Experimental and theoretical study on bio-inspired functional surfaces with tunable surface energy and geometry

Abstract

The control of surface energy () and geometry of solid surfaces has been shown by many natural creatures such as lotus leaf, Namibian beetles, geckos, thorny devil lizards, etc. With the combination of surface energy selection and unique structural patterns on their surfaces, the functional surfaces by natural designs have proved to be effective in many natural creatures until now. In this dissertation, we investigate the role of surface energy and geometry in (de)-wetting behavior of water on solid surfaces and dry adhesion behavior of bio-inspired nanohairy structures. For (de)-wetting studies, we demonstrate a method to fabricate hierarchical structures covered with low surface energy materials ( = 24. mJ/m2) for superhydrophobicity. The combination of top-down process with replica molding of polydimethyl siloxane (PDMS) together with bottom-up process by film deposition is proposed. For the surfaces we fabricated, we derive the general models to express the role of surface energy and geometry on static wetting angle based on the integration of Wenzel and Cassie laws on micro- and nanoscale roughness. We found out that there are 4 wetting states in general dual-scale system: Cm-Cn, Cm-Wn, Wm-Cn, and Wm-Wn, where C, W, m, n are Cassie states, Wenzel states, micro-, and nano-, respectively. Next, we demonstrate the application of low surface energy materials with hierarchical structures for calf pulmonary artery endothelial (CPAE) cells adhesion behavior. We found that the 3-dimensional (3-D) patterned superhydrophobic diamond-like carbon (DLC) coating fabricated using previous method exhibits excellent anti-biofouling properties against non-specific cell adhesion. Therefore, the gradient hydrophobicity of the prepared surfaces with optimum design can be used as a template to control the adhesion of specific cells for potential biomedical devices. For dry adhesion studies, we demonstrate a method to control the surface energy from 21.3 to 71.9 mJ/m2 on nanohairy structures using broad ion beam irradiation. The set of as-prepared vertical nanohairs are exposed to argon ion beam irradiation, resulting in a uniform bending of nanohairy structures. Subsequently, the nanohairs are exposed to oxygen ion beam irradiation to change the surface energy. We further establish the model to show the relationship between surface energy and tilting angle of nanohairs to the adhesion strength of nanohairs to solid surfaces. With the variation of surface energy, we compared our experimental results with established theories of Jonhson-Kendall-Roberts (JKR), Side contact, Kendall peeling, and peel zone (PZ) models.