The phenomenon of hydrodynamic focusing in a flow cytometer is investigated using a computational approach. In this work, a three-dimensional two-fluid theoretical model was established to describe the flow transport behaviour and the interaction of two fluids coflowing at different velocities. Treating both sample and sheath fluids as laminar, incompressible, and isothermal flows, the analysis encompasses two sets of three-dimensional unsteady equations for conservation of mass and momentum, with consideration of interfacial momentum exchange. Governing equations are solved numerically through an iterative semi-implicit method for pressure-linked equations consistent algorithm to determine the flow variables. For code validation, both focused width and length in the two-dimensional configuration are predicted at a broad range of u{sub}(sh)/u{sub}s ratios and are compared with Lee et al.'s measured data. Subsequently, the work extends to examine the three-dimensional hydrodynamic focusing process and the time required for completion of one focusing event. To explore the feasibility of the proposed flow cytometer in applications, the focused properties are determined by varying the ratio of sheath velocity to sample velocity from 10 to 80. Ten numerical experiments were also conducted to examine the effects of the fluid properties on the length and width of the focused sample stream.
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