Qin, Mian
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Publication Microfluidic Pumping With Surface Tension Force and Magnetohydrodynamic Drive(2011-08-12) Qin, MianMicropumping is difficult to design and control as compared to their macro-scale counterparts due to the size limitation. The first part of this dissertation focuses on micropumping with surface tension forces. A simple, single-action, capillary pump/valve consisting of a bi-phase slug confined in a non-uniform conduit is described. At low temperatures, the slug is solid and seals the conduit. Once heated above its melting temperature, the liquid slug moves spontaneously along a predetermined path due to surface tension forces imbalance. This technique can be easily combined with other propulsion mechanisms such as pressure and magnetohydrodynamics (MHD). The second part of this dissertation focuses on MHD micropumping, which provides a convenient, programmable means for propelling liquids and controlling fluid flow without a need for mechanical pumps and valves. Firstly, we examined the response of a model one dimensional electrochemical thin film to time-independent and time-dependent applied polarizations, using the Nernst-Planck (NP) model with electroneutrality and the Poisson-Nernst-Planck (PNP) model without electro -neutrality, respectively. The NP model with well designed boundary conditions was v developed, proved capable of describing the bulk behavior as accurate as the full PNP model. Secondly, we studied the MHD propelled liquid motion in a uniform conduit patterned with cylinders. We proved equivalence in MHD and pressure driven flow patterns under certain conditions. We examined the effect of interior obstacles on the electric current flow in the conduit and showed the existence of particular pillar geometry that maximizes the current. Thirdly, we looked at MHD flow of a binary electrolyte between concentric cylinders. The base flow was similar to the pressure driven flow in the same setup. The first order perturbation fields, however, behave differently as the traditional Dean’s flow. We carried out one-dimensional linear stability analysis for the unbounded small gap situation and solved it as an eigenvalue problem. Two-dimensional nonlinear simulation was performed for finite gap size or bounded situations. We observed strong directionality of the applied electric field for the onset of stability. Results in this study could help enhance the stability of the system or introduce secondary motion depending on the nature of the applications.Publication When MHD-Based Microfluidics is Equivalent to Pressure-Driven Flow(2011-01-01) Qin, Mian; Bau, Haim HMagnetohydrodynamics (MHD) provides a convenient, programmable means for propelling liquids and controlling fluid flow in microfluidic devices without a need for mechanical pumps and valves. When the magnetic field is uniform and the electric field in the electrolyte solution is confined to a plane that is perpendicular to the direction of the magnetic field, the Lorentz body force is irrotational and one can define a “Lorentz” potential. Since the MHD-induced flow field under these circumstances is identical to that of pressure-driven flow, one can utilize the large available body of knowledge about pressure-driven flows to predict MHD flows and infer MHD flow patterns. In this note, we prove the equivalence between MHD flows and pressure-driven flows under certain conditions other than flow in straight conduits with rectangular cross-sections. We determine the velocity profile and the efficiency of MHD pumps, accounting for current transport in the electrolyte solutions. Then, we demonstrate how data available for pressure driven flow can be utilized to study various MHD flows, in particular, in a conduit patterned with pillars such as may be useful for liquid chromatography and chemical reactors. Additionally, we examine the effect of interior obstacles on the electric current flow in the conduit and show the existence of a particular pillar geometry that maximizes the current.