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Study of Droplet Splitting in an Electrowetting Based Digital Microfluidic System STUDY OF DROPLET SPLITTING IN AN ELECTROWETTING BASED DIGITAL MICROFLUIDIC SYSTEM by BIDDUT BHATTACHARJEE B.Sc., Bangladesh University of Engineering & Technology, 2004 M.Sc., Bangladesh University of Engineering & Technology, 2007 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE COLLEGE OF GRADUATE STUDIES (Applied Science) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) September 2012 © Biddut Bhattacharjee, 2012 Abstract This thesis focuses on the symmetric and asymmetric splitting of droplets, which is a prominent fluidic operation in a digital microfluidic system (DMFS). The prerequisite part of the investigation of droplet splitting is to understand the electrowetting-on-dielectric (EWOD) based droplet actuation. This thesis demonstrates that not only the EWOD actuation is a self-feedback system - implying that the actuation force depends on the position of the droplet, but also the size of the droplet affects the magnitude of actuation force. However, a sensing mechanism is essential for complex operations, e.g. dispensing and splitting. One contribution of this thesis is a novel method of sensing the droplet position that requires connections to the two adjacent electrodes in the lower plate only. For the fabrication of prototype DMFS, a new polymeric material, cyanoethyl pullulan (CEP), is proposed as the dielectric layer resulting in a simple and low-cost fabrication of DMFS. The required voltage for droplet manipulation is drastically reduced owing to high relative permittivity of CEP. Droplet splitting is investigated both numerically and experimentally. Numerical investigation of droplet splitting in FLOW-3D®, a commercial computational fluid dynamics software, revealed that the strength of viscous forces relative to the surface tension force determines the success of splitting. For successful asymmetric splitting, performed by applying voltages of unequal magnitude to left-hand and right-hand sides of the droplet, there exists a minimum voltage for the low-voltage side that guarantees splitting. This minimum voltage increases if the aspect ratio (i.e., diameter to height) of the droplet is reduced while keeping the diameter of the droplet constant. Investigation of the asymmetric splitting with different ratios of applied voltage revealed that the ratio between the volumes of accumulated liquid on either sides increases with voltage ratio. The feasibility of asymmetric splitting as well as the effects of different ratios of applied voltages were studied in prototype DMFS. The results verify the existence of a minimum voltage for successful splitting. The ratio between the volumes of the sister droplets increases with that of the applied voltages. Moreover, the general characteristics of flow-rates and liquid accumulation were found to be similar to those in simulations. ii Preface This thesis presents the results of research conducted in the Advanced Control and Intelligent Systems (ACIS) laboratory at the Okanagan School of Engineering, UBC, under the supervision of Dr. Homayoun Najjaran. Part of the thesis has been published in peer reviewed journals and conferences. The highlights of contributions of this thesis are as follows: The simulation results of Section 1 of Chapter 2 were published in two journals (aBhattacharjee and Najjaran 2010, bBhattacharjee and Najjaran 2010). The droplet dynamic system was modelled by considering the droplet as a rigid body and incorporating the empirical functions for the resistive forces. A simple controller and generic actuation and sensing mechanisms were also considered for the control of droplet position between two adjacent electrodes. Results show the relative strength of different resistive forces, transient and steady- state response in relation to controller gain and geometrical parameters. My responsibility was to investigate the modelling, computer simulation and manuscript preparation. Another contribution of this thesis is a novel method of droplet position sensing by measuring the capacitance between two coplanar electrodes. An analytical model of coplanar capacitance given by a quadratic function of the droplet position is proposed and verified by both numerical and experimental results, presented in Section 2 of Chapter 2. This new sensing method can be easily incorporated in feedback control of droplet operations from simple motion to complex splitting. Results of Section 3 of Chapter 2 were published in a conference (cBhattacharjee and Najjaran 2009) and my task was to analyse the EWOD-actuated droplet, develop simulation model of droplet motion, process and organize simulation results and prepare the manuscript. Under the assumption of a cylindrical conductive droplet, the EWOD-actuated droplet was demonstrated as a self-feedback system where the magnitude and direction of the generated actuation force depends on the position of the droplet with respect to the energized electrode. Moreover, the profile of actuation force also depends on the size of the droplet. Results in terms of transition time for the droplet from one electrode to the next as a function of electrode size, dielectric thickness and dielectric constant were also presented. The results of the last section of Chapter 2 were published in another conference (dBhattacharjee and Najjaran 2010). I was responsible for researching the material, fabricating the device and setting up instruments for measurement and writing the manuscript. The dielectric material, cyanoethyl pullulan, reported in this paper possesses high dielectric constant and can be iii deposited using a spin-coater only. Thus, digital microfluidic devices, which are fabricated using this new material, can be operated at significantly lower voltages than the values commonly reported in the literature. One of the major contributions of this thesis is the extensive study of the coupled electrohydrodynamics of symmetric and asymmetric splitting of droplets through CFD simulations (eBhattacharjee and Najjaran 2011). Results shown in Chapter 3 elucidate the distribution of electric field, charges, pressure and velocities during symmetric splitting. The relative strength of viscous shear with surface tension force determines the result of splitting. For a given size of the droplet, material properties and geometry, there exists a voltage level below which splitting is infeasible. Identification of the key parameters in determining the success of asymmetric splitting and the extent to which each of the parameters influence the splitting process is another major contribution of this thesis and is presented in Chapter 4. There exists a minimum required voltage for the low voltage side in asymmetric splitting for a given electrode size and aspect ratio of the droplet. The required voltage increases with increasing droplet height while the electrode size remains constant. For a given geometry and droplet aspect ratio, the ratio between the flow-rates to the right-hand and left-hand sides increases with the ratio between the applied voltages till the breakup of the neck. The final contribution of this thesis is the experimental investigation of asymmetric splitting. The results, as presented in Chapter 5, verify the characteristic pattern of exponentially decaying liquid flow-rates and capillary numbers. The ratio between the volumes of sister droplets resulting from asymmetric splitting increases with that of the applied voltages. Moreover, the volume ratio is greater for higher base voltages on the low voltage side. List of Publications Bhattacharjee B. and Najjaran H., 2012, “Droplet sensing by measuring the capacitance between coplanar electrodes in a digital microfluidic system”, Lab on a Chip (Accepted) Bhattacharjee B. and Najjaran H., 2010, “Simulation of droplet position control in digital microfluidics systems”, Journal of Dynamic System, Measurement and Control, 132, 014501. Bhattacharjee B. and Najjaran H., 2010, “Droplet position control in digital microfluidic systems”, Biomedical Microdevices, 12 ,115-124. Bhattacharjee B. and Najjaran H., 2011, “Modeling and simulation of unequal droplet splitting in electrowetting based digital microfluidics”, ASME ICNMM, Edmonton. iv Bhattacharjee B. and Najjaran H., 2011, “Low-cost, low-voltage microfluidic biochip based on electrowetting actuation of droplets”, Proceedings of Microtechnologies in Medicine and Biology, Lucerne, Switzerland. Bhattacharjee B. and Najjaran H., 2010, “Effects of the properties of dielectric materials on the fabrication and operation of digital microfluidic systems”, Proceedings of ASME IMECE, Vancouver. Bhattacharjee B. and Najjaran H., 2009, “Size dependant droplet actuation in Digital Microfluidic system”, Proceedings of SPIE conference on Micro- Nanotechnology Sensors, Systems and Application, Orlando, 7318. v Table of Contents Abstract .......................................................................................................................................... ii Preface .......................................................................................................................................... iii Table of Contents .......................................................................................................................... vi List of Tables .............................................................................................................................. viii List of Figures ..............................................................................................................................
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