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Matter on earth exists mostly in three different phases of solid, liquid, and gas. With extreme amounts of energy, temperature, or pressure, a matter can be changed between the phases. Six different types of phase-change phenomena are possible: freezing (the substance changes from a liquid to a solid), melting (solid to liquid), condensation (gas to liquid), vaporization (liquid to gas), sublimation (solid to gas), and desublimation (gas to solid). Another form of phase change which will be discussed here is the wetting or dewetting transitions of a superhydrophobic surface, in which the phase residing within the surface structure switches between vapor and liquid. Phase transition phenomena frequently occur in our daily life; examples include: a ``liquid'' to ``solid'' transition when cars decrease their distance at a traffic light, solidification of liquids droplets during winter months, and the dancing of droplets on a non-sticking pan. In this dissertation we will address seven different phase-change problems occurring in nature. We unveil completely new forms of phase-change phenomena that exhibit rich physical behavior. For example, during traffic flow, drivers keep a large distance from the vehicle in front of them to ensure safe driving. When vehicles come to a stop, for example at a red light, drivers voluntarily induce a ``phase transition'' from this ``liquid phase'' to a close-packed ``solid phase''. This phase transition is motivated by the intuition that traveling as far as possible before stopping will minimize the overall travel time. However, we are going to investigate this phase-change process and show that this long standing intuition is wrong. Phase-change of solidification will be discussed for different problems. Moreover, the complex physics of oil as it wicks up sheets of frost and freezing of bubble unveil completely new forms of multiphase flows that exhibit rich physical behavior. Finally, the ``Cassie'' to ``Wenzel'' transition will be investigated for layered nano-textured surfaces. These phenomena will be modeled using thermodynamics and fluid mechanics equations. / Doctor of Philosophy / The main focus of this dissertation is on the dynamical phase change phenomena occurring in nature. First, we study the solid to liquid phase change of group of people moving from rest. We show that increasing the packing density of vehicles at a stop-and-go motion (e.g., vehicles at a traffic light) would not increase the efficiency of the flow once it is resumed. Second, we present a passive anti-frosting surfaces just by using the chemistry of ice. We show how the in-plane frost growth can be passively suppressed by patterning arrays of microscopic ice stripes across a surface. Third, we elucidate how bubbles deposited on a chilled and icy substrate freeze in different ambient conditions. We reveal the various phenomena that govern how soap bubbles freeze and produce a variety of beautiful effects. Fourth, we will study oil-ice interactions which are important for the emerging science of using oil-impregnated surfaces for anti-icing and anti-frosting applications, where oil drainage from the surface due to wicking onto ice is a pressing issue. We observe oil as it wicks up sheets of frost grown on aluminum surfaces of varying wettability: superhydrophilic, hydrophilic, hydrophobic, and superhydrophobic. Fifth, we study the effect of topography of the nanopillars on dynamics of jumping droplets. The critical diameter for jumping to occur was observed to be highly dependent on the height and diameter of the nanopillars, with droplets as small as 2 µm jumping on the surface with the tallest and most slender pillars. Sixth, we show that micrometric condensate spontaneously launches several millimeters from a wheat leaf’s surface, taking adhered pathogenic spores with it. We quantify spore liberation rates of order 10 cm⁻² hr⁻¹ during a dew cycle. Finally, inspired by duck feathers, two-tier porous superhydrophobic surfaces were fabricated to serve as synthetic mimics with a controlled surface structure. We show the effect of layers of feathers on energy barrier for the wetting transition.

A superhydrophobic coating can be produced using a hydrophobic material textured with surface roughness on the micro-/nano-scale. Such a coating on the outside of a submersible body may result in reduced skin-friction drag due to a trapped layer of air in the coating. However, this layer may become unstable when subjected to elevated hydrostatic pressures, and a coating’s performance is compromised beyond a certain threshold (critical pressure). This thesis presents a numerical model for predicting the pressure tolerances of superhydrophobic coatings comprised of randomly deposited hydrophobic particles or fibers. We have also derived a set of force-balance-based analytical equations for predicting critical pressure in surfaces with ordered roughness, and compared our numerical model against it, observing reasonable agreement. The numerical model was then applied in a large parameter study, predicting critical pressure for coatings with a given set of microstructure properties.

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Ice accretion on various surfaces can cause destructive effect of our lives, from cars, aircrafts, to infrastructure, power line, cooling and transportation systems. There are plenty of methods to overcome the icing problems including electrical, thermal and mechanical process to remove already accumulated ice on the surfaces and to reduce the risk of further operation. But all these process required substantial amount of energy and high cost of operation. To save the global energy and to improvement the safety issue in many infrastructure and transportation systems we have to introduce some passive anti-icing coating known as ice-phobic coating to reduce the ice-formation and ice adhesion onto the surface. Ice-phobic coatings mostly devoted to utilizing lotus-leaf-inspired superhydrophobic coatings. These surfaces show promising behavior due to the low contact area between the impacting water droplets and the surface. In this present study we investigate systematically the influence of chemical composition and functionality as well as structure of surfaces on wetting properties and later on icing behavior of surfaces. Robust anti-icing coating has been prepared by using modified silica particles as a particles film. Polymer brushes were synthesized on flat, particle surfaces by using Surface initiated ATRP. We have also investigated the effect of anti-icing behavior on the surfaces by varying surface chemistry and textures by using different sizes of particles. This approach is based on the reducing ice accumulation on the surfaces by reducing contact angle hysteresis. This is achieved by introducing nano to micro structured rough surfaces with varying surface chemistry on different substrates. Freezing and melting dynamics of water has been investigated on different surfaces by water vapour condensation in a high humidity (80%) condition ranging from super hydrophilic to super hydrophobic surfaces below the freezing point of water. Kinetics of frost formation and ice adhesion strength measurements were also performed for all samples. All these experiments were carried out in a custom humidity and temperature controlled chamber. We prepared a superhydrophobic surface by using Poly dimethyl siloxane (PDMS) modified fumed silica which display very low ice-adhesion strength almost 10 times lower than the unmodified surface. Also it has self-cleaning behavior after melting of ice since whole ice layer was folded out from the surface to remove the ice during melting. Systematic investigation of the effect of three parameters as surface energy, surface textures (structure, geometry and roughness) and mechanical properties of polymers (soft and stiff) on icing behavior has also been reported.

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.

Since metals in general, and steels in particular, are vital construction materials in our modern society, the corrosion protection of said materials is of great importance, both to ensure safety and to reduce costs associated to corrosion. Previously, chromium (VI) and other harmful substances were effectively used to provide corrosion protection to steel, but since their use was heavily regulated around year 2000, no coating has yet been developed that, in a fully satisfactory manner, replaces their corrosion protective properties.In this thesis, the use of hydrophobic and superhydrophobic surface coatings as part of corrosion protective coating systems has been studied. Since the corrosion mechanism relies on the presence of water to take place, the use of a superhydrophobic coating to retard the penetration of water to an underlying metal surface is intuitive. The evaluation of corrosion protective properties of the hydrophobic and superhydrophobic surfaces was performed using mainly contact angle measurements and electrochemical measurements in severely corrosive 3 wt% NaCl water solution.First, the differences in corrosion protection achieved when employing different hydrophobic wetting states were investigated using a model alkyl ketene dimer wax system. It was found that superhydrophobicity in the Lotus state is superior to the other states, when considering fairly short immersion times of less than ten days. This is due to the continuous air film that can form between such a superhydrophobic surface and the electrolyte, which can retard the transport of electrolyte containing corrosive ions to the metal surface to the point where the electrical circuit is broken. Since corrosion cannot occur unless an electrical current is flowing, this is a very efficient way of suppressing corrosion.An air layer on an immersed superhydrophobic surface is, however, not stable over long time, and to investigate long-term corrosion protection using hydrophobic coatings a polydimethylsiloxane formulation containing hydrophobic silica nanoparticles was developed. This system showed enhancement in corrosion protective properties with increasing particles loads, up until the point where the particle load instead causes the coating to crack (at 40 wt%). The conclusion is that the hydrophobicity of the matrix and filler, in combination with the elongatedivdiffusion path supplied by the addition of particles, enhanced the corrosion protection of the underlying substrate.To further understand how hydrophobicity and particle addition affect the corrosion protective properties of a coating a three layer composite coating system was developed. Using this coating system, consisting of a polyester acrylate base coating, covered by TiO2 particles (with diameter &lt; 100 nm) and finally coated with a thin hexamethyl disiloxane coating, it was found that both hydrophobicity and particles are needed to reach a great enhancement in corrosion protective properties also for this system. / Eftersom metaller, och då särskilt stål, är viktigta konstruktionsmaterial i vårt moderna samhälle är korrosionsskydd av stor betydelse, både för att garantera säkerhet och för att minska kostnader som uppkommer i samband med korrosion. Tidigare har sexvärt krom och andra skadliga ämnen använts för att på ett effektivt sätt skydda stål från korrosion, men efter att deras användning kraftigt reglerades runt år 2000 har ännu ingen beläggning utvecklats som helt kan ersätta krombeläggningarna med avseende på funktion.I denna avhandling har hydrofoba och superhydrofoba ytbeläggningar och deras möjliga applikation som en del av ett korrosionsskyddande beläggningssystem studerats. Eftersom korrosionsmekanismen är beroende av närvaron av vatten, är användandet av en superhydrofob beläggning för att fördröja transporten av vatten till den underliggande metallytan intuitiv. De korrosionsskyddande egenskaperna hos superhydrofoba ytbeläggningar utvärderades här främst med hjälp av kontaktvinkelmätningar och elektrokemisk utvärdering i korrosiv lösning bestående av 3 vikts% NaCl i vatten.Först undersöktes skillnaden i korrosionsskydd som uppnås vid användandet av ytbeläggningar med olika hydrofoba vätningsregimer med hjälp av ett modellsystem bestående av ett alkylketendimer vax. Det konstaterades att superhydrofobicitet i Lotusregimen är överlägset bättre än de andra hydrofoba vätningsregimerna, i alla fall när man ser till relativt korta exponeringstider, typiskt mindre än tio dagar. Detta beror på att den kontinuerliga luftfilm som kan bildas på en sådan typ av superhydrofob yta kan minska transporten av elektrolyt (som innehåller korrosiva joner) till metallytan till den grad att den elektriska kretsen bryts. Eftersom korrosion inte kan ske utan en sluten elektrisk krets är detta ett mycket effektivt sätt att förhindra korrosion från att ske.Ett luftskikt på en superhydrofob yta nedsänkt i vatten är dock inte stabilt under lång tid. För att undersöka möjligheten till korrosionsskydd under längre tid med hjälp av hydrofoba beläggningar utvecklades en hydrofob ytbeläggning bestående av polydimetylsiloxan och hydrofoba nanopartiklar av kiseldioxid. Detta system visade en förbättring av korrosionsskyddet vid ökat partikelinnehåll upp till den koncentration (40 wt%) där i stället sprickbildning i ytbeläggningen observerades. Från detta system kunde slutsatsen dras att matrisens och partiklarnasvihydrofobicitet i kombination med den längre diffusionsvägen som partiklarna orsakade förbättrade korrosionsskyddet av den underliggande metallen.För att ytterligare förstå hur hydrofobicitet och partikeltillsatser påverkar en ytbeläggnings korrosionsskyddande egenskaper har dessutom ett treskikts kompositbeläggningssystem utvecklats. Genom att använda detta beläggningssystem, som består av en basbeläggning av polyesterakrylat, ett lager TiO2-partiklar (med en diameter på &lt;100 nm) slutligen belagt med ett tunt ytskikt bestående av hexametyldisiloxan så kunde slutsatsen dras att både en hydrofob matris och partiklar behövs för att nå en markant förbättring av ytbeläggningens korrosionsskyddande egenskaper. / <p>QC 20151015</p>

Surfaces with static contact angle greater than 150 degrees are typically classified as superhydrophobic. Such coatings have been inspired by the lotus leaf. As water flows over a superhydrophobic surface, "slip effect" is produced resulting in a reduction in the skin-friction drag exerted on the surface. Slip flow is caused by the entrapment of a layer of air between water and the surface. Superhydrophobicity could be utilized to design surfaces for applications such as energy conservation, noise reduction, laminar-to-turbulent-transition delay, and mixing enhancement. A popular method of manufacturing a superhydrophobic surface is microfabrication in which well-designed microgrooves and/or poles are placed on a surface in a regular configuration. This method is a costly process and cannot easily be applied to large-scale objects with arbitrary shapes. In this work, we fabricated and characterized simpler low-cost superhydrophobic coatings based on controlling the volume of entrapped air in order to enhance durability (longevity) and the properties of the coating bringing the technology closer to large-scale submerged bodies such as submarines and ships. Two different low-cost fabricating techniques have been utilized: (i) random deposition of hydrophobic aerogel microparticles; and (ii) deposition of hydrophobic polymer micro- and nanofibers using DC-biased AC-electrospinning. The present study is aimed at providing experimental, numerical, and analytical models to characterize the superhydrophobicity and longevity of the coatings depending on the morphology of the surfaces and the concentration of the hydrophobic materials. The surface's micro/nanostructure were observed by field emission scanning electron microscopy. The degree of hydrophobicity of the coatings was estimated using drag-reduction and contact-angle measurements using a rheometer and a goniometer respectively. Furthermore, We have advanced and calibrated a novel optical technique to noninvasively measure the longevity of submerged superhydrophobic coatings subjected to different environmental conditions. We have also modeled the performance of superhydrophobic surfaces comprised of randomly distributed roughness. The numerical simulations are aimed at improving our understanding of the drag-reduction effect and the stability of the air–water interface against pressure in terms of the microstructure parameters. Moreover, we have experimentally characterized the terminal pressure (i.e. the pressure at which the air–water interface completely fails) of aerogel coatings with different morphologies.

The goal of this study is to quantify the transient performance of microfabricated superhydrophobic surfaces when used in underwater applications. A mathematical framework is developed and used to predict the stability, longevity, and drag reduction benefits of submerged superhydrophobic surfaces with two- or three-dimensional micro-textures. In addition, a novel design is proposed to improve the drag-reduction benefits of lubricant-infused surfaces, by placing a layer of trapped air underneath the lubricant layer. The new design is referred to as lubricant–infused surfaces with trapped air, and it is designed to eliminate the long-lasting longevity problem of submerged superhydrophobic surfaces. The effectiveness of liquid-infused surface with trapped air design was examined via numerical simulation, and it was found to outperform its liquid-infused surface counterpart by about 37%.