Given the fact that most of the reactions that take place in microreactors occur on the surface, it is crucial to keep the reactants close to the reactive wall. One of the effective techniques that have been proposed in this respect is the single-phase hydrodynamic focusing. This method, however, has the main drawback that the percentage of reactants penetrating the sheath fluid is high. To circumvent this defect, the notion of two-phase hydrodynamic focusing is introduced in the present work. The whole idea is to use a highly viscous sheath fluid to establish a barrier against the penetration of reactants into the sheath flow. To show the effectiveness of the proposed technique, an analytical modeling is performed by considering a parallel-plate microreactor with an irreversible first-order reaction. The sample and sheath fluids are assumed to flow side-by-side and the hydrodynamic entry length is neglected. The results indicate the effectiveness of the two-phase focusing: efficiency, defined to be the number of reactants located in the sample flow divided by the whole reactants, may be kept close to unity and the fully-developed Sherwood number may be increased by one order of magnitude assuming a sheath-to-sample viscosity ratio of 10 under certain conditions.
