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Wetting and Evaporating Drops on Superhydrophobic Surfaces

  • Author / Creator
    Bozorg Bigdeli, Masoud
  • Superhydrophobic interfaces, due to their unique water repellent and self-cleaning properties, are attracting a wide-spread interest for implementation in a variety of applications, including self-assembly based fabrication methods, nano/microfluidics, and solar energy harvesting. To facilitate the scaling-up of the superhydrophobic technology and its use in a diverse range of applications, there is a need for further understanding of the wetting and evaporation process of droplets on such surfaces. The wetting and drying of droplets of pure liquids on hydrophobic surfaces have been extensively studied. On the contrary, the studies concerned with the wetting and evaporation of droplets of complex fluids on superhydrophobic surfaces are still lacking. The objective of this work is to provide experimental data of the wetting states and evaporation dynamics of simple and complex fluids on superhydrophobic surfaces. Systematic experiments for contact angle measurement of water droplets on superhydrophobic surfaces of different roughness levels have been conducted. Drops placed on surfaces with high roughness values always maintained a none-wetting Cassie-Baster state, while on low roughness level surfaces, droplets rest at a Wenzel wetting state. Drops on surfaces with moderate roughness levels, on the other hand, demonstrated a metastable behavior. Furthermore, the droplets evaporating on these structures revealed a three step process, namely constant contact radius, constant contact angle, and mixed modes, similar to that of microstructured and flat hydrophobic substrates. Moreover, the wetting and contact angle behavior of aqueous surfactant solutions (0 to 1 CMC) on superhydrophobic microstructures with high and moderate roughness levels has been studied. The droplets deposited on the high roughness level surface maintained a Cassie-Baxter wetting state for concentrations below 0.5 CMC and a Wenzel state for concentrations above 0.5 CMC. Droplets deposited on the surface with moderate roughness level, however, showed a metastable behavior. For both surfaces, the contact angles of droplets in Cassie-Baxter state were nearly constant with surfactant concentration. The droplets in Wenzel state, adversely, demonstrated a strong dependence on concentration. Concerning the evaporation of particle laden fluids, aqueous droplets laden with 5wt\% and 60wt\% of silica nanoparticles were left to evaporate on a microstructured superhydrophobic surface. Low concentration droplets resulted in toroidal light diffracting structures, while quasi-spherical photonic structures were obtained from high concentration droplets. The behavior of droplets was monitored through side and bottom views visualization. From bottom view visualization it was observed that the color of contact area changes from red to green. A Bragg-Snell analysis was conducted to analyze the reflection of the light from the bottom view. The model developed can predict the wavelengths reflected from nanoparticle assemblies made of particles of different sizes and materials. In brief, this thesis work contributes to better understanding of the wetting and evaporation behavior of some complex fluids on superhydrophobic surfaces of different roughness levels. In particular, additive surfactants and nanoparticles have strong effects on wetting and evaporation behavior at high concentrations.

  • Subjects / Keywords
  • Graduation date
    Spring 2018
  • Type of Item
    Thesis
  • Degree
    Master of Science
  • DOI
    https://doi.org/10.7939/R3RR1Q264
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.