Investigating wetting characteristics on microstructured surfaces for superhydrophobicity and metal microcasting

The engineering of liquid behavior on surfaces is important for infrastructure, transportation, manufacturing, and sensing. Surfaces can be rendered superhydrophobic by microstructuring, and superhydrophobic devices could lead to practical corrosion inhibition, self-cleaning, fluid flow control, and surface drag reduction. To more fully understand how liquid interacts with microstructured surfaces, this dissertation introduces a direct method for determining droplet solid-liquid-vapor interfacial geometry on microstructured surfaces. The technique performs metrology on molten metal droplets deposited onto microstructured surfaces and then frozen. Unlike other techniques, this visualization technique can be used on large areas of curved and opaque microstructured surfaces to determine contact line. This dissertation also presents measurements and models for how curvature and flexing of microstructured polymers affects hydrophobicity. Increasing curvature of microstructured surfaces leads to decreased slide angle for liquid droplets suspended on the surface asperities. For a surface with regularly spaced asperities, as curvature becomes more positive, droplets suspended on the tops of asperities are suspended on fewer asperities. Curvature affects superhydrophobicity because microscopic curvature changes solid-liquid interaction, pitch is altered, and curvature changes the shape of the three phase contact line. This dissertation presents a model of droplet interactions with curved microstructured surfaces that can be used to design microstructure geometries that maintain the suspension of a droplet when curved surfaces are covered with microstructured polymers. Controlling droplet dynamics could improve microfluidic devices and the shedding of liquids from expensive equipment, preventing corrosion and detrimental performance. This dissertation demonstrates redirection of dynamic droplet spray with anisotropic microstructures. Superhydrophobic microstructured surfaces can be economically fabricated using metal embossing masters, so this dissertation describes casting-based microfabrication of metal microstructures and nanostructures. Low melting temperature metal was cast into flexible silicone molds which were themselves cast from microfabricated silicon templates. The flexibility of the silicone mold permits casting of curved surfaces, which this dissertation demonstrates by fabricating a cylindrical metal roller with microstructures. The metal microstructures can be in turn used as a reusable molding tool. This dissertation also describes an industrial investment casting process to produce aluminum molds having integrated microstructures. Unlike conventional micromolding tools, the aluminum mold was large and had complex curved surfaces. The aluminum was cast into curved microstructured ceramic molds which were themselves cast from curved microstructured rubber. Many structures were successfully cast into the aluminum with excellent replication fidelity, including circular, square, and triangular holes. This dissertation demonstrates molding of large, curved surfaces having surface microstructures using the aluminum mold. This work contributes a more full understanding of the phenomenon of superhydrophobicity and techniques for the economic fabrication of superhydrophobic microstructures.