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Blown Away: The Shedding and Oscillation of Sessile Drops by Cross Flowing Air Open Access


Other title
Drag Force
Pressure Gradient
Adhesion Force
Boundary Layer
Shear Sensor
Poly(methyl methacrylate) PMMA
Air Velocity
Cross Flowing Air
Reynolds Number
Floating Element
Incipient Motion
Contact Angle
Coefficient of Drag
Dispersion Relationship
Sessile Drop
Differential Drag
Superhydrophobic Surface
Type of item
Degree grantor
University of Alberta
Author or creator
Milne, Andrew J. B.
Supervisor and department
Amirfazli, Alidad (Mechanical Engineering, University of Alberta and York University)
Fleck, Brian (Mechanical Engineering)
Examining committee member and department
Elliott, Janet (Mechanical Engineering)
Flynn, Morris (Mechanical Engineering)
Bhattacharjee, Subir (Mechanical Engineering)
Ashgriz, Nasser (Mechanical and Industrial Engineering, University of Toronto)
Department of Mechanical Engineering

Date accepted
Graduation date
Doctor of Philosophy
Degree level
For drops sessile on a solid surface, cross flowing air can drive drop oscillation or shedding, based on the balance and interaction of aerodynamic drag force (based on drop size/shape and air speed) and adhesion/capillary forces (based on surface tension and drop size/shape). Better understanding of the above has applications to, e.g., fuel cell flooding, airfoil icing, and visibility in rain. To understand the basic physics, experiments studying individual sessile drops in a low speed wind tunnel were performed in this thesis. Analysis of high speed video gave time resolved profiles and airspeed for shedding. Testing 0.5 µl to 100 µl drops of water and hexadecane on poly(methyl methacrylate) PMMA, Teflon, and a superhydrophobic surface (SHS) yielded a master curve describing critical airspeed for shedding for water drops on all surface tested. This curve predicts behavior for new surfaces, and explains experimental results published previously. It also indicates that the higher contact angle leads to easier shedding due to decreased adhesion and increased drag. Developing a novel floating element differential drag sensor gave the first measurements of the microNewton drag force experienced by drops. Forces magnitude is comparable to gravitational shedding from a tilted plate and to simplified models for drop adhesion, with deviations that suggest effects due to the air flow. Fluid properties are seen to have little effect on drag versus airspeed, and decreased adhesion is seen to be more important than increased drag for easing shedding. The relation between drag coefficient and Reynolds number increases slightly with liquid-solid contact angle, and with drop volume. Results suggest that the drop experiences increased drag compared to similarly shaped solid bodies due to drop oscillations aeroelasticly coupling into the otherwise laminar flow. The bulk and surface oscillations of sessile drops in cross flow was also studied, using a full profile analysis technique to determine mode shapes. Oscillation frequency/mode shape is similar for cross flow and quiescent drops. The highest order models collected from the diffuse literature are seen to be reasonably accurate, except at maximum and minimum ranges of contact angle.
Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.
Citation for previous publication
Milne, A. J. B.; Amirfazli, A. Langmuir 2009, 25, 14155–14164.

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