Controlling the injection of picoliter volumes in droplet microfluidics
Document typeMaster thesis
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An electrode based pico-injector chip is a microfluidic device aimed at injecting a precise amount of volume into every picoliter droplet of a continuous emulsion flow. This is possible because of the application of an electric field disturbance near the injection junction. In this study, two pico-injector designs have been proposed, modelled, simulated and analysed. First of all, the manufacturing process of pico-injector chips, including the electrode positioning and implementation, have been studied in order to produce the two said designs. In addition, an estimation of the voltage signal parameters required to disrupt the electric field has been done. Secondly, the behaviour of the chip has been studied through fluidic modelling. Fluidic systems can be easily modelled by a linear equation system using an electrical analogy, where the flow passing through a channel is directly proportional to the pressure drop divided by the fluidic resistance. However, the dispersed phase (droplets) introduces nonlinear effects into this system, making both the pressure drop and the fluidic resistance vary depending on the number and volume of all droplets inside the channel at a given time. In other words, the flow rates do not only depend on the inlet pressure inputs, but also on the current droplet distribution of the channel. Adding this modification yields a nonlinear two dimensional system, whose solution provides all the system variables to control (flow rates, droplet generation frequency and volume, injected volume) given the three input pressures (water, oil and injection pressures). This system has been used to: Find the equilibrium points that will eventually be reached given a specific pressure input configuration. Generate a discrete computational model implemented by the software Matlab, which simulates transient states between two equilibrium points produced by pressure input step changes. In addition, these equilibrium points have been studied by means of the analysis of a simplified and linearized continuous model, which proves the stability of all equilibria 4 since small disturbances producing alterations in the state of the system will be dimmed and the system will come back to the equilibrium point. Finally, five different simulations involving different input pressure variations have been performed, including a water pressure step response, an oil pressure step response, an injection pressure step response and a combined pressure step response. With the results provided by the simulations, a final dynamic analysis of time constants and overshoots in variables such as flow rates and injected volume has been conducted.
DegreeMÀSTER UNIVERSITARI EN ENGINYERIA INDUSTRIAL (Pla 2014)
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