College of Engineering at UMass Dartmouth

Mechanical Engineering MS Thesis Defense by Mr. Daniel J. O'Coin DATE: July 23, 2024 TIME: 2:00 p.m. - 4:00 p.m. LOCATIONS: Science & Engineering (SENG) Building, Room 110 and on Zoom: https://umassd.zoom.us/j/94136706089?pwd=xwKSJQdtQ0iDdJ5gj0XU4cqfQBweNY.1 (Password: 443450) TOPIC: An Experimental Study of Bubble Formation on Super-Hydrophobic Surfaces ABSTRACT: This thesis experimentally studied the bubble formation on a superhydrophobic surface (SHS), which had a large equilibrium water contact angle (>150). Bubble formation is a crucial process for many industrial and biomedical applications, for example, pool boiling heat transfer, froth floatation, surface cleaning, and drug deliver. In this thesis, we captured the bubble formation under constant gas flow rates by using a high-speed camera. The SHS was fabricated by first sandblasting an aluminum surface and then coating the rough surface with hydrophobic nanoparticles. We systemically investigated the impacts of radius of SHS (RSHS), gas flow rate (Q), and surface tension () on bubble formation and bubble detached volume (Vd). First, we found that depending on RSHS, bubble formation followed two different modes: Mode A and Mode B. In Mode A for small RSHS, the contact line quickly pined at the rim of SHS after an initial expansion, and Vd increased as increasing RSHS. In Mode B for large RSHS, the contact line continuously expanded as the bubble grows. Second, we found that Vd increased as increasing Q, and the relation between Vd and Q followed similar trends after proper normalizations, regardless of the types of surfaces and the values of equilibrium contact angle. During the necking, the bubble volume was nearly constant for small Q but increased significantly for large Q. Third, we found that as reducing , the equilibrium contact angle and surface area covered by gas reduced, leading to a smaller bubble base radius and smaller Vd. Moreover, we performed a force balance analysis and found that the main forces acting on the bubble were one lifting force (pressure force) and two retaining forces (surface tension force and buoyancy force). We found that the necking radius and time to pinch-off followed a power-law relation, which agreed well with that for the pinch-off of bubble on a nozzle. Last, we found that the Tate volume, derived based on the balance between surface tension and buoyancy, well predicted Vd. Overall, our results provided a better understanding of bubble formation on SHS and can be applied for: (i) the control of bubble generation by using complexed surfaces; and (ii) restoration of gas layer and extension of the longevity of SHS for applications such as drag reduction, anti-icing, anti-biofouling, and anti-corrosion. ADVISORS: - Dr. Hangjian Ling, Assistant Professor, Department of Mechanical Engineering, UMass Dartmouth COMMITTEE MEMBER: - Dr. Sankha Bhowmick, Professor, Department of Mechanical Engineering, UMass Dartmouth - Dr. Mehdi Raessi, Professor, Department of Mechanical Engineering, UMass Dartmouth Open to the public. All MNE students are encouraged to attend. For more information, please contact Dr. Hangjian Ling (hling1@umassd.edu).

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