Date of Award

2014

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Engineering

Faculty

Health, Engineering and Science

First Advisor

Dr Majid Rad

Second Advisor

Dr Zonghan Xie

Third Advisor

Dr Jeremy Shaw

Abstract

Highly evolved, efficient and sophisticated biological systems can be used as models for scientific innovations. This research explored specific surface structures on plant leaves with respect to their hydrophobicity in the context of the often arid Australian climate. The relationships between leaf surface structures and their hydrophobicity could inform the making of artificial surfaces with specially designed hydrophobicity.

Moderate hydrophobicity and strong surface adhesion were discovered on many study plant leaves. Scanning Electron Microscopy (SEM) revealed that their surface morphologies could be categorized into four groups while their water-repellent mechanisms were considered at an individual species level. Specifically, physical models were built based on the topography of several Eucalyptus species. Wetting robustness and surface free energy analyses were performed with these models to study wetting transitions on surfaces with specific microscopic features.

In the fabrication component of the study, a convenient self-assembly procedure of oxysilane successfully converted a hydrophilic glass slide into a hydrophobic surface, with the measured contact angle changing from 30.8 to more than 1000. Atomic force microscope (AFM) images showed randomly distributed roughness at a micrometre scale on these self-assembled hydrophobic surfaces. Samples with square arrays of micro-posts were also fabricated following a sophisticated photo-lithography process. Wetting properties similar to some leaves, namely moderate hydrophobicity and strong surface adhesion, were observed with these fabricated samples. Anisotropic wetting, liquid-surface contact footprints and base lengths on these micro-textured surfaces were also investigated.

Finally, fluorine containing diamond-like carbon (F-DLC) coatings were examined because of their chemical inertness, mechanical durability, and low surface energy. F-DLC films were prepared by closed-field, unbalanced, magnetron sputtering (CFUBMS) on silicon substrate to study their wetting and mechanical properties. The influences of CF4 and C2H2 gas addition during fabrication on these properties were explored by measuring contact angles, fluorine contents, surface roughness, Young's modulus and hardness. Simulation from Finite Element Analysis with COMSOL software was also conducted to confirm the mechanical results obtained in nano-indentation experiments.

The leaf surface geometries revealed in this study could trigger further relevant research and applications. Surface free energy analysis on the built models could lead to a deeper theoretical understanding of wetting state transition for these geometries. The preliminary results on the self-assembly of oxysilane at ambient conditions could contribute to the development of cost-efficient and environmentally friendly methods for fabricating durable hydrophobic coatings. The results of F-DLC coatings could be beneificial for manipulating surface free energy and mechanical properties, to match specific requirements for certain applications.

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