Hydrodynamics of droplet dislodgement and breakup in confined microfluidic geometries
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Droplet dynamics in confined geometries is a problem of fundamental importance in droplet microfluidics and oil recovery. In droplet microfluidics, the reagents can be stored inside the droplets. These reagents can vary from chemical to biological samples. Therefore, these technologies provide a platform for several applications including chemical reactions engineering, liquid-liquid extraction, drug screening, cellular assays, DNA analysis and protein crystallization. In these applications, it is necessary to generate and manipulate the behaviors of the droplets in the microchannels. Manipulating the behaviors of the droplets include: breakup, coalescence, mixing, dilution and sorting. The design of the microfluidic devices requires a thorough understanding and a mechanistic insight into the droplet dynamics. On the other hand, in oil recovery the purpose is to extract oil from the reservoirs with the maximum efficiency. Oil droplets are trapped inside porous network in confined configurations. In this field, a secondary fluid is injected to mobilize the oil droplets in the micro-pores. The efficiency of the droplet mobilization depends on the fluids and flow properties as well as the droplet confinement and porosity. Therefore, understanding the mechanism of droplet dislodgement is important to increase the efficiency of oil recovery. My dissertation focuses on understanding the dynamics of the confined droplets in the microfluidics. Here, we used three-dimensional, pore-scale, Volume-of-Fluid simulations. We studied the hydrodynamics of droplet generation at a microfluidic T-junction, droplet mobilization in a constricted microchannel and droplet breakup in a microchannel with an obstacle. As the droplets move in the confined microfluidic geometries, there exists corner flows around the droplets, also called as gutter flows. Our results show that the gutter flows control the mechanisms of droplets behaviors through their hydrodynamic resistance. The resistance links between the pressure drop and the flow rate in any fluidic system. The resistance of the gutters depends on their size and interfacial mobility. As the size of the gutter and the interfacial mobility decreases, the hydrodynamic resistance increases. Droplets with higher viscosity have less interfacial mobility. On the other hand, larger droplets and droplets in the channels with smaller heights, have smaller gutters. Therefore, by increasing the droplet viscosity and size and decreasing the channel height, the resistance increases. The higher resistance means that at a fixed flow rate, the pressure drop in the system is higher. These findings explain two of our major observations: firstly, highly viscous droplets get mobilized and break-up at smaller imposed flow rate. Secondly, large droplets and droplets in channels with small aspect ratios, get mobilized and break-up at smaller flow rates. During the process of droplet breakup, there is a critical time, after which the breakup is inevitable, even if the flow is stopped. The process of breakup without the continuous phase flow is called autonomous breakup. We found that the autonomous breakup time depends linearly on the viscosity of the confined droplets. By increasing the drop viscosity, the breakup time increases linearly. This observation is in agreement with the breakup of unconfined droplets. Therefore, it indicates that when the gutter flows are not present in the microfluidic systems, the droplet behaves similar to unconfined droplets. Additionally, the linear increase in the breakup time with the viscosity, results in generation of larger droplets at microfluidic T-junction. As the viscosity of the dispersed phase increases, the breakup time also increases. Therefore, the droplet gets filled-up longer before the pinch-off. This leads to generation of larger droplets.