Modeling and analysis of biomimetic structures with self-cleaning properties fabricated by ultra-precision machining

Abstract

Biomimetic structures with hydrophobic properties have enormous potential for applications on artificial self-cleaning surfaces, such as the next generation of vehicle windshields, exterior paint for buildings, and solar panels, due to their excellent water repellence properties. Although most of the recent studies focus on the fabrication methods of hydrophobic surfaces for self-cleaning, and the wetting behavior on self-cleaning surfaces, there are still great challenges in applying one-step manufacturing methods to fabricate bare hydrophobic micro-patterned surfaces with good sliding and optical performance for mass production. This involves fabricating, analyzing, and characterizing micro-patterned surfaces taking into account the wetting characteristics, droplet wetting states, and optical performance for advanced self-cleaning surfaces. This thesis is divided into four parts. In the first part, optimization modeling of the surface geometries of biomimetic structures for self-cleaning has been developed based on an optical function and a wetting model of micro-patterned surfaces, considering hydrophobicity and optical performance. The second part of the thesis describes investigations on the wetting characteristics of bare hydrophobic micro-patterned cyclic olefin copolymer surfaces machined by a one-step fabrication method, ultra-precision raster milling (UPRM), which is potentially applicable for the mass production of plastic injection moldings. Water droplets are governed by the Cassie and Baxter regime, and are stabilized by the sharp edges induced by the numerical-controlled tool path in the material removal process in the mechanical machining of the top asperities of the micro-patterns after deposition on the surfaces. This results in a good sliding performance on the micro-grooved surfaces. In the third part, a characterization method for the droplet wetting states, significantly affecting its sliding performance on hydrophobic surfaces, is proposed, based on observations of the droplet contact lines on the micro-grooved surfaces. This characterization method is capable of identifying intermediate wetting states but it is difficult to use traditional verification methods for such identification. The partial wetting state is found to be the sliding boundary of the micro-grooved surfaces, whereas droplets contacting the side walls of the grooves still form a favorable wetting state for the anisotropic sliding on the micro-grooved surfaces. In the fourth part, the anisotropic wetting of micro-micro hierarchical structured surfaces fabricated by ultra-precision machining, including regular, irregular, and multi-level hierarchical structures, have been investigated. It is found that applying irregular hierarchical structures and multi-level design can further enhance the water static contact angle to over 150{493} and 161{493} respectively. This is achieved by inducing more energy barriers, reducing the contact area of the solid-liquid interface, and reserving more buffers to air pocket formation. The originality and significance of this research include: (i) an optimization modeling for the surface geometries of biomimetic structures for advanced self-cleaning surfaces has been established; (ii) highly controllable one-step fabrication methods, potentially applicable for mass production in the plastic injection molding of bare micro-patterned surfaces with good sliding and optical performance, are proposed through study of the wetting characteristics; (iii) a characterization method for the droplet wetting states, that significantly affect the sliding performance on hydrophobic surfaces, is firstly proposed from observations of the droplet contact lines on micro-patterned surfaces; (iv) this characterization method is capable of identifying intermediate wetting states which show the sliding boundary and the favorable wetting state for anisotropic sliding on the micro-grooved surfaces. This provides more flexibility for the design of advanced self-cleaning surfaces.