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Viscoelastic Free Surface Instabilities During Exponential Stretching Viscoelastic Free Surface Instabilities During Exponential Stretching by Ryan D. Welsh B.S., Mechanical Engineering University of Cincinnati, 2000 Submitted to the Department of Mechanical Engineering in partial fulfillment of the Requirements for the Degree of Master of Science in Mechanical Engineering at the Massachusetts Institute of Technology December 2001 2001 Massachusetts Institute of Technology All Rights reserved Signature of Author ……………………………………………………………………….. Department of Mechanical Engineering December 20, 2001 Certified by ………………………………………………………………………………... Gareth H. McKinley Professor of Mechanical Engineering Thesis Supervisor Accepted by ……………………………………………………………………………….. Ain A. Sonin Chairman, Department Committee on Graduate Students Acknowledgements The person who I owe the most for helping me to accomplish this thesis is Canee, who was patient enough to put off getting a ‘real’ place to live and who agreed to spend two more years living like a student; I am happy that I will now finally be able to spend more time with her and Zachary. I thank my advisor, Gareth McKinley, for his guidance throughout this project. I feel very lucky to have worked with Gareth over the past year and a half. Although he is constantly busy, he always took time to answer my questions, and I am grateful to him for making my time here at MIT both an enjoyable and enriching experience. I was able to do and learn much more than I expected while here, thanks to Gareth. I am also heavily indebted to José Bico, my friend and coworker on this project, for all of his help, good ideas, and insights. This thesis would not be what it is without his input. José will make a great professor next year. Thanks to everyone in the lab for making our lab such a fun and interesting place to work. I really hate to leave such a great environment, but then I think of moving all of the equipment back to Building 3 and I come back to my senses! Anyway, I have a lot of friends here at MIT and it is definitely hard to leave for that reason. Finally, I have to acknowledge my financial support. The bulk of the funding, I think, has come from my parents, who have generously kept us going for the past couple years. In addition, I appreciate the funding I got from the Department in the form of the Rohsenow Fellowship and I thank NASA Glenn for funding this research. Viscoelastic Free Surface Instabilities During Exponential Stretching by Ryan D. Welsh Submitted to the Department of Mechanical Engineering on December 20, 2001 in partial fulfillment of the requirements for the Degree of Master of Science in Mechanical Engineering ABSTRACT A viscous Newtonian fluid and two model polystyrene Boger fluids are used to investigate the evolution of free-surface instabilities during the debonding of two cylindrical parallel rigid surfaces. A filament stretching rheometer is used to separate the surfaces at an exponential rate and simultaneously measure the evolution in the axial force. The experimental configuration also allows for simultaneous imaging of the fluid sample from beneath the endplate during the test. The test method is similar to both the probe-tack test used in adhesive testing and filament stretching rheometry experiments. Several geometric aspect ratios (defined as the ratio of initial sample height to radius) are used to vary the degree of sample confinement. These geometries bridge the gap between conventional adhesive testing and extensional rheometry. Three types of instabilities are seen in the experiments. The first is similar to the classical Saffman-Taylor instability, in which fingers of a less viscous fluid penetrate into a more viscous one during pressure driven flow. In the present three-dimensional analog, the instability occurs at a critical rate of plate separation for a given geometry and fluid. Larger pressure gradients result from higher fluid viscosity, a more confined geometry and greater rate of plate separation. A second instability is caused by the extensional stress growth in the elastic fluids and occurs at a critical Hencky strain for a given Deborah number. This instability is characterized by the growth of fingers radially outward from the base of a stable, cylindrical fluid column. As this instability progresses, the fingers can repeatedly bifurcate resulting in complex surface morphologies. Finally, cavitation in the test fluids occurs below a critical negative gage pressure within the fluid. Modal interactions of the three instabilities are also observed and lead to complex evolution of the elastic instability. Thesis Supervisor: Gareth H. McKinley Title: Professor of Mechanical Engineering Table of Contents Chapter 1 Introduction and Background.............................................................................. 9 1.1 Non-Newtonian Fluids..................................................................................................... 9 1.1.1 Flow Behavior ................................................................................................... 11 1.1.2 The Role of Molecular Structure....................................................................... 13 1.2 Characterization of Non-Newtonian Fluids................................................................... 15 1.2.1 Shear Rheometry: Cone and Plate Rheometer................................................... 18 1.2.2 Extensional Rheometry: Filament Stretching Rheometry ................................. 19 1.3 Pressure-Senstive Adhesives (PSAs)............................................................................. 22 1.3.1 Literature Review .............................................................................................. 23 1.3.2 The Probe-Tack Test ......................................................................................... 27 1.4 A Modified Probe-Tack Experiment ............................................................................. 28 1.5 Summary........................................................................................................................ 31 Chapter 2 Experimental Setup and Testing Method.......................................................... 33 2.1 Experiment..................................................................................................................... 33 2.1.1 Flow Geometry and Experimental Setup........................................................... 34 2.1.2 Sample Preparation and Loading....................................................................... 36 2.1.3 Temperature....................................................................................................... 37 2.2 FISER II Filament Stretching Rheometer ....................................................................... 37 2.2.1 Motor Platens..................................................................................................... 37 2.2.2 Endplates and Endplate Assemblies.................................................................. 41 2.2.3 Data Acquisition................................................................................................ 42 2.2.4 Force Measurements.......................................................................................... 43 2.2.5 Dynamic Testing of Futek load cells, Model L2338-Q10419........................... 47 2.2.6 Video Imaging................................................................................................... 49 2.3 Data Analysis................................................................................................................. 50 2.3.1 Force Data ......................................................................................................... 51 2.4 Conclusion ..................................................................................................................... 55 Chapter 3 Test Fluids ............................................................................................................ 57 3.1 Styrene-based fluids....................................................................................................... 58 3.1.1 Fluid Composition............................................................................................. 59 3.1.2 Shear Rheometry ............................................................................................... 61 3.2 Polybutadiene Fluid ....................................................................................................... 66 3.2.1 Fluid Composition............................................................................................. 66 3.2.2 Shear Rheometry ............................................................................................... 67 Chapter 4 Experimental Results .......................................................................................... 73 4.1 Elastic Fingering Instability........................................................................................... 75 4.1.1 Background........................................................................................................ 77 4.1.2 Results and Discussion ...................................................................................... 80 4.2 Saffman-Taylor Fingering Instability ............................................................................ 87 4.2.1 Planar Saffman-Taylor Instability ..................................................................... 88 4.2.2 Scaling Arguments for 3-D Radial Case ..........................................................
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