Low-friction surfaces with recoverable super-hydrophobicity for water transmission
Creators: name: Capril-Carniere Auwerter, Liliane
Publisher: name: Imperial College London
Year: 2019

description: Super-hydrophobic surfaces are of considerable interest as they can reduce friction losses and thus lead to substantial reductions in the energy consumption of water distribution networks. However, super-hydrophobic surfaces are not durable, as they are known to lose their friction-reducing properties rapidly due to loss of small air-pockets in the roughness elements of the surface. This thesis describes the development and testing of a novel porous low-friction surface with recoverable super-hydrophobicity for use under fully turbulent water flow conditions. Preliminary experiments carried out as part of this dissertation showed that samples with super-hydrophobic coatings immersed in water lost their super-hydrophobicity quickly due to depletion of trapped air. Furthermore, the loss of particles on the surface that form the surface structure can lead to irreversible loss of super-hydrophobicity. Therefore a durable super-hydrophobic surface was designed and manufactured, using sintered soda-lime glass granules that provided an increased level of particle bonding compared to coatings. The surface of the materials was made hydrophobic through a reaction with one of several surface modifying agents. Combinations of granules of five different particle sizes and three different surface modifying agents (hexamethyldisilazane, trichloro(1H,1H,2H,2H-perfluorooctyl)silane and 1H,1H,2H,2H-perfluorooctyltriethoxysilane) were considered. The hydrophobicity of the materials was analysed by measuring the water contact angle. The best performing material, sample ST, showed a mean water contact angle of 153°. The surface and structure of the best performing samples were characterised using scanning electron microscopy (SEM), x-ray tomography and Fourier Transformed Infra-red (FTIR) spectroscopy. FTIR spectroscopy confirmed the reaction between trichloro- (1H,1H,2H,2H-perfluorooctyl)silane and the soda-lime glass substrate. SEM micrographs showed that samples formed with smaller particle sizes presented 30% less peak areas than surfaces formed with larger particle sizes, and x-ray tomography showed that samples formed with smaller particle sizes presented higher relative surface roughness, with features up to 30 µm size. Lower peak area and high relative surface roughness affect the area of contact on the solid-liquid interface, and consequently the attachment of a liquid on a solid surface. X-ray tomography analysis showed that samples were porous throughout the material, with tile porosity in the range of 23-36% with mean pore size of 8-15 micro m. The porosity of the material allowed for replenishment of the air pockets by pressurising an air layer on the other side of the surface which forces air through the material. This can take place under fully immersed conditions, allowing the material to recover its super-hydrophobicity. The ability of the material to recover its super-hydrophobic properties was investigated experimentally in fully immersed setups under quiescent conditions and under turbulent flow conditions. Fully immersed conditions results showed recovery of the air pockets at the liquid-solid interface under both quiescent and turbulent flow conditions. Under quiescent conditions the experiment was performed by exposing the samples to static water to a depth of 10 mm and applying an air pressure of 0.04 bar. The recovery of air pockets was analysed qualitatively by capturing the formation of air pockets and bubbles on the surface. The turbulent flow experiments measured the friction factor fd as a function of time. Values of fd were obtained by measuring the pressure drop and volume flux. The Reynolds number of the flow was approximately 4.5×104 for all experiments. Turbulent flow results showed reductions in fd of 31-45% relative to a hydraulically smooth surface. The Cassie-Baxter state could be recovered by blowing air through the porous surface for 10 minutes. The durability of the drag-reduction, as quantified by the relaxation time T in which the surface loses its super-hydrophobic characteristics, were measured to be between 10-60 minutes depending on the initial head. In contrast to models based on Darcy flow through a porous medium, this study indicates that there seems to be a critical water pressure beyond which the Cassie Baxter state cannot be sustained for the material under consideration, which limits its use for water distribution purposes.