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Experiments were performed to characterize hydraulic jumps that form due to liquid jet impingement on superhydrophobic surfaces with alternating micro-ribs and cavities. If the surface is unimmersed, a surface tension based transition into droplets occurs, so a known depth of water was imposed downstream from the hydraulic jump to ensure the existence of a hydraulic jump. The surfaces are characterized by the cavity fraction, which is defined as the width of a cavity divided by the combined width of a cavity and an adjoining rib. Four different surface designs were studied, with respective cavity fractions of 0 (smooth surface), 0.5, 0.8, and 0.93. Each surface was tested in its naturally hydrophilic state where water was allowed to flood the cavities, as well as with a hydrophobic coating which prevented water from entering the cavities and created a liquid-gas interface over much of the surface. The experimental data spans a Weber number range (based on the jet velocity and radius) of 3x102 to 1.05x103 and a corresponding Reynolds number range of 1.15x104 to 2.14x104. While smooth surfaces always result in circular transitions, for any rib and cavity patterned surface the flow exhibits a nearly elliptical transition from the thin film, where the major axis of the ellipse is parallel to the ribs, concomitant with greater slip in that direction. When the downstream depth is small and a superhydrophobic surface is used, the water is completely expelled from the surface, and the thin film breaks up into droplets due to surface tension interactions. When the downstream depth is large or the surface is hydrophilic a hydraulic jump exists. When the water depth downstream of the jump increases, the major and minor axis of the jump decreases due to an increase in hydrostatic force, following classical hydraulic jump behavior. The experimental results indicate that for a given cavity fraction and downstream depth, the radius of the jump increases with increasing Reynolds number. The jump radius perpendicular to the ribs is notably less than that for a smooth surface, and this radius decreases with increasing cavity fraction. When comparing flow over superhydrophobic (coated) surfaces to patterned, hydrophilic (uncoated) surfaces, a general increase is seen in the radial location of the hydraulic jump in the direction of the ribs, while no statistically significant change is seen in the direction perpendicular to the ribs.

<p>A computer model based on minimization of the free energy, capable to predict contact angles and spreading transitions between Wenzel and Cassie mode for drops placed on surfaces with different topography were implemented in matlab. Simulations were compared with experiments documented in the literature. These showed that reported transitions between Cassie and Wenzel mode can be explained by minimization of the free energy. In this report, a study on the possibility of constructing water repellent paper surfaces with a combination of treatment with octadecyltrichlorosilane and topography changes, is included.</p>

A computer model based on minimization of the free energy, capable to predict contact angles and spreading transitions between Wenzel and Cassie mode for drops placed on surfaces with different topography were implemented in matlab. Simulations were compared with experiments documented in the literature. These showed that reported transitions between Cassie and Wenzel mode can be explained by minimization of the free energy. In this report, a study on the possibility of constructing water repellent paper surfaces with a combination of treatment with octadecyltrichlorosilane and topography changes, is included.

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Water droplets tend to bead up on rough or textured hydrophobic surfaces by trapping air on the crevices underneath resulting in “Cassie” state of wetting. When a textured hydrophobic surface is immersed in water, the resulting underwater “Cassie” state can lead to significant drag reduction. The entrapped air pockets act as shear free regions and the composite interface consisting of alternate no slip and no shear regions thus formed can deliver substantial drag reduction during flow. The magnitude of drag reduction depends not only on the fractional coverage of air on the surface, but also on the size of the air pockets, with larger sized air pockets facilitating larger drag reduction. It is a common observance that Lotus leaf when kept immersed in water for a few minutes loses its water repellency due to the loss of entrapped air on the surface. Underwater Cassie state on textured hydrophobic surfaces is also not sustainable because of the depletion of air pockets caused by the diffusion of trapped air into water. This causes the drag reduction to diminish with time. Rate of diffusion of air across the water–air interface depends on the concentration gradient of air across the interface. Under flow conditions, removal of entrapped air is further enhanced by convection, leading to more rapid shrinkage of the air pockets. In order to sustain the Cassie state, it is thus necessary to continuously supply air to these air pockets. In this work, we explore the possibility of supplying air to the cavities on the textured surface inside a microchannel by controlling the solubility of air in water close to the surface. The solubility is varied by i) Controlling the absolute pressure inside the channel and ii) Localized heating of the surface To trap uniform air pockets, a textured surface containing a regular array of blind holes is used. The textured surface is generated by photo etching of brass and is rendered hydrophobic through a self-assembled monolayer. The sustainability of the underwater Cassie state of wetting on the surface is studied at various flow conditions. The air trapped on the textured surface is visualized using total internal reflection based technique, with the pressure drop (or drag) being simultaneously measured. Water which is initially saturated with air at atmospheric conditions, when subjected to sub-atmospheric pressures within the channel becomes supersaturated causing the air bubbles to grow in size. Further growth causes the bubbles to merge and eventually detach from the surface. The growth and subsequent merging of the air bubbles leads to a substantial increase in the pressure drop because as the air pockets grow in size, they project into the flow and start obstructing the flow. On the other hand, a pressure above the atmospheric pressure within the channel makes the water undersaturated with air, leading to gradual shrinkage and eventual disappearance of air bubbles. In this case, the air bubbles do cause reduction in the pressure drop with the minimum pressure drop (or maximum drag reduction) occurring when the bubbles are flush with the surface. The rate of growth or decay of air bubbles is found to be significantly dependent on the absolute pressure in the channel. Hence by carefully controlling the absolute pressure, the Cassie state of wetting can be sustained for extended periods of time. A drag reduction of up to 15% was achieved and sustained for a period of over 5 hours. Temperature of water also influences the solubility of air in water with higher temperatures resulting in reduced solubility. Thus locally heating the textured hydrophobic surface causes the air bubbles to grow, with the rate of growth being dependent on the heat input. The effect of trapped air bubbles on thermal transport is also determined by measuring the heat transfer rate through the surface in the presence and absence of trapped air bubbles. Even though the trapped air bubbles do cause a reduction in the heat transfer coefficient by about 10%, a large pressure drop reduction of up to 15% obtained during the experiments helps in circumventing this disadvantage. Hence for the same pressure drop across the channel, the textured hydrophobic surface helps to augment the heat transfer rate. The experiments show that, by varying the solubility of air in water either by controlling the pressure or by local heating, underwater Cassie state of wetting can be sustained on textured hydrophobic surfaces, thus delivering up to 15% drag reduction in both cases for extended periods of time. The results obtained hold important implications towards achieving sustained drag reduction in microfluidic applications.

This thesis deals with cultural sights reconstruction, particularly of sacral character, in villages of Český Krumlov district - in a region formerly inhabited by German population. The introductory parts focus on the development of Czech-German co-existence in Czech lands with its historical chapters, as well as on the impact of the 1939&#8209;1989 events on cultural sights condition in the Czech borderland, particularly in the region of Český Krumlov. The following part is dedicated to history and mainly to the reconstruction of these sights. These reconstructions are a common venture of a cross-border cooperation of Czechs with expatriated German compatriots. The final part of the thesis is enriched by brief biographies of significant people who have bonds to this region.

This thesis is divided into two main parts. The First cultural-historical part is dedicated to introduction of cultural-historical context between Bohemian kingdom and Silesia. Historical development of Silesia and its joining into the union of Lands of Bohemian crown is also shortly discussed. The second art-historical chapter in first part brings the resume of literature dedicated to phenomenon of beautiful style. The second part brings resume of literature dedicated particularly to the Silesian centre of beautiful style. And the last part of this chapter consists of a catalogue of chosen works of beautiful style from Wroclaw. Those are Pietà of St. Elisabeth in Wroclaw, Pietà of Virgin Mary on Sand church in Wroclaw, Man of sorrows from Goldsmith's altar, Pietà from St. Stanislaw and Wenzel church in Sweidnitz, Pietà from St. Matthew church. For each of these works a detailed formal analysis is made and existing research outputs are summarized. As a conclusion of researches mentioned above an attempt is made to improve accuracy of dating and to answer some questions related to those works of art.