Abstract
The hydrodynamic skin friction in turbulent flows contributes to 60-70% of the total drag of most surface and subsurface vessels. Super-hydrophobic surface (SHS) is a new passive method to reduce the friction drag in turbulent flows, due to its ability to trap a thin layer of gas (or plastron) within the surface microstructures. However, the application of SHS in real engineering systems, e.g., marine vessels, is still a challenge for the reason that the SHS may lose the gas and thereby the drag-reducing property under turbulent flows. It is unclear what is the optimal surface texture for achieving sustained drag reduction by SHS. To address this challenge, this thesis has made three contributions. First, we developed a simple method to fabricate SHSs with controlled roughness heights based on superimposing nanosized hydrophobic silica particles on top of the sandpapers. The surface roughness was controlled by using sandpapers of different grit sizes. We found that the coated sandpapers with grit sizes of 240, 400, 800, 1000, and 1500 exhibited super-hydrophobicity, while other coated sandpapers with grit sizes of 60, 120, and 600 did not show superhydrophobicity. The fabricated SHS remained in the partial Cassie-Baxter state at the highest pressure (2.4 atm), although the percentage of surface area covered by gas reduces with increasing pressure. Second, we studied the impact of surface roughness on the stability and drag reduction of SHS fabricated on sandpapers in turbulent flows. Multiple SHSs with different roughness heights were tested in a turbulent channel flow facility. We found a strong correlation between drag reduction and k+ ᵣₘₛ=kᵣₘₛ/δυ, where δυ is the viscous length scale and kᵣₘₛ is the root-mean-square roughness height (the surface roughness can be partially characterized by the root-mean-square value of roughness height). For k+ᵣₘₛ ≤1, drag reduction was independent of k+ᵣₘₛ and was nearly a constant (∼ 47%) as increasing Reynolds number. For 1 ≤k+ᵣₘₛ ≤ 2, less drag reduction was observed due to the roughness effect. And for k+ᵣₘₛ ≥ 2, the SHSs caused an increase in drag. We also found that surface roughness influenced the trend of gas depletion. By increasing Reynolds number, the gas fraction (ϕg) is reduced gradually for SHSs with large kᵣₘₛ, but reduced rapidly and maintained as a constant for SHSs with small kᵣₘₛ. Last, we investigated the effects of texture size and texture shape on the stability of SHS consisting of transverse grooves in turbulent flows. We systematically varied the groove width (g), texture height (h), and texture wavelength (λ) in the range of 200 to 800 µm. The experiments were performed in a turbulent channel flow facility, and the status of the gas layer on SHS was imaged by reflected-light microscopy. We found that by increasing Reynolds number, the SHS experienced a sudden wetting transition from the Cassie-Baxter state to the Wenzel state. A metastable state where the liquid partially filled the grooves was not observed. We found that the wetting transition was delayed or occurred at a higher Reynolds number as increasing h and reducing g, which indicates that a larger energy barrier between the Cassie-Baxter state and Wenzel state led to a more stable interface. The trend between g and the critical Reynolds number Recᵣ for wetting transition was well captured by theoretical models based on the force balance at the gas-liquid interface. We also showed that grooves with a T-shape geometry maintained a more stable plastron in turbulent flows.