Simulation Studies on Microdroplet Generation and Trapping In T-Junction Devices

Abstract

In this thesis we present results of simulation studies on the generation of microdroplets due to the flow of two immiscible liquids in microfluidic T-junction devices and their trapping in Microwells built into the devices. We have studied devices, of the Slip-Chip type, having a single T-junction and double T-junction. We have used ANSYS Fluent Solver, which employs the Volume of Fluid (VOF) method, for the numerical simulations. The generation of water droplets in mineral oil has been studied for the case of steady flow of the two liquids as well as for the case of pulsed flow of water, the dispersed phase liquid. Droplet generation has been investigated in two different regimes - viz., squeezing and squeezing to dripping transition regime. The effect of fluid flow rates and surfactant concentration (Span 80 in mineral oil, the continuous phase liquid) on the droplet generation has been investigated. The scaling of the droplet length with the flow rate of either of the two liquids has been shown to be similar to results reported in the literature. It is seen that addition of a surfactant leads to formation of smaller droplets and an increase in the frequency of droplet generation. To understand droplet generation, we have plotted the pressure, within the two liquids, close to the T-junction as a function of time. These plots have given us a good insight into the process of droplet generation. Droplets generation by pulsed flow of water, keeping the flowrate of oil constant, revealed very interesting behaviour. In the absence of a surfactant, only single droplets are generated during each pulse duration. However, when the surfactant concentration exceeds a certain value, a pair of smaller droplets are generated for each pulse. The generation of droplet pairs happens at a higher surfactant concentration if the width of the side-channel, carrying the dispersed phase liquid, is reduced. Further, if the pulse width is reduced, only one, relatively small, droplet is generated per pulse, irrespective of the surfactant concentration. In many applications, merging of two (or more) droplets is required. To explore the merging of two droplets, we studied droplet generation in a double T-junction device. Depending on the geometry of the device, it is seen that the generated droplets either merge at the junction or travel further without merging. Merging of the droplets happens for relatively smaller width of the side-channel. With gradual increase in capillary number, droplets are generated in alternate regime. Our studies on droplet trapping in microwells indicate viii that inclusion of additional structures, such as a shallow circular pit or a pinhole, in the main channel of the device, enhances the chance of trapping. It is seen that the droplets traveling faster than a critical trapping velocity (Ucr) do not get trapped in the microwell. The addition of a surfactant is seen to lead to a significant reduction in Ucr for droplet trapping. The trapping of droplets traveling at velocities close to Ucr depends strongly on the alignment of the microwell and the pit or pinhole in the two plates of the Slip- Chip device. Dependence of droplet trapping on the dimensions of the pit or pinhole, the liquid flowrates and the surfactant concentration has been investigated. It is seen that droplet trapping does not happen if the surfactant concentration exceeds a certain value depending on the flowrates of the two liquids. The investigations reported in this thesis clearly indicate that extensive numerical simulations are required for arriving at an optimum design for a Microfluidic device for any application.