Capillarity and convection-controlled assembly in the spreading of particulate suspensions on an air-liquid interface By Rajesh Ranjan A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Department of Chemical Engineering and Applied Chemistry University of Toronto © Copyright by Rajesh Ranjan 2018 i Capillarity and convection-controlled assembly in the spreading of particulate suspensions on an air-liquid interface Master of Applied Science Rajesh Ranjan Department of Chemical Engineering and Applied Chemistry University of Toronto 2018 Abstract Self-assembly of particles at interfaces has immense potential for printing and coating applications in biological and industrial processes. Several studies on the spreading of pure fluids on an air- liquid interface have been conducted; however, none have examined the spreading characteristics of two-phase fluid materials. In this work, a drop of concentrated suspension of PMMA particles in silicone oil was placed on an aqueous glycerol solution – air interface. Depending on the initial rate of spreading of the suspension, two outcomes were observed: the particles were either swept away by the spreading suspension or organized into an array of two-dimensional networks. The two outcomes were explained by describing the particle motion as being a result of a competition between fluid convection and capillary attraction. This description was confirmed by performing experiments for different particle sizes, volume fractions, the viscosity and salinity of the substrate on the spreading behavior and pattern. ii Acknowledgement I would like to express my appreciation to all those people who have contributed to make this work possible through their help and support along the way. My deepest gratitude goes to my supervisor Dr. Arun Ramachandran for giving me the opportunity to work on this interesting project and for his valuable guidance, inspiration, constant encouragement and support through all the phases of the project. I feel indebted to my supervisor for giving abundant freedom to me for pursuing new ideas. I take this opportunity to express my deep sense of gratitude to Dr. Julia A. Kornfield from California Institute of Technology and Dr. Kathleen J. Stebe from University of Pennsylvania for thoughtful advice and discussions. I extend my thanks to my summer student, Srishti Sehgal, for help with the experiments and my lab members for their inputs that helped in the completion of this project. iii Table of Contents Acknowledgement ......................................................................................................................... iii Table of Contents ........................................................................................................................... iv List of Figures ................................................................................................................................ vi List of Tables ............................................................................................................................... viii List of Videos ................................................................................................................................. ix 1. Motivation ............................................................................................................................... 1 2. Literature Review .................................................................................................................... 8 3. Experiment............................................................................................................................. 13 3.1 Materials and Methods ........................................................................................................ 13 3.2 Experimental Conditions and Systems Investigated ........................................................... 14 4. Result and Discussions .......................................................................................................... 16 4.1 Effect of Volume Fraction of the PMMA Suspension ................................................... 32 4.2 Effect of Particle Size ..................................................................................................... 33 4.3 Effect of Viscosity of Suspending Fluid ........................................................................ 34 4.4 Effect of Addition of Salt in the Substrate ..................................................................... 36 4.5 Effect of Dish Area ........................................................................................................ 37 4.6 Summary of the Behavior of Systems ............................................................................ 38 5. Conclusions and Future Work ............................................................................................... 42 Appendix ....................................................................................................................................... 44 iv Appendix A: Determination of dependence of radius as a function of time ............................. 44 Appendix B: Shape of the interface around individual PMMA particle ................................... 46 Appendix C: Precursor film experiments .................................................................................. 49 Appendix D: Theoretical prediction of the area coverage ........................................................ 50 Appendix E: Scaling analysis to understand the force balance ................................................. 53 Appendix F: Dependence of the critical radius (Rc) on particle size and volume of the suspension introduced during the spreading process ................................................................................... 55 References ..................................................................................................................................... 56 v List of Figures Figure 1: Wetting conditions and their relationship to the contact angles ...................................... 1 Figure 2: Self-cleansing properties of lotus leaf due to surface roughness and hydrophobicity .... 2 Figure 3: Spreading behavior can be categorized based on fluidity of the substrate, the volatility of the drop phase, and the volume fraction of particles in the suspending medium ........................... 5 Figure 4: Drop of ethanol mixed with dye defragments into myriads of droplets over a layer of mineral oil ....................................................................................................................................... 6 Figure 5: Liquid drop spreading over a solid substrate ................................................................... 8 Figure 6: Suspension drop introduced over glycerol b: Subsequent spreading of the suspension drop ............................................................................................................................................... 13 Figure 7: Image of the PMMA aggregates under 4X magnification b: Image of extended network of aggregate under 4X magnification ........................................................................................... 16 Figure 8: Image sequence for 40% suspension, 20 μm particles on pure glycerol (initial part of the spreading process) Last frame: after a critical radius, the particle ring/ dam can no longer be sustained, and it fragments ............................................................................................................ 17 Figure 9: Area shows linear dependence on time during the spreading process ......................... 18 Figure 10: The spreading front is circular during the initial stage (the spreading front is nearly circular) ......................................................................................................................................... 18 Figure 11: The spreading front during the later stage of the spreading process (the spreading front starts to deviate from circularity) .................................................................................................. 19 Figure 12: Image sequence for 40% suspension, 20 μm particles on glycerol (later part of the spreading process once the instability sets in) .............................................................................. 20 Figure 13: Schematic of the cross-sectional shape of the spreading drop showing the confinement of the particles and the pure fluid region. The inset shows the shape of interface around PMMA particles sitting at the silicone oil – glycerol interface ................................................................. 20 Figure 14: Comparison of the change in outer radius of the suspension front to inner core radius ....................................................................................................................................................... 21 Figure 15: Regions in the spreading drop phase before the instability ......................................... 22 Figure 16: Regions in the spreading drop phase after the instability ............................................ 22 Figure 17: Confocal experiments indicates the presence of silicone oil around the PMMA particles ....................................................................................................................................................... 25 vi Figure 18: The size of the polygons increases radially outward from