In one embodiment as described in this section, an apparatus for mixing of microfluidic streams on a chip is presented, which comprises a micro-channel and a plurality of magnetic valves on the chip. A guiding magnet produces a proximal magnetic field gradient to exert a force on a bead in a cavity when placed at in a vicinity of the chip. The bead-cavity combination form a magnetic valve. In one embodiment, the mouth of the cavity is tapered so to prevent the magnetic bead from completely blocking the corresponding micro-channel section to enhance the mixing of microfluidic streams at the narrowed fluid path. In one embodiment, magnetically actuated valves direct the flow in a microfluidic system in one of several flow paths wherein the mixing characteristics of the paths are different.