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Line Tension and Entropy for Molecularly Thin Liquid Crystal Films at Temperatures Corresponding to Less-Ordered Bulk Phases

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Line Tension and Entropy for Molecularly Thin Liquid Crystal Films at Temperatures Corresponding to Less-Ordered Bulk Phases LINE TENSION AND ENTROPY FOR MOLECULARLY THIN LIQUID CRYSTAL FILMS AT TEMPERATURES CORRESPONDING TO LESS-ORDERED BULK PHASES A thesis submitted to Kent State University in partial fulfillment of the requirements for the degree of Master of Science by Joseph S. Yarzebinski August 2016 © Copyright All right reserved Except for previously published materials Thesis written by Joseph S. Yarzebinski B.S., Slippery Rock University of Pennsylvania, 2010 M.S., Kent State University, 2016 Approved by Dr. Elizabeth Mann , Chair, Master’s Thesis Committee Dr. Hamza Balci , Members, Master’s Thesis Committee Dr. John Portman Accepted by Dr. James T. Gleeson , Chair, Department of Physics Dr. James L. Blank , Dean, College of Arts and Sciences TABLE OF CONTENTS LIST OF FIGURES ........................................................................................................ VI ACKNOWLEDGEMENTS ........................................................................................... IX CHAPTER 1 INTRODUCTION ..................................................................................... 1 1.1 Summary ................................................................................................................... 2 1.2 Molecularly Thin Films ............................................................................................. 3 1.3 Liquid Crystalline Films ............................................................................................ 5 1.4 Hydrodynamics Associated With Domain Relaxation in Langmuir Films ............... 6 1.5 Thin Film Thermodynamics .................................................................................... 10 1.5.1 General Surface and Line Thermodynamics ................................................. 10 1.5.2 Surface Excess and Line Excess ................................................................... 13 1.5.3 Determining Line Entropy ............................................................................ 13 1.6 Phases ...................................................................................................................... 17 1.6.1 Classifying Phase .......................................................................................... 17 1.6.2 Liquid Crystalline Phases .............................................................................. 17 1.7 Phases in 8CB Langmuir Films ............................................................................... 20 1.7.1 8CB Isotherms and Early BAM Imaging ...................................................... 20 1.7.2 Surface Freezing and Surface Melting .......................................................... 22 1.8 Previous Experimental Results for 8CB Line Tension ........................................... 23 1.9 Summary and Organization of Thesis ..................................................................... 26 iii CHAPTER 2 MATERIALS AND METHODS ............................................................ 27 2.1 4′-n-octyl-4-cyano-biphenyl (8CB) ......................................................................... 27 2.1.1 General Properties ......................................................................................... 27 2.1.2 The Sample .................................................................................................... 28 2.2 Brewster Angle Microscopy .................................................................................... 29 2.2.1 Different Techniques for Imaging Thin Films .............................................. 29 2.2.2 Operating Principle of the Brewster Angle Microscope ............................... 30 2.2.3 Minimum Optical Requirements ................................................................... 33 2.2.4 Beam Path ..................................................................................................... 39 2.2.5 Laser and Polarizer ........................................................................................ 41 2.2.6 Optocoupler and Fiber ................................................................................... 42 2.2.7 Imaging System ............................................................................................. 43 2.2.8 Optical Limitations and Considerations ........................................................ 45 2.3 Measuring Pixel Dimensions .................................................................................. 47 2.4 The Procedure ......................................................................................................... 47 2.4.1 Cleaning ........................................................................................................ 47 2.4.2 Preparing the Subfluid and Floating the Table .............................................. 50 2.4.3 Aligning the Polarizer and the Incident Angle .............................................. 50 2.4.4 Initial Deposition of the Sample and Surface Pressure Measurements ......... 51 2.4.5 Intensity and Focus Control and Final Sample Deposition ........................... 53 2.4.6 Controlling the Temperature ......................................................................... 53 2.4.7 Stretching Domains and Capturing Relaxation Events ................................. 54 CHAPTER 3 RELAXATION ANALYSIS ................................................................... 55 iv 3.1 Simulation Methods ................................................................................................ 55 3.2 Using the Simulation ............................................................................................... 56 3.2.1 Choosing Events ............................................................................................ 57 3.2.2 Selecting Frames ........................................................................................... 60 3.2.3 Cleaning Frames ............................................................................................ 63 3.2.4 Marking the Edge .......................................................................................... 63 3.2.5 Tracing By Hand ........................................................................................... 63 3.2.6 Operating the Program .................................................................................. 65 3.3 Simulation Results ................................................................................................... 65 3.3.1 Checking Results for Model Agreement ....................................................... 65 3.3.2 Determining the Line Tension....................................................................... 67 CHAPTER 4 DATA ........................................................................................................ 68 4.1 Line Tension vs. Temperature ................................................................................. 68 4.1.1 Measurement Uncertainty ............................................................................. 69 4.1.2 Uncertainties Due to Deviations from the Model ......................................... 73 4.2 Relaxation Event Summary ..................................................................................... 73 4.3 Data Interpretation ................................................................................................... 77 CHAPTER 5 DISCUSSION .......................................................................................... 78 REFERENCES ................................................................................................................ 81 v LIST OF FIGURES Figure 1.1 Brewster angle microscope images of domain relaxation ..........................................2 Figure 1.2 Cellular membrane cartoon .........................................................................................3 Figure 1.3 Vesicle image, vesicle 3D cartoon, and 2D vesicle bilayer cartoon ...........................4 Figure 1.4 Cartoon of 8CB monolayer-trilayer boundary ............................................................6 Figure 1.5 BAM image of 8CB on water with domains of several different thicknesses ............6 Figure 1.6 Fluid motion during domain relaxation ......................................................................7 Figure 1.7 Flow underneath a hole closing in a monolayer .........................................................9 Figure 1.8 Defining thermodynamic phase boundaries ..............................................................12 Figure 1.9 Side-view diagram of an 8CB film on water with two discrete phase thicknesses showing a smooth transition profile between them ..................................................12 Figure 1.10 Cross section through the surface system showing the placement of the Gibb’s dividing surface and Gibb’s dividing line .................................................................14 Figure 1.11 Interface placement using the Gibbs convention. .....................................................14 Figure 1.12 Interface placement choices for surfactant/water solution open to the air ................15 Figure 1.13 Alignment characteristics of some basic liquid crystalline phases ...........................18 Figure 1.14 Molecular dynamics simulation of bulk 8CB ...........................................................19 Figure 1.15 3D carbon dioxide phase diagram showing several pressure-volume isotherms .....21 Figure 1.16 Isotherm for 8CB labeled with embedded diagrams indicating the stacking phases present on a surface
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