Two neighboring sheath flows are used to constrain the central sample flow laterally (horizontally) within the center of the microchannel. Hydrodynamic forces provide a convenient means of adjusting the focused (center) stream width by regulating the relative volumetric flow rates of the inlet streams. Typically, the hydrodynamic flow focusing techniques employed in the applications above can be classified as either: (i) multi-phase flow focusing (i.e. liquid-liquid or liquid-gas) [1] or (ii) liquid single-phase flow focusing [2]. Recently, hydrodynamic focusing have been successfully demonstrated in a wide variety of microfluidic applications, including in micro-flow cytometers for cells/particles counting and sorting [3], rapid diffusion-based micro-mixers for the kinetic studies of protein folding, microfluidic optical waveguides [4] and fluorescent light sources, micro-droplet and bubble generators [1], micro- and nano-particles productions and cell encapsulations etc.