News & Press Releases

Take part in the New Bedford Historical Society's communal reading of Frederick Douglass' fiery 1852 speech, "What to the Slave is the Fourth of July". The Society invites SouthCoast residents to read Douglass together starting at 6pm on (Wed) July 17 at the Seamen's Bethel, 15 Johnny Cake Hill, in New Bedford. CONTACT: Lee Blake 508-979-8828

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Financial Aid Services wants to remind all students to file their FAFSA! Join Financial Aid Services for FAFSA Help Labs via Zoom on Fridays from 2-3pm for help filing your FAFSA and learning more about financial aid. https://umassd.zoom.us/j/96235213250?pwd=ZE1FSWtNek05dnNEQWVqdnZyNHVDUT09 Contact Mark Yanni myanni@umassd.edu

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Financial Aid Services wants to remind all students to file their FAFSA! Join Financial Aid Services for FAFSA Help Labs in LARTS 203 on Tuesdays from 10am-11am for help filing your FAFSA and learning more about financial aid. Contact Mark Yanni myanni@umassd.edu

Liberal Arts Building 203

EAS Doctoral Dissertation Defense by Shabnam Mohammadshahi Date: Tuesday, July 23, 2024 Time: 11:00 a.m. Topic: Experimental Study of the Stability of Super-Hydrophobic Surface in Turbulent Flow Location: LIB 314 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 micro-structures. 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 krms+=krms/v, where v is the viscous length scale and krms is the root-mean-square roughness height. For krms+<1, drag reduction was independent of krms+ and was nearly a constant (~47%) as increasing Reynolds number. For 12, the SHSs caused an increase in drag. We also found that surface roughness influenced the trend of gas depletion. As increasing Reynolds number, the gas fraction (g) 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 () in the range of 200 to 800 m. 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 as 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. ADVISOR(S): Dr. Hangjian Ling, Department of Mechanical Engineering (hling1@umassd.edu) COMMITTEE MEMBERS: Dr. Banafsheh Seyedaghazadeh, Dept of Mechanical Engineering Dr. Caiwei Shen, Dept of Mechanical Engineering Dr. Geoffrey W. Cowles, Dept of Marine Science & Technology NOTE: All EAS Students are ENCOURAGED to attend.

LIB-314

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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Financial Aid Services wants to remind all students to file their FAFSA! Join Financial Aid Services for FAFSA Help Labs via Zoom on Fridays from 2-3pm for help filing your FAFSA and learning more about financial aid. https://umassd.zoom.us/j/96235213250?pwd=ZE1FSWtNek05dnNEQWVqdnZyNHVDUT09 Contact Mark Yanni myanni@umassd.edu

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