Biomimetic Surface Coatings for Anisotropic Wetting Properties

Abstract

Inspired by the anisotropic wetting behaviour observed on the integument of the Texas horned lizard, biomimetic functional coatings were designed and fabricated through a novel patterning and growth technique to replicate this phenomenon. Flame spray pyrolysis was first utilised to produce zinc oxide nanoparticles to serve as seeds for subsequent hydrothermal growth. An electrospraying technique with a stencil mask was then used to deposit these seed nanoparticles in biomimetic patterns. The electrospray duration, feed rate, and voltage were varied to analyse their impact on the surface coverage, roughness, and dimensional accuracy of the seed layer. The density of the seed layer was found to increase with increasing electrospray duration and feed rate but no clear trend with voltage was discerned. A feed rate of 2 ml/hr demonstrated optimal results, yielding the greatest surface coverage and minimum roughness. Measurements of the deposited biomimetic pattern found a size reduction of 21%, possibly due to charge buildup or the mask thickness. Hydrothermal synthesis was then used to grow zinc oxide nanorods from the biomimetic pattern. The impact of the growth duration, temperature, and molarity on the height, morphology, and wetting behaviour of the ZnO nanorod layer was investigated. An increase in any of the three parameters saw an increase in the nanorod growth rate. All samples exhibited a nanorod layer height between 1 and 2.74 µm, with a growth temperature of 95°C producing the tallest layer. Two distinct morphologies were observed, a hexagonal Wurtzite structure and a fine nanotip structure. All samples displayed hydrophilic wetting behaviour with water contact angles below 90°, and super hydrophilic wetting behaviour was observed on samples grown at 95°C. Samples with biomimetic ZnO nanorod patterns underwent testing for anisotropic wetting in both a horizontal and vertical position, but no compelling evidence of it occurring was observed. This lack of success is attributed to the reduction in pattern dimensions during the electrospraying process, excessive porosity of the nanorod layers, and the insufficient height of the layer when compared with the natural structure.