Robust and Programmable Superhydrophobic Surfaces

This thesis deals with design, fabrication and characterization of robust and programmable superhydrophobic surfaces. The content is divided into two main categories. The first part is dedicated to fabrication of durable superhydrophobic surfaces and their performance characterization, including blood compatibility. The second part focuses on programmable superhydrophobic/superhydrophilic surfaces. A new fabrication process was introduced to produce durable superhydrophobic silicon surfaces, based on geometrical modification of the surface without any hydrophobic coating. The fabrication process is based on a combination of inductively coupled plasma-reactive ion etching (ICP-DRIE) and metal-assisted chemical etching (MaCE). Elimination of hydrophobic coating made the surface chemically and thermally robust, but due to use of silicon, the mechanical fragility issue remained. A biomimetic approach inspired by the exoskeleton of insects such as Armadillidium, was introduced to tackle the mechanical robustness. The new hybrid material consists of elastomeric overhang nanostructures covered by a thin layer of metal oxide. The elastomer part can bend and deform without breaking, while the hard metal oxide layer protects the surface from mechanical damage. We used atomic layer deposition (ALD) and replica moulding to produce such hybrid PDMS/titania surfaces. Our new fabrication process is based on sacrificial etching of the aluminum template and the transfer of titania film from the template to the PDMS. Subjecting the surfaces to a battery of mechanical, thermal, chemical and radiation tests demonstrated extremely durable superhydrophobicity. As an extension to replica moulding/sacrificial template process, we introduce a new way to make hydrophobic and hemophobic elastomeric tubes. The tubes showed a dramatic drag reduction for water and blood droplets compared to control tubes. A programmable superhydrophobic surface was introduced using hierarchical silicon micro and nanostructures covered by photo-switchable materials. Combination of ICP-DRIE and RIE were used to produce silicon T-shaped microstructures. Colloidal deposition was done to introduce nanoscale roughness. These structures were then conformally coated with photoactive titania using ALD. Fast and reversible transition of superhydrophobic to hydrophobic, hydrophilic and superhydrophilic state was demonstrated using UV-exposure and thermal annealing.