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The Development and Improvement of Instructions MICROFLUIDIC EMULSIFICATION A Dissertation by PENG HE Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY December 2011 Major Subject: Chemical Engineering Microfluidic Emulsification Copyright 2011 Peng He MICROFLUIDIC EMULSIFICATION A Dissertation by PENG HE Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Approved by: Chair of Committee, Zhengdong Cheng Committee Members, Debjyoti Banerjee Arul Jayaraman Victor Ugaz Head of Department, Charles Glover December 2011 Major Subject: Chemical Engineering iii ABSTRACT Microfluidic Emulsification. (December 2011) Peng He, B.E., University of Science and Technology of China Chair of Advisory Committee: Dr. Zhengdong Cheng This dissertation investigates the emulsification of aqueous liquid in immiscible organic liquid in various microfluidic environments, and addresses both experimental characterization and theoretical interpretation of the dynamics and design guidelines, as well as an application of microfluidic emulsification in fabrication of disk-like colloidal particle suspensions for studying its sedimentation behavior. In an attempt to understand the dynamics of drop formation in flow-focusing microfluidic channels, especially for an explanation of a transition from unique drop size to bimodal oscillating drop sizes as observed in the experiments, numerical simulation is developed to use the volume-of-fraction method to model the drop formation, and the simulation results help to interpret the transition in the theory of saddle-node transition in drop formation, as well as show the importance of selecting proper orifice length in flow-focusing microfluidic channel design. The electric technique for controlling of microfluidic emulsification is explored by a detailed study on low-frequency alternating-current electro-flow-focusing (EFF) emulsification in microfluidic channels. It is found that the droplet size variation is not a monotonic function of the electric field as in the case of direct-current EFF emulsification, which originates from the relaxation oscillation of the flow rate through the Taylor cone, and a power-law droplet size distribution was obtained at the voltage ramping-up stage. This emulsification process was modeled in analog to the charge iv accumulation and release in a resistor-capacitor electric circuit with an adjustable resistor, and the simulated data exhibit good agreement with the experiments. As an application of the microfluidic emulsification, a method of fabricating disk-like wax colloidal particle suspensions using electrospray is reported. Based on this technique, the first measurement of the hindrance function for sedimentation and creaming of disk-shaped colloids via the analytical centrifugation is reported. Disks align with the external flow right above the volume fraction of a few percent and this effect is extremely sensitive to the aspect ratio of disks. Due to this alignment effect, disk sedimentation/creaming demonstrate distinct trends in dilute and semi-dilute region. v DEDICATION To my parents. vi ACKNOWLEDGEMENTS It is a rewarding experience of pursuing my doctor degree in Texas A&M University and having Prof. Zhengdong Cheng as my research supervisor. Besides his broad knowledge in various physical and chemical subjects, his profound and keen insights into complicated problems are great and essential help for me to finish the journey of these research topics. As a good mentor, he also helps me form good habits in many aspects of personality during the graduate life which would be an invaluable and enduring memory for the future. I want express my grateful acknowledgments to Prof. Victor Ugaz, Prof. Arul Jayaraman, and Prof. D. Banerjee for the guidance and stimulations in research; Prof. Michael Bevan and Prof. Hung-Jue Sue for the inspiring discussion; Prof. Shantz, Prof. Jeong, and Prof. Cremer for their support in experiments. I also gracefully appreciate to current and former members of Complex Fluids Group for their helpful advices and constant contributions. Many thanks to Dr. Gang Liang and Dr. Orla Wilson for their technical assistances at Material Characterization Facility; Dr. Inho Lee, Dr. Richard Beckham, Dr. Dazhi Sun, Dr. Qingsheng Wang, Dr. Xin Qu, Dr. Zuyi Huang, Dr. Xiaole Wang, and many other friends for their help. I want to express sincere gratitude to Dr. Jingyi Shen for her spiritual advice and practical guidance. Finally, thanks to my parents for their encouragement and support, and to my wife, Weiwei Wang for her patience and love. vii NOMENCLATURE AC alternating current CFD computational fluid dynamics DC direct current DLVO Derjaguin-Landau-Verwey-Overbeek EFF electro-flow-focusing ES electrospray FF flow focusing PDMS polydimethylsiloxane viii TABLE OF CONTENTS Page ABSTRACT ............................................................................................................. iii DEDICATION ......................................................................................................... v ACKNOWLEDGEMENT ....................................................................................... vi NOMENCLATURE ................................................................................................. vii TABLE OF CONTENTS ......................................................................................... viii LIST OF FIGURES .................................................................................................. x LIST OF TABLES ................................................................................................... xiv 1. INTRODUCTION ........................................................................................ 1 1.1 Microfluidic Emulsification ................................................................. 1 1.2 Colloidal Suspensions .......................................................................... 15 1.3 Dissertation Overview .......................................................................... 36 2. DYNAMIC MODES OF DROP FORMATION IN MICROFLUIDICS .... 38 2.1 Summary .............................................................................................. 38 2.2 Introduction .......................................................................................... 38 2.3 Experimental Observation .................................................................... 43 2.4 CFD Simulation .................................................................................... 46 2.5 Results and Discussion ......................................................................... 57 2.6 Conclusion ............................................................................................ 74 3. AC ELECTRIC FIELD IN MICROFLUIDIC EMULSIFICATION ........... 75 3.1 Summary .............................................................................................. 75 3.2 Introduction .......................................................................................... 75 3.3 Experimental Setup .............................................................................. 76 3.4 Results and Discussion ......................................................................... 78 3.5 Conclusion ............................................................................................ 91 ix Page 4. SEDIMENTATION OF COLLOIDAL DISKS ............................................ 93 4.1 Summary .............................................................................................. 93 4.2 Introduction .......................................................................................... 93 4.3 Colloidal Disk Fabrication ................................................................... 95 4.4 Characterizations and Data Analysis .................................................... 101 4.5 Results and Discussion ......................................................................... 104 4.6 Conclusion ............................................................................................ 119 5. SUMMARY .................................................................................................. 120 REFERENCES ............................................................................................................ 123 APPENDIX A ............................................................................................................. 137 APPENDIX B ............................................................................................................. 168 APPENDIX C ............................................................................................................. 170 VITA ............................................................................................................................ 173 x LIST OF FIGURES FIGURE Page 1 Single-cell array of molded PDMS wells for biological study. ................ 4 2 CMOS/microfluidic hybrid cell manipulator. ............................................ 5 3 Chemical gradient generated in microfluidic networks. ............................ 7 4 Examples of various droplet-based microfluidic systems. ......................... 8 5 Multiple-emulsion capillary device. ........................................................... 10 6 Scale chart of typical particle sizes in the colloidal domain. ..................... 17 7 Tracings of the motion of colloidal
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