Multiscale dynamics of bubbles and droplets in microfluidic networks

MULTISCALE DYNAMICS OF BUBBLES AND DROPLETS IN MICROFLUIDIC NETWORKS PRAVIEN PARTHIBAN NATIONAL UNIVERSITY OF SINGAPORE 2013 MULTISCALE DYNAMICS OF BUBBLES AND DROPLETS IN MICROFLUIDIC NETWORKS PRAVIEN PARTHIBAN (B.Tech, Anna University, India) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2013 DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all sources of information which have been used in the thesis This thesis has also not been submitted for any degree in any university previously Pravien Parthiban 07 March 2013 Acknowledgements My life immersed in experiments over the past six years would not have been as enjoyable and invigorating as it has been, if not for my supervisor’s constant support and encouragement. Dr.Khan, you have truly been an inspiration. Im greatly indebted to you for always lending a patient ear, and above all for giving me the freedom to fail and the space to explore and learn. Im grateful for all the support provided by the Department of Chemical and Biomolecular Engineering and the University. I would also like to thank Dr. Ramam for facilitating access to the clean rooms at IMRE. Friends near and far have been a crucial part of this journey, I will forever be in their debt. I owe special thanks to my Iranian brethren for always providing me with a place to call home. Suhanya, Zahra, Abhinav, Dr. Rahman, and Sophia, my old comrades in arms, thanks for making the lab an engaging place to be. I would be remiss, if I fail to mention the new comers to the lab, Arpi, Prasanna, Reno, Sweekun, Abu its been wonderful getting to know you guys. Sathvi, these past two years would have been truly difficult if not for you. Lastly, I wouldn’t be here if not for my brother, and my parents. My gratitude towards them will remain till eternities die. Contents Acknowledgements i Summary v List of Figures vii List of Symbols xvii Microscale Multiphase Flows 1.1 1.2 1.3 Multiphase Flow In Porous and Fractured media . . . . . . . . 1.1.1 Carbon Dioxide Sequestration . . . . . . . . . . . . . . 1.1.2 Enhanced Oil Recovery . . . . . . . . . . . . . . . . . . 1.1.3 Contaminant Transport in Porous media . . . . . . . . 12 Multiphase Flow in Physiological Systems . . . . . . . . . . . 13 1.2.1 Respiratory System . . . . . . . . . . . . . . . . . . . . 13 1.2.2 Microcirculation in the Cardiovascular System . . . . . 15 Multiphase Flows in Structured Microchannels . . . . . . . . . 17 1.3.1 1.4 Microfluidics . . . . . . . . . . . . . . . . . . . . . . . . 17 Engineering multiphase flows in microchannel networks . . . . 22 Contents 1.5 Confined Bubble and Droplet Dynamics In Microscale Systems - an Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 1.6 1.5.1 Pressure Drop . . . . . . . . . . . . . . . . . . . . . . . 28 1.5.2 Confined Bubble and Droplet Transport in Junctions . 40 Thesis Aims and Scope . . . . . . . . . . . . . . . . . . . . . . 44 Experimental Methods 2.1 46 Device Fabrication . . . . . . . . . . . . . . . . . . . . . . . . 46 2.1.1 Photolithography . . . . . . . . . . . . . . . . . . . . . 46 2.1.2 Replica Molding . . . . . . . . . . . . . . . . . . . . . . 48 2.2 Pressure Measurements . . . . . . . . . . . . . . . . . . . . . . 50 2.3 Measuring Number of Bubbles, Bubble Length, Bubble Velocity, and Liquid Slug Length . . . . . . . . . . . . . . . . . . . 51 2.4 Measurement Errors and Error Propagation . . . . . . . . . . 54 Transport of non-wetting bubbles and droplets in microchannels 3.1 3.2 3.3 56 Confined bubble transport . . . . . . . . . . . . . . . . . . . . 57 3.1.1 Experimental methods . . . . . . . . . . . . . . . . . . 58 3.1.2 Results and discussions . . . . . . . . . . . . . . . . . . 61 Confined droplet transport . . . . . . . . . . . . . . . . . . . . 70 3.2.1 Experimental methods . . . . . . . . . . . . . . . . . . 70 3.2.2 Results and discussions . . . . . . . . . . . . . . . . . . 75 Hydrodynamic resistance of a microchannel filled with bubbles or inviscid droplets . . . . . . . . . . . . . . . . . . . . . . . . 81 3.3.1 Results and Discussion . . . . . . . . . . . . . . . . . . 82 iii Contents 3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Effects of Channel Surface Wettability 92 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 4.2 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . 93 4.3 Results and Discussions . . . . . . . . . . . . . . . . . . . . . 96 4.4 4.3.1 Wetting Transition . . . . . . . . . . . . . . . . . . . . 96 4.3.2 Pressure drops across partially lubricated bubbles . . . 105 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 Traffic of Bubble and Droplet Trains at Microfluidic Junctions 5.1 111 Filtering of Droplet and Bubble Trains at a Symmetric Microfluidic Junction . . . . . . . . . . . . . . . . . . . . . . . . 113 5.2 5.1.1 Experimental Methods . . . . . . . . . . . . . . . . . . 113 5.1.2 Results and Discussions . . . . . . . . . . . . . . . . . 119 Bistable Filtering of Droplet and Bubble Trains in Asymmetric Microfluidic Junctions . . . . . . . . . . . . . . . . . . . . . . 132 5.3 5.2.1 Experimental Methods . . . . . . . . . . . . . . . . . . 132 5.2.2 Results and Discussions . . . . . . . . . . . . . . . . . 135 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Epilogue 146 6.1 Principle Thesis Contributions . . . . . . . . . . . . . . . . . . 146 6.2 Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . 148 Bibliography 151 iv Summary Understanding the flow of confined bubbles and droplets within natural or man-made microchannel networks is crucial to a broad range of technologies ranging from enhanced oil recovery to microfluidic chip-based chemical analysis, synthesis and discovery. The traffic of droplet or bubble ensembles through even elementary microchannel networks is complex and nonlinear. This makes it challenging to both design and engineer new networks and to predict the dynamical behavior of a known network. This thesis broadly aims to advance the current understanding of such phenomena by conducting rigorous and detailed experimental measurements of bubble and droplet dynamics in simple yet prototypical microchannel topologies. Specifically, this thesis studies bubble/droplet-scale hydrodynamics in terms of the pressure drop across confined bubbles and droplets translating through lithographically defined microchannels of rectangular cross-section under a variety of conditions, and how such local phenomena dictate the global behavior of trains of monodisperse bubbles or droplets as they flow through prototypical network components such as junctions. The pressure drop across confined bubbles and droplets translating through rectangular microchannels is first studied, and the modification of the hy- drodynamic resistance to flow through a microchannel due to the presence of bubbles and droplets is addressed. In the ideal case where the microchannel is filled with a completely wetting continuous liquid phase, it is found that there are readily accessible conditions wherein the presence of bubbles or droplets reduces the hydrodynamic resistance of the microchannel – a rarely documented phenomenon in the multiphase microfluidics literature. Remarkably, even a slight variation from the ideal case of complete wetting in bubble flows is shown to dramatically increase the hydrodynamic resistance of the microchannel, highlighting the crucial role played by the wettability of channel surfaces. In such systems, a rich variety of bubble morphologies are observed, governed by the speed of propagation of the bubble and its size. Finally a counterintuitive bistable behavior in the traffic of bubble and droplet trains at a simple microfluidic junction, wherein the incoming bubble/droplet train can exclusively and entirely sort into either arm of the junction, is investigated. Furthermore the existence of this bistability is exploited to flexibly regulate bubble or droplet traffic at a microfluidic junction. The studies conducted in this thesis provide important and new insights that advance the understanding, prediction and regulation of multiphase flows in porous media or multifunctional, multiplexed microfluidic devices. vi List of Figures 1.1 Representation of different flow patterns, A - Bubbly Flow, B - Segmented Flow, C - Transition to Churn Flow, D - Churn Flow, E - Annular Flow . . . . . . . . . . . . . . . . . . . . . 1.2 Representation of the front cap of a moving bubble showing the spherical, transition and uniform film regions . . . . . . . 32 2.1 Micrograph of the cross-section of a representative micro-channel in PDMS. The scale bar represents 100 microns. . . . . . . . . 48 Chapter Epilogue Understanding the hydrodynamics of simple flows like that of confined bubbles and droplets in prototypical network geometries such as a single straight microchannel or an elementary channel bifurcation is fundamental to engineer multiphase flows in more complex microchannel networks that may occur in geological formations or high throughput, multifunctional microfluidic devices. This thesis aimed to advance the current understanding of such prototype flows. 6.1 Principle Thesis Contributions The transport of confined bubbles in rectangular microchannels filled with liquids that completely wet the channel walls were first studied. The pressure drop across a single confined bubble was experimentally determined from the overall pressure drop across a microchannel filled with n bubbles by using a unit cell pressure drop model. Even with an implicit assumption that the Principle Thesis Contributions flow across the entire liquid slug is fully developed, the measured pressure drop across the bubble was just 30% higher than the theoretical predictions given by Wong. Hodges and coworkers showed theoretically that droplets with a viscosity substantially lower than that of the surrounding continuous phase liquid, can be treated as being inviscid. In essence an inviscid droplet should behave like a bubble. We show that this is indeed the case; to the best of our knowledge this is the first experimental validation of Hodges theoretical hypothesis. We subsequently developed a model for the hydrodynamic resistance of a microchannel filled with droplets or inviscid bubbles. This model was seen to predict our experimental measurements to within ±10%. Crucially this model identified the conditions where the hydrodynamic resistance of a microchannel decreases with an increase in the number of bubbles or droplets present. This is in contrast to existing models for the hydrodynamic resistance of microchannel, wherein it is always assumed that droplets increase the resistance to flow. The implications of the situation wherein bubbles and droplets reduce the resistance to flow were highlighted by studying the traffic of bubble and droplet trains in symmetric and asymmetric junctions. In contrast to previous experiments where droplets had the ability to filter only into the shorter arm of an asymmetric junction at high dilution of the incoming train, we found the existence of filter regimes even in symmetric junctions. Interestingly, the bubbles and droplets also had the ability to filter into the longer arm of an asymmetric junction, surprisingly this was favored in trains with low dilutions. 147 Future Directions Slight deviations from the ideal case of complete wetting of channel walls by the liquid in confined bubble transport were examined. A rich bubble morphology was observed, depending on the speed and bubble length, the front and rear end of the translating bubble could be comprised moving three phase contact lines, or the bubble could be either partially or completely lubricated by a deposited film of liquid. Curvature driven flows from the film to the liquid filled channel corners were used to rationalize the existence of the latter two regimes. In partially encapsulated bubbles, the pressure drop across the bubble could be nearly a couple of orders greater than an equivalent bubble translating through a completely wetting fluid. 6.2 Future Directions In this section we catalogue a set of interesting questions that remain unanswered. The implications of conditions wherein bubbles lower the resistance to flow have been investigated in traffic of bubble trains at microfluidic junctions. It would be of interest to see what happens if such conditions are accessed during the filling of a long microchannel with bubbles. If the flow is pressure driven and the channel is initially filled completely with the liquid, if the first bubble that flows into the channel has a length and speed such that it decreases the resistance of the microchannel, it can be expected that subsequent bubbles that are formed will be progressively larger and move faster. If the resistance of the microchannel in contrast is increased by the first bubble, the subsequent bubbles that are formed will be progressively 148 Future Directions smaller and move slower. If the channel is initially filled with the gas instead, different start up dynamics can be expected. An intriguing question to ask would be if the under same input conditions of gas and liquid pressures, if the channel is initially gas filled against liquid filled, will the bubble train formed at steady state be identical. Investigations into the effects of the compressibility of bubbles would be another worthwhile venture. In long microchannels the bubbles expand and accelerate along the length of the microchannel as expected, but preliminary experiments conducted in our lab show that the presence of non-ideal channel geometries can excite interesting dynamical response in long bubble trains. If the bubble train is forced to squeeze through a constriction, violent oscillations in flow speeds can occur upstream, these oscillations die out after a finite length. If sharp turns are included in the microchannel layout, the bubbles expand and contract as they traverse the turn, this can in turn have interesting implications in the overall profile of bubble lengths and speeds across a long microchannel composed of dozens of such turns. The effect of surfactants on bubble and droplet transport is another topic of interest. Past studies have shown that the relative speed between confined bubbles and the surrounding liquid can be dramatically impacted by the presence of surfactants. Trace quantities of surfactants should also increase the pressure drop per bubble by a factor of 42/3 . In preliminary experiments carried out, we have seen that in systems with minute quantities of surfactants (10% of the critical micellar concentration), the range of flow rates where ordered droplet formation is possible is narrower than that for surfactant-less systems (even when the surface tension and fluid viscosities are nearly iden149 Future Directions tical). Additionally in surfactant systems, it was observed that the droplets catch up with each other and coalesce as the travel along the microchannel. Elucidation of mechanisms that govern such phenomenon would be valued, as in a lot of microscale multiphase flows surfactants are present. In studies with droplet and bubble traffic in junctions, situations where bubbles and droplets not break up at the junction have been exclusively investigated. 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Durlofsky, Annuual Review of Fluid Mechanics 37, 211 (2005). 161 [...]... sequestration of carbon dioxide, and spread of contaminants in aquifers often involve the transport of multiple phases in complex networks of microchannels [1–3] , so do physiological flows in the microvascular system and pulmonary airways [4–6] Confinement in such flows promotes the dominance of surface forces over those of viscous, inertial or gravitational origin; this and other benefits that go hand in hand... measurement of P , showing that in contrast to single phase flows the pressure drop in multiphase flows can remain constant with increasing flowrates (a-inset) Plot showing the variation of flow ratio qG /qL with increasing Qtot (b) The number of bubbles in the microchannel is seen to first slightly increase and then decrease dramatically with increasing Qtot (b-inset) The pressure drop per unit cell increases...List of Figures 2.2 Micrographs showing bubble train flow in the single channel device used for measuring pressure drop (a) and through the microfluidic loop (f) (b, g) Binary images of the grayscale micrographs shown in (a) and (f ) respectively (c, h) Cropped binary images showing the region of interest (d, i) Plot of the digitized signal obtained by column wise scanning of the cropped binary images... flowrate qW , and ionic liquid at a flow rate qIL using syringe pumps They meet at a T-junction to generate mono-disperse droplets of water in ionic liquid The pressure pf at the gas inlet is measured 72 3.8 (a) Micrograph, showing the formation and flow of droplets in the microchannel (b-e) Variations in lB , lS , U and f of the droplets and liquid slugs generated in a period of 2 minutes... to generate mono-disperse bubbles of nitrogen in ethanol The pressure pf at the gas inlet is measured 59 viii List of Figures 3.2 (a) Micrograph, showing the formation and flow of bubbles in the microchannel (b-e) Variations in lB , lS , U and f of the bubbles and liquid slugs generated in a period of 2 minutes Only very rarely do the sizes, speeds and frequency go beyond ±2% of the mean ... the need to study the physics of bubble and droplet transport in microchannel networks by describing in brief the various scenarios and solutions where such flows are inherent, and highlighting 3 Multiphase Flow In Porous and Fractured media the importance of such systems and challenges in engineering them 1.1 Multiphase Flow In Porous and Fractured media A vast majority of the earths subsurface environment... phases in a number of situations contaminant transport in aquifers, enhanced oil recovery processes, and CO2 entrapment and storage in saline aquifers [1,3,18–23] These flows are challenging to understand and model; in part because of the nonlinearities introduced by the presence of interfaces, and due to the fact that porous media are highly heterogenous in multiple length scales, from the scale of roughness... protection of these water sources from contaminants or remediation of sources already contaminated is crucial Thus, understanding the transport and entrapment of non-aqueous phase liquids in geological formations is of the utmost importance The petroleum and oil industry is one of the biggest sources for such contaminants, the very process of drilling and extraction of oil can lead to contamination of aquifers... (or droplet) train flow are used interchangeably 3 Churn Flow : At higher superficial velocities of both the wetting and non wetting fluid satellite droplets or bubbles appear at the tail of the non wetting fluid segments 4 Annular flow : At high velocities and low fractions of the wetting fluid the wetting fluid forms a thin film flowing along the wall of the channel with the non wetting fluid flowing as a core... prosperity, current and future, remains inextricably linked to the availability of energy, and fossil fuels currently supply nearly 85% of the energy needed for industrial activity, and are far cheaper than cleaner alternatives The United States, China, Russia and India, incidentally a group containing some of the worlds largest and fastest growing economies, together hold two-thirds of the words coal . MULTISCALE DYNAMICS OF BUBBLES AND DROPLETS IN MICROFLUIDIC NETWORKS PRAVIEN PARTHIBAN NATIONAL UNIVERSITY OF SINGAPORE 2013 MULTISCALE DYNAMICS OF BUBBLES AND DROPLETS IN MICROFLUIDIC NETWORKS PRAVIEN. Micrograph, showing the formation and flow of bubbles in the microchannel. (b-e) Variations in l B , l S , U and f of the bubbles and liquid slugs generated in a period of 2 minutes. Only very. Micrograph, showing the formation and flow of droplets in the microchannel. (b-e) Variations in l B , l S , U and f of the droplets and liquid slugs generated in a period of 2 minutes. Only very