Using the method of separation variables to solve the governing equation of this flow system, the focused stream width or the position of two-fluid interface can be expressed as: W=w2/w=fun.(α,γ,ε), where α is the side stream to center stream flow rate ratio, γ is the viscosity ratio of the two fluids, and ε is the aspect ratio of the microchannels. In the limiting case, i.e. ε