Investigating the Performance of Randomly Patterned Superhydrophobic Surfaces in Laminar and Turbulent Channel Flows

• Author / Creator

  Wilkinson, Daren K.

• Superhydrophobic surfaces (SHSs) have been investigated based on their success in water repellency, anti-fouling, and drag reducing effects seen in nature (Neinhuis & Bartlott 1997; Kreuz et al. 2001). SHSs utilize a low surface energy material with a microscale surface roughness to prevent water from entering cavities between the roughness elements. When a SHS is exposed to a liquid, the cavities remain filled with air, described as the air layer, which inhibits the droplet from achieving a wetted state on the surface. When exposed to flows, the water-air interface is shear-free, thus lowering the skin-friction compared to a smooth surface. Due to the number of different possible surface geometries and roughness as well as different flow characteristics, a relationship between the SHS performance and the flow parameters has not fully modeled. SHS performance characteristics include slip velocity, slip length, and drag reduction. This study aimed to finish characterizing the trend between a previously studied SHSs and the flow conditions, as well as characterize a SHS and determine its performance at a certain flow condition. The first part of the study focused on a commercially available spray coating NeverWet (Rustoleum) which has previously been studied by Aljallis et al. (2014), Zhang et al. (2015), and Abu Rowin et al. (2017). The surfaces were tested in a laminar channel with 180 mm length (L), 20 mm width (W), and 2 mm height (H) at bulk Reynolds numbers (Reb) ranging from 50-450. The drag reducing capabilities at different flow conditions were measured through a 2D shadowgraphic particle tracking velocimetry (shadow-PTV) setup. There was a linear trend between slip velocity, us, and Reb determined to be us = 0.077Reb up to a Reb of 250. The surfaces showed a maximum drag reduction of 16%. The 2D shadow-PTV measurements at Reb > 250 showed a decrease in the SHS performance with the lowest being a drag reduction of 11%. The decrease in the SHS performance was attributed to a combination of the low hydrostatic pressure and the high Reb. This agrees with the experiment of Gose et al. (2018) when suggested that pressures below atmospheric extract the air layer away from the surface. With gauge pressures below atmospheric and higher streamwise velocities, portions of the air layer can detach from the surface. It was believed that after Reb = 250, the channel pressure and the flow conditions allowed for partial detachment of the air layer and therefore a reduction in the SHS performance. The slip length measurements also confirm this trend as they remained the same at 71 µm ± 1 µm while at Reb = 450 the slip length decreased to 48.8 µm ± 0.3 µm.The second part of this study focused on comparing the performance of four randomly patterned SHSs with a varying surface roughness in turbulent channel flow. Characterizations of the surface topography through a scanning electron microscope and surface roughness through profilometer were performed. Based on their roughness value normalized by the inner scaling, k+rms, the surfaces were referred to as SHS0.35, SHS0.23, SHS0.18, and SHS0.14. The surfaces had dimensions of 234 × 36 mm2 (L×W) and were tested in a closed loop channel with a rectangular cross-section of 40 × 6 mm2 (W×H) and a length of 1200 mm. The Reb = 8000 flow was captured utilizing a planar micro-PTV setup operated over 6 seconds at a rate of 10 kHz. The 2D PTV measurement showed that the normalized slip velocity by the inner scaling, us+, was positively related to the surface roughness with a second order power relationship found, us+ = 0.051/(k+rms)2. It was observed that us+ increased exponentially with the decrease of the surface roughness. This trend was also consistent with the drag reduction measurement which was obtained from the reduction of the velocity gradient. The drag reduction over the SHSs started as the highest at 21% over SHS0.35 and reached a drag increase of 19% over SHS0.14. Therefore, the drag reduction and the slip boundary condition over the SHS depends on the surface roughness where the large posts of the surface can disturb the shear-free regions and results in loss of the performance as suggested by the simulation of Alame & Mahesh (2018).

• Subjects / Keywords

  • Particle Tracking Velocimetry
  • Particle Image Velocimetry
  • Superhydrophobic
  • Laminar Channel Flow
  • Drag Reduction
  • Turbulent Channel Flow

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-18xt-dx53

• License

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