Semester of Graduation

Fall 2018

Degree

Master of Materials Science and Engineering (MMatSE)

Department

Department of Mechanical and Industrial Engineering

Document Type

Thesis

Abstract

Asymmetric surfaces been shown to inducing unidirectional motion in the Leidenfrost regime; however, very minimal research has been conducted to investigate whether these surface can enhance condensation through the same means. The investigation of heat transfer of ratchets in condensation is a relatively untapped area of study, specifically ratchets with superhydrophobic properties. Anticipated difficulty lies in creating surfaces features or coatings that retain the ratchets and can adequately sustain optimal wetting state of Cassie-Baxter required to improve heat transfer performance during condensation. This study serves to investigate whether ratchets are a feasible surface feature to enhance condensation heat transfer. First, fabrication protocols for microscale superhydrophobic asymmetric structures with high throughput and at low cost are devised. Then, droplet behavior during condensation is directly observed under an optical microscope and quantified. Finally, heat transfer is performance is investigated using a custom built chamber and a liquid crystal thermography technique. Superhydrophobic coating reduced the droplet area coverage by about 25% as well as reduced the average droplet area which remained consistent regardless of ratchet period,. For non-ejected coalescence, displacement vectors showed a slight increase in preferring either ratchet direction or opposite of ratchet direction with increasing ratchet period. In the case of coalescence induced ejection, average ejection diameter remained constant at approximately 10um yet event count increased with increasing ratchet period seemingly attributed to the increased potential configurations due to the increased ratchet surface area. No individual droplets were readily identified using the surface plots extracted from MatLab image processing, however heat transfer coefficients were found and showed a trend opposite to what was anticipated. Uncoated PMMA without ratchets yielded the highest HTC and the coated 10um ratchets yielded the lowest. xi Potential investigations into surface manipulation without the use of coatings can eliminate the added thermal resistance potentially produced by the coating itself, this would also eliminate the need to have a strong bonding between the substrate surface and the coating eliminating a source of mechanical instability. Finally, improvements to the condensation chamber design can be made, specifically including a reservoir in which the flow rate of vapor that the substrate is exposed to can be controlled.

Date

11-6-2018

Committee Chair

Sunggook Park

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