Chemical and Biomolecular Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2751
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Item Dynamics of Elastic Capsules in Cross-Junction and T-Junction Microfluidic Channels(2017) Mputu udipabu, Pompon; Dimitrakopoulos, Panagiotis; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation, we investigate via numerical computations the dynamicsof elastic capsules (made from a thin strain-hardening elastic membrane) in two microfluidic channels of cross-junction and T-junction geometries. For the cross-junction microfluidic channel, we consider an initially spherical capsule with a size smaller than the cross-section of the square channels comprising the cross-junction, and investigate the effects of the capsule size, flow rate, and lateral flow rates on the transient dynamics and deformation of low-viscosity and equiviscous capsules. In addition, we also study the effects of viscosity ratio on the transient capsule dynamics and deformation. Our investigation shows that the intersecting lateral flows at the cross-junction act like a constriction. Larger capsules, higher flow rates and higher intersecting lateral flows result in stronger hydrodynamic forces that cause a significant capsule deformation, i.e., the capsule’s length increases while its height decreases significantly. The capsule obtains different dynamic shape transitions due to the asymmetric shape of the cross-junction. Larger capsules take more time to pass through the cross-junction owning to the higher flow blocking. As the viscosity ratio decreases, the capsule’s transient deformation increases and tail formation develops transiently, especially for low-viscosity capsules owing to the normal-stress effects of the surrounding fluid on the capsule’s interface. However, the viscosity ratio does not affect much the capsule velocity due to a weak inner circulation. Our findings suggest that the tail formation of low-viscosity capsule may promote membrane breaking and thus drug release of pharmaceutical capsules in the microcirculation. Furthermore, we investigate via numerical computations the motion of an elastic capsule (made from an elastic membrane obeying the strain-hardening Skalak law) flowing inside a microfluidic T-junction device. In particular, we consider the effects of the capsule size, flow rate, lateral flow rate, and fluid viscosity ratio on the motion of the capsule in the T-junction micro-channel. As the capsule’s initial lateral position increases, the capsule moves faster and reaches different final lateral positions. As the capsule size increases, the gap between the capsule’s surface and the channel wall decreases. This results in the development of stronger hydrodynamic forces and a decrease in the capsule velocity due to flow blocking. As the capsule size increases, there is a small lateral migration towards the micro-channel centerline, which is the low-shear region of the T-junction micro-channel. This migration is in agreement with experimental and numerical studies on non-inertial lateral migration of vesicles in bounded Poiseuille flow by Coupier et al. [13] who showed that the combined effects of the walls and of the curvature of the velocity profile induce a lateral migration toward the centerline of the channel. As the capillary number Ca increases, the stronger hydrodynamic forces cause the capsule to extend along the flow direction (i.e., the capsule’s length Lx increases as the capsule enters the T-junctions and decreases as the capsule exits the T-junction). There is a small lateral migration away from the micro-channel centerline as the flow rate Ca increases. The capsule lateral position zc, main-flow velocity Ux and migration velocity Uz are practically not affected by the fluids viscosity ratio λ. As the channel’s lateral flow rate increases, the capsule migrates downwards towards the bottom of the device. Our findings on the lateral migration in the T-junction micro-channel suggest that there is a great potential for designing a T-junction microfluidic device that can be used to manipulate artificial and biological capsules.Item DYNAMICS OF CAPSULES IN COMPLEX MICROFLUIDIC DEVICES(2018) Koolivand, Abdollah; Dimitrakopoulos, Panagiotis; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The dynamics of micro-capsules has attracted a lot of attention in the last decade due to their vast applications in different industrial sectors such as cosmetic products, food industry, chemical processes, reaction systems, cell modeling, drug delivery, and medical processes. Additionally, biological cells such as red blood cells can be modeled as capsules. Understanding the rheological behavior of these cells provides great physical insight for early diagnosis of relevant diseases. The main objective of this research is to investigate the effects of physical and geometrical parameters on the hydrodynamics of simple and multiple capsules in complex mi- crofluidic devices. For this purpose, we have developed the mathematical formulation needed for modeling multiple capsules with or without complex internal structures. The developed framework provides an enormous flexibility in problem definition, and facilitates the investigation of the hydrodynamics of a wide class of capsules in microfluidic channels and vascular capillaries. We first study the deformation of a spherical capsule in a T-junction channel. It is shown that an initially spherical capsule develops a bean shape at low flow rates and an inverse kayak shape at high flow rates. Based on the non-trivial deformation of the capsule, a new methodology for the determination of membrane moduli is proposed. For an accurate determination of the membrane moduli, it is paramount to measure the capsule dimensions precisely, which is easier in the proposed device owning to the stagnation-point flow of the T-junction. To determine the membrane moduli, one needs to do a single experiment for different flow rates, and compare the experimental measurements of the capsule steady-state dimensions with the provided computational data. We then consider the flow dynamics of non-spherical capsules and investi- gate the effects spheroidity and initial orientation on the steady-state shape. It is found that a non-spherical capsule, placed with a non-zero initial orientation angle along the centerline of a microchannel, does not practically rotate during deforma- tion. Thus, precise instrumentation is required for proper alignment of the capsule which influences the deformation and steady-state shape. This behavior may explain possible inconsistencies between measured (experimental) and calculated (compu- tational) shapes. We then study the lateral migration of capsules with different size in a mi- crofluidic channel with a trapezoidal cross-section. Owing to the emergence of 3D printing technology, fabrication of a channel with trapezoidal cross-section is fea- sible. Based on our computational data, we proposed an optimized geometry that could be utilized for separation of capsules or cells with different size. The main advantage of the proposed geometry is its inexpensive fabrication cost without the need for incorporating complicated inner structures, which automatically eliminates the risk of channel clogging. Moreover, the simple structure of the trapezoidal mi- crochannel allows an easy scale out through parallelization and reduction of the cell sorting time. In addition, we investigate the complex behavior of two (equal or unequal sized) capsules flowing in a square microfluidic channel. Capsules merging process controls the on-demand drug release and reaction. Thus, we identified the hydro- dynamic conditions that facilitates or hinders the merging of the capsules. The merging process is commonly accompanied by the drainage of existing liquid film between two particles. We observed that the capsules merging in most cases is ac- companied by the formation of dimple surfaces, and thus a simplified flat lubrication surface assumption which is widely-used in the theoretical studies might not be an ideal choice for modeling the film drainage time in merging process.Item Breakage of Single Droplets in 2-D Inertial Flows(2018) Ko, Derrick I.; Calabrese, Richard V; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Droplet break-up research has traditionally focused on droplets in: 1) generally uniform flow fields (constant strain rates or constant turbulence dissipation rates) that are easier to characterize and study, and in 2) highly complex flow fields generated by mixing devices in which the evolution of an entire droplet population with time is of interest. The current work adds to the existing body of knowledge by investigating the effect of short-term high-intensity deformation events on the break-up of single large droplets in both turbulent and inertial laminar flows. This approach consists of two components: high-speed imaging of droplets as they pass through a 2-D slit orifice and CFD simulations of the orifice flow field. The experimental trajectories of the droplets are combined with the CFD-generated flow field to determine the deformation history of the droplet prior to break-up. In turbulent 2-D orifice flows, droplets and bubbles on the order of the macroscale of turbulence were studied. For these large droplets and bubbles, it was found that the product of strain rate magnitude and un-deformed diameter (essentially the velocity difference across the droplet) was a more suitable velocity scale. A new form of locally-derived, trajectory-dependent Weber number, consisting of the maximum average strain rate magnitude over an exposure time equal to 0.04 multiplied by the Stokes particle relaxation time, was used to develop a break-up probability model that can be applied to the break-up of both liquid droplets and gas bubbles. The model adds weight to the break-up interpretation of Levich (1962); break-up occurs due to the internal pressure fluctuations overcoming the interfacial stresses of the deformed droplet. In inertial laminar 2-D orifice flows, the break-up of water droplets in oil was studied at two viscosity ratios. The recommended local Weber number was again based on the maximum average strain rate magnitude over a particular exposure time, but this exposure time was instead 8 multiplied by the oscillation time scale. Using the maximum length achieved by the droplet as the length scale was also found to better represent the potential for break-up. With these modifications, and an associated capillary number-based model for predicting the drop draw ratio, two local Weber number thresholds were defined. First, the threshold for break-up is at Welocal = 30. Second, the threshold for producing large daughter droplets, termed fracturing in this work, is at Welocal = 1,000. Between these thresholds, droplets may fracture or undergo a mechanism termed erosion in this work, where a small number of tiny droplets break off from the main body of the droplet. Both of these break-up types are based on an elongative end-pinching mechanism.