Programmable and Responsive Superhydrophobic Surfaces

Abstract

Progress in the field of superhydrophobic surfaces requires precise characterization techniques and synthesis of surfaces that exhibit robust non-wettability. In this thesis, microfabrication techniques are used to produce static as well as bioinspired responsive superhydrophobic structures. In addition, transverse droplet oscillations are implemented to accurately evaluate superhydrophobicity of micropillared surfaces. Variations in surface properties that influence the degree of superhydrophobicity were successfully probed by relating friction and viscous dissipation of dynamic droplets to pattern density and chemical coating. Additionally, controlling the normal force exerted on the water-based ferrofluid droplet allows the measurement of impalement pressure necessary to induce wetting transition for a droplet in motion. A new fabrication process was introduced for rapid prototyping of cilia-inspired magnetic micropillars. The fabricated array of sub-10 µm diameter pillars are based on polydimethylsiloxane (PDMS) loaded with carbonyl iron particles (CIP). Lubricating in silicone oil allowed controlled droplet motion at the sub-mm scale facilitated by fast actuation and superhydrophobicity of the oil infused PDMS magnetic micropillars. Lack of mechanical stability due to flexibility of the high aspect ratio PDMS micropillars restricted the application of the array to liquid media only. Thiol-ene based magnetic micropillar arrays were introduced to address the stability issue of the high-aspect ratio micropillars. The remarkable properties of thiol-ene including tunability of surface and mechanical properties allowed topography modification of the magnetic micropillars using photo-induced thiol-ene click coupling. Decorating the surface of the high aspect ratio thiol-ene micropillars with polyvinyltrimethoxysilane (PVTMS) colloidal micro- and nanoparticles enhanced the mechanical stability of the flexible micropillars without compromising powerful bending actuation. This allowed actuation of the micropillar arrays in air as well as in liquid media. The magnetic micropillars were rendered superhydrophobic by grafting hydrophobic self-assemnled monolayer onto the PVTMS micro- and nanoparticles that are covalently bonded to the surface. This enabled directed water droplet motion by repetitive bending and recovery of the micropillars. Combining mechanical stability with robust superhydrophobicity can lead to numerous practical applications of cilia-inspired thiol-ene magnetic micropillars.