Computational Analysis Of Magnetic Droplet Generation And Manipulation In Microfluidic Devices
Amiri Roodan, Venoos
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Droplet based microfluidics involves the generation and manipulation of discrete volume of fluid (droplets or dispersed phase) in an immiscible (continuous) phase. The interest in this technology had grown dramatically as it holds great potential to provide innovative solutions for numerous applications that range from fast analytical systems of the synthesis of advanced materials to biological assays for living cells. However, to realize the full potential of this technology, precise control of the droplet volumes and reliable manipulation of individual droplets including coalescence, mixing and sorting etc. are needed. In this study, we discuss magnetic control of ferromagnetic liquid droplets. Magnetic separation has proven a useful and elegant method for manipulating magnetic particles or magnetically labelled biomaterials in microfluidic devices. Magnetic fluids known as ferrofluids are colloidal suspensions in which magnetic nanoparticles (MNP) are dispersed in a carrier fluid. Magnetic forces, provided by an external field source are commonly used as dominant controlling factors to manipulate the behavior of these fluids in laminar flow streams.In this work, we present a computational fluid dynamic (CFD) model to investigate the dynamics of oil-based ferrofluid droplets within an aqueous continuous phase under an external inhomogeneous magnetic field. CFD modelling is performed using the volume-of-fluid (VOF) method as implemented in the commercial program FLOW-3D (www.flow3d.com). The flow solver was linked to a custom FORTRAN subroutine that analytically calculates the magnetic field due to a rare-earth permanent magnet field source and the corresponding magnetization using a Langevin function as well as the magnetic force exerted on the droplets. For droplet generation, a flow focusing junction design is studied. The effect of flow rates and chip dimensions on the droplet size was first investigated to determine the optimum conditions. The magnetically driven separation of the generated droplets was studied at a T-junction outlet. Parameters such as magnet size and location were optimized in order to achieve adequate separation efficiencies.Our results show that the effect of these factors can be tuned and optimized through various chip designs, depth of the channel, and hydrodynamics of the both oil and aqueous phases. The developed model includes the integration of the magnetic analysis with computational fluid dynamic (CFD)-based models that accurately describe the droplet generation and motion under different magnetic field and flow conditions. Overall, the model enables better understanding of physical phenomena involved in the continuous droplet processing and serves as an efficient parametric analysis and optimization platform. Furthermore, it is useful for the rational design of magnetically functional microfluidic chip systems.