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Superhydrophobic Surfaces:

Helmut Rathgen (Broschiert, Englisch)

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In this thesis optical diffraction was used to study the static and dynamic properties of microscopic liquid-gas interfaces that span between adjacent ridges of a superhydrophobic surface. An observed interference phenomenon at grazing incident angle led to the development of optical gratings with a large spectral bandwidth, and an observed sensitive response of the liquid-gas interfaces to ultrasound led to the development of a superhydrophobic fiber optical micro cavity that enables interferometric detection of the motion of a single microscopic meniscus. In Chapter 3 angle resolved diffraction from superhydrophobic optical gratings was used to measure the shape of the microscopic liquid-gas interfaces with nanometer resolution, under an applied hydrostatic pressure, under ambient as well as water-vapor conditions. A transition from the superhydrophobic to the impregnated state was observed in situ on the microscopic level. It was shown that the macroscopic collapse is triggered by depinning of the microscopic contact lines at a threshold angle that is in close agreement with Young’s macroscopic contact angle. The results confirmed experimentally that the assumption of a local contact angle, that forms the basis of several more recent contact angle models, is satisfied well below the micrometer scale. The experimental results are explained with a simple model that is based on the macroscopic laws of Laplace and Young, allowing the formulation of stability criteria for the design of optimal superhydrophobic and superoleophobic surfaces. Prospects of superhydrophobic surfaces for switchable and tunable diffractive optical elements were evaluated through numerical simulations. It was shown that a superhydrophobic optical grating fabricated from glass could provide an immersed grating with near 100% diffraction efficiency and a metallic grating with a larger grating period could provide a spectrally tunable grating with an on-off ratio close to one. In Chapter 4 possible conformation of a drop on a superhydrophobic surface were considered. Competing definitions of a macroscopic apparent contact angle were reviewed and the role of metastable drop states and external forcing was illustrated. Recent experimental data was reviewed, that shows a deficiency of the Cassie-Baxter model to describe measured contact angles on connected surface patterns. The considerations underline the need for solving more recent ’local contact angle’ models for drops on superhydrophobic surfaces, that appreciate the smallness of thermal fluctuations and the consequent local validity of Young’s condition and associated metastability of drop states. Further, they express the need for selecting from the set of allowed metastable states a suitable ensemble that provides a realistic and physical definition of a macroscopic apparent contact angle. In Chapter 5 dielectric optical gratings were studied through numerical simulations, with focus on the achievable spectral bandwidth. Grating geometries were considered (i) in transmission, (ii) ’buried’ grating between two glass bodies, (iii) TIR grating geometry. The effect of a high refractive index grating layer as well as slanted grating lamella was analyzed. It was shown that a suitable high refractive index grating layer dramatically improves the spectral bandwidth. Dielectric optical gratings with octave spanning -1dB bandwidth were devised, providing a larger spectral bandwidth than any grating known to date. Dielectric transmission gratings with 100% peak efficiency were devised by introducing a double layer grating. Slanted fused silica TIR gratings were studied. It was shown that these devices provide their largest efficiency not when operated in Littrow configuration but in a substantially different scattering geometry. Today, all grating designs are optimized for Littrow configuration. The observation recalls the theoretical foundation of the Littrow configuration and highlights that it is rigorously applicable only to symmetric grating profiles. In Chapter 6, the collective dynamics of arrays of microscopic liquid-gas interfaces formed at the openings of cylindrical hydrophobic micro-cavities were studied through time resolved optical diffraction. Their resonance behavior was described in Chapter 8 with a model that solves the unsteady Stokes flow equations for a single cavity-meniscus system and accounts for hydrodynamic interaction through a monopole approximation. It was shown that the collective dynamics are governed by hydrodynamic interaction, that results in a pronounced decrease of the resonance frequency of an array of micromenisci as compared to a single micromeniscus. The resonance behavior of a single cavity-meniscus system was studied experimentally in Chapter 7 through a hydrophobic optical micro-cavity fabricated on the end face of an optical fiber. The results confirmed quantitatively the unsteady Stokes flow model for a single cavity-meniscus system. By contrast, potential flow theory including dissipation integrals overestimated the heights of the resonance curves, i.e. underestimated dissipation, showing that viscous dissipation is dominated by vorticity generation inside the boundary layer. The experiments showed prospects for low noise ultrasound detection based on a single cavity-meniscus system on an optical fiber. Weniger zeigen

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Produktinformationen

Technische Daten

Erscheinungsdatum

05.12.2008

Sprache

Englisch

EAN

9783868440935

Herausgeber

sierke VERLAG - Sierke WWS GmbH

Sonderedition

Nein

Autor

Helmut Rathgen

Seitenanzahl

226

Auflage

Einbandart

Broschiert

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