Ph.D Thesis

Ph.D StudentAlexandra Cohen
SubjectBio-Inspired Growth of Wax Crystals for the Control
of Surface Wettability
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Pokroy Boaz
Full Thesis textFull thesis text - English Version


One of the most fascinating properties of materials in nature is the superhydrophobic and self-cleaning capabilities of different plant surfaces. This is achieved by an array of hydrophobic wax crystals on plant cuticles, which provide both required elements necessary to attain superhydrophobicity (surface roughness and intrinsic chemical hydrophobicity). Drawing inspiration from such natural systems, paraffin wax films were deposited via thermal evaporation and show that the surface microstructure evolves in time via self-assembly. This leads to a dramatic change in the wetting properties with a transition from mere hydrophobic to superhydrophobic characteristics even at room temperature. This phenomenon was investigated and on this basis a mechanism of formation was proposed, namely, strain induced coarsening. Based on the understanding of this process we were able to formulate a simple route for the formation of hierarchical roughness by the successive deposition of various waxes of increasing molecular weight. We show that, compared to unary wax crystal surfaces, these hierarchical structures have the advantage of increased surface stability expressed by the ability to maintain smaller drops in the Cassie-Baxter state, as well as the ability to support liquids having lower surface tensions with contact angles that exceed 150̊.

The influence of intrinsic hydrophobicity was investigated by the alternation of the hydrocarbon wax to a low-surface-energy fluorinated wax. In this case thermal evaporation enabled a single-step and single-component fabrication method of hierarchical superoleophobic surfaces combined with re-entrant curvature. The achieved texture in combination with the low surface energy resulted in high contact angles and low contact-angle hysteresis values even of low-surface-tension liquids (as low as that for ethanol). Moreover in these cases exceptional surface stability over many months was observed.

Last but not least we examined the ability of our surfaces to inhibit biofilm formation; a huge challenge nowadays in different industries. Our surfaces indeed proved to facilitate passive prevention of surface biofilm attachment that resulted in inhibition (of up to 99.9%) of the biofilm formation over a 7-day period.