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Xu, Zejia

University of British Columbia

Droplet dynamics under shear flow on horizontal surfaces including wettability-homogeneous and wettability-heterogeneous surfaces are investigated. The investigated influencing factors include air velocity, droplet volume and surface wettability. The O2 plasma procession and chemical vapor deposition are used to turn hydrophilic areas (HI) to hydrophobic (HO), and turn textured superhydrophilic areas to superhydrophobic (SHO). The hydrophobic areas acquired from the chemical vapors of 1H,1H,2H,2H-Perfluorooctyltriethoxysilane and 1,3-dichlorotetramethyldisiloxane are referred to as HO-POTS and HO-PDMS respectively. The HO-POTS areas have a larger contact angle hysteresis (CAH) than the HO-PDMS areas. The two-duct method is employed to achieve designed air velocity instantly, and a high-speed camera is utilized to capture droplet behaviors. On wettability-homogeneous surfaces, the motion of droplets with small deformation can be divided into two stages (Acceleration and Constant Speed). Results show that the critical air velocity for the onset of droplet motion, as well as the velocity that droplet can reach at Constant Speed Stage, is affected by the air velocity, droplet volume and surface wettability. The mathematical expression of retention force is derived, and forces are calculated to verify the force balances at Constant Speed Stage. For wettability-heterogeneous surfaces, the critical airflow velocities for droplets to pass the boundaries between various areas are measured. The HI+SHO and HI+HO-POTS surfaces have the largest critical airflow velocities, while HO-PDMS+SHO feature the smallest ones. The droplet behaviors when crossing the boundaries between areas with different wettability are investigated. On the HI+SHO and HI+HO-POTS surfaces, baby droplet(s) separate from main droplet and stay still on the surface after the main droplet crosses the border. There are no small droplets remaining on HO-POTS+SHO, HO-PDMS+SHO and HI+HO-PDMS. The HO-PDMS+SHO and HI+HO-PDMS surfaces have a better performance of droplet removal than HO-POTS+SHO and HI+HO-POTS respectively, due to a smaller CAH of the HO-PDMS areas. It can be concluded that adding a superhydrophobic area to a hydrophilic surface will assist in droplet removal. However, in order to decrease the critical airflow velocity for boundary crossing and avoid baby droplet(s) remaining on surface, a hydrophobic area with a small CAH should be added between the HI and SHO areas.

Text

2024-08-28

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/89067

Mechanical Engineering

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/4.0/