reduction is then proportional to the difference between the two virtual origins, reminiscent of drag reduction using riblets. The validity of slip-length models to represent textured superhydrophobic surfaces can resultantly be extended up to $L^+ \lesssim 20$. However, for $L^+ \gtrsim 25$ a non-linear interaction with the texture-coherent flow alters the dynamics of the background turbulence, with a reduction in coherence of large streamwise lengthscales. This non-linear interaction causes an increase in Reynolds stress up to $y^+ \lesssim 25$, and decreases the obtained drag reduction compared to that predicted from homogeneous slip-length models.

Superhydrophobic surfaces, which display water contact angles of larger than 150°, have attracted more and more attention due to their importance in both fundamental research and practical applications. This dissertation is mainly focused on the fundamental understanding and exploring applications of superhydrophobic surfaces. First, some specific examples of superhydrophobic surface fabrication were given, which include superoleophobic Si surface, robust superhydrophobic SiC surface, and reversible wettability nanocomposite films. Based on the study of superhydrophobic surfaces, the application of superhydrophobic surfaces in electronic packaging were explored. Superhydrophobic silica/epoxy nanocomposite coating serves as an encapsulant to improve the electronic device reliability. Such superhydrophobic coating showed good stability under humidity at elevated temperatures and was applied on the triple track resistors test coupons. In addition, the applications of superhydrophobic surfaces in solar cells were studied. Two multi-functional hierarchical structure solar cells with self-cleaning, low reflection and high efficiency properties were built up by coating or etching methods.

An aircraft may experience inlight ice accretion and corresponding reductions in performance and control when the vehicle encounters clouds of super-cooled water droplets. The EADS-IW Surface Engineering Group is investigating passive anti-icing possibilities, such as functional and ice phobic coatings. Ice-resistant coatings require investigating droplet impact on dry surfaces and wet films, including microscopic effects such as droplet splashing. To investigate droplet impacts, a volume of fluid (VOF) flow solver was used for droplets impacting dry and wetted hydrophobic and superhydrophobic surfaces, focusing on meso-scale simulations. The effects of structured, micro-scale surface roughness and the effects of a thin wet film on the surface, corresponding to a saturated surface under high humidity conditions, were investigated. Axisymmetric domains produced acceptable results for smooth, dry surfaces. It was determined that in order to properly predict behavior of droplets impacting surfaces with structured micro-scale roughness, three-dimensional simulations are recommended.

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As the fluidic devices are miniaturized to improve portability, the friction of the microchannel becomes intrinsically high and a high pumping power will be required to drive the fluid. Since the pumping power delivered by portable devices is limited, one method to reduce this is to render the surface to become slippery. This can be achieved by roughening up the microchannel wall and form a bed of air pockets between the roughness elements, which is known as the superhydrophobic Cassie-Baxter state. While the study on superhydrophobic microchannels are focused mainly in maximizing the friction reduction effects and maintaining the stability of the air pockets, less attention has been given to characterizing the microchannel friction under a metastable state, where partial flooding of the micro-textures may be present, and under heated conditions, where the air pockets are trapped between the micro-textures. In order to quantify the frictional characteristics, microchannels with micron-sized trenches on the side walls were fabricated and tested under varying inlet pressures and heating conditions. By measuring the hydrodynamic resistance and comparing with numerical simulations, results suggest that (1) the air-water interface behaves close to a no-slip boundary condition, (2) friction becomes insensitive to the wetting degree once the micro-trenches become highly wetting, (3) the fully wetted micro-trench may be beneficial over the de-wetted ones in order to achieve friction reduction effects and (4) heating the micro-trenches to induce a highly de-wetting state may actually be detrimental to the microchannel flow due the excessive growth of the air layer. As part of the future work to characterize heat transfer in superhydrophobic microchannels, a rectangular microchannel with microheaters embedded close to the side walls was fabricated and the corresponding heat transfer rates were measured through dual fluorescence thermometry. Results suggested that significant heat is lost through the environment despite the high thermal resistance of the microchannel material. An extra insulation is suggested prior to characterizing the convective heat transfer coefficients in the superhydrophobic microchannel flow. / text

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.

Superhydrophobic surfaces have gained attention as a potential mechanism for increasing condensation heat transfer rates. Various aspects related to condensation heat transfer are explored. Adiabatic, air-water mixtures are used to explore the influence of hydrophobicity on two-phase flows and the hydrodynamics which might be present in flow condensation environments. Pressure drop measurements in a rectangular channel with one superhydrophobic wall (cross-section approximately 0.37 X 10 mm) are obtained, revealing a reduction in the pressure drop for two-phase flow compared to a control scenario. The observed reduction is approximately 10% greater than the reduction that is observed for single-phase flow (relative to a classical channel). Carbon nanotubes have been used to create superhydrophobic coatings due to their ability to offer a relatively uniform nanostructure. However, as-grown carbon nanotubes often require the addition of a thin-film hydrophobic coating to render them superhydrophobic, and fine control of the overall nanostructure is difficult. This work demonstrates the utility of using carbon infiltration to layer amorphous carbon on multi-walled nanotubes to achieve superhydrophobic behavior with tunable geometry. The native surface can be rendered superhydrophobic with a vacuum pyrolysis treatment, with contact angles as high as 160 degrees and contact angle hysteresis less than 2-3 degrees. Drop-size distribution is an important aspect of heat transfer modeling that is difficult to measure for small drop sizes. The present work uses a numerical simulation of condensation to explore the influence of nucleation site distribution approach, nucleation site density, contact angle, maximum drop size, heat transfer modeling to individual drops, and minimum jumping size on the distribution function and overall heat transfer rate. The simulation incorporates the possibility of coalescence-induced jumping over a range of sizes. Results of the simulation are compared with previous theoretical models and the impact of the assumptions used in those models is explored. Results from the simulation suggest that when the contact angle is large, as on superhydrophobic surfaces, the heat transfer may not be as sensitive to the maximum drop-size as previously supposed. Furthermore, previous drop-size distribution models may under-predict the heat transfer rate at high contact angles. Condensate drop behavior (jumping, non-jumping, and flooding) and size distribution are shown to be dependent on the degree of subcooling and nanostructure size. Drop-size distributions for surfaces experiencing coalescence-induced jumping are obtained experimentally. Understanding the drop-size distribution in the departure region is important since drops in this size are expected to contribute significantly to the overall heat transfer rate.

Superhydrophobic surfaces are a class of surfaces that have a microscale roughness imposed on an already hydrophobic surface, akin to a lotus leaf. These surfaces have been shown to produce significant drag reduction for both laminar and turbulent flows of water through large and small-scale channels. The goal of this thesis was to explore how these surfaces alter the vortex shedding dynamics of a cylindrical body when coated on its surface, thus leading to an alteration in drag and lift on these surfaces. A cylindrical body was chosen as it is a very nice representative bluff body and sets the stage for predicting the behavior of hydrofoils and other bluff bodies under flow with a slip boundary condition. In this work, a series of experiments were performed which investigated the effect of superhydrophobic-induced slip on the flow past a circular cylinder. In these experiments, circular cylinders were coated with a series of superhydrophobic surfaces fabricated from PDMS with well-defined micron-sized patterns of surface roughness or random slip surfaces fabricated by sanding Teflon cylinders or spray painting superhydrophobic paint on a smooth cylinder. The presence of the superhydrophobic surface was found to have a significant effect on the vortex shedding dynamics in the wake of the circular cylinder. When compared to a smooth, no-slip cylinder, cylinders coated with superhydrophobic surfaces were found to delay the onset of vortex shedding and increase the length of the recirculation region in the wake of the cylinder. For superhydrophobic surfaces with ridges aligned in the flow direction the separation point was found to move further upstream towards the front stagnation point of the cylinder and the vortex shedding frequency was found to increase. For superhydrophobic surfaces with ridges running normal to the flow direction, the separation point and shedding frequency trends were reversed. The vortices shed from these surfaces were found to be weaker and less interlaced leading to reduced circulation and lift forces on these cylinders. The effect of slip on bluff bodies and separating flow was dealt with in detail in this thesis and the results could be used to predict the impact of these surfaces on the flow past hydrofoils which combine skin friction dominated flow with separating flow.

This thesis examines the time resolved heat transfer to drops rolling or sliding along inclined, subcritical heated non-wetting surfaces. Results were experimentally obtained using IR imaging for a smooth hydrophobic surface and post as well as rib structured superhydrophobic surfaces of varying solid fraction (f_s = 0.06 - 0.5). Tests were performed at varying inclination angle (α = 10, 15, 20, and 25°), drop volume (12, 20, 30, and 40 μL), and surface temperature (T_w = 50, 65, and 80 °C). Rib structured superhydrophobic surfaces were explored for drops moving parallel and perpendicular to the rib structures. The findings indicate that transient heat transfer is predominantly influenced by the surface’s solid fraction and the velocity of the drops, with a secondary dependence on drop volume. Surfaces with low solid fraction show a significant reduction in initial heating rate (up to 80% reduction) to the drop, when compared with that of the smooth surface. The drop velocity depends on surface solid fraction and inclination angle, with drop volume exerting smaller influence. Rib structured surfaces impact heat transfer by enhancing heat transfer rate for drops that move along the rib direction compared with drops that move perpendicular to the ribs. The difference is likely due to increased drop velocity that exists for the parallel rib orientation.

Millions of years of evolution have led to a wealth of highly adapted functional surfaces in nature. Among the most fascinating are superhydrophobic surfaces which are highly water-repellent and shed drops very easily owing to their chemical hydrophobicity combined with micropatterning. Superhydrophobic materials have attracted a lot of attention due to their practical applications as ultra-low friction surfaces for ships and pipes, water harvesters, de-humidifiers and cooling systems. At small length scales, where surface tension dominates over gravity, these surfaces show a wealth of phenomena interesting to physicists, such as directional flow, rolling, and drop bouncing. This thesis focuses on two examples of dynamic drop interactions with micropatterned surfaces and studies them by means of a lattice Boltzmann simulation approach. Inspired by recent experiments, we investigate the phenomenon of the self-propelled bouncing of coalescing droplets. On highly hydrophobic patterned surfaces drop coalescence can lead to an out-of-plane jump of the composite drop. We discuss the importance of energy dissipation to the jumping process and identify an anisotropy of the jumping ability with respect to surface features. We show that Gibbs' pinning is the source of this anisotropy and explain how it leads to the inhibition of coalescence-induced jumping. The second example we study is the novel phenomenon of pancake bouncing. Conventionally, a drop falling onto a superhydrophobic surface spreads due to its inertia, retracts due to its surface tension, and bounces off the surface. Here we explain a different pathway to bouncing that has been observed in recent experiments: A drop may spread upon impact, but leave the surface whilst still in an elongated shape. This new behaviour, which occurs transiently for certain impact and surface parameters, is due to reversible liquid imbibition into the superhydrophobic substrate. We develop a theoretical model and test it on data from experiments and simulations. The theoretical model is used to explain pancake bouncing in detail.