Liquid-liquid fluorescent waveguides using microfluidic-drifting-induced hydrodynamic focusing

We demonstrate fluorescent liquid-core/liquid-cladding (L (2)) waveguides focused in three-dimensional (3-D) space based on a 3-D hydrodynamic focusing technique. In the proposed system, the core and vertical cladding streams are passed through a curved 90A degrees corner in a microfluidic channel, leading to the formation of a pair of counter rotating vortices known as the Dean vortex. As a result, the core fluid is completely confined within the cladding fluid and does not touch the top and bottom poly(dimethylsiloxane) (PDMS) surfaces of the microfluidic channel. Because the core stream was not in contact with the PDMS channel, whose refractive index contrast and optical smoothness with the core fluid are lower than that between the core and the cladding fluids, the 3-D focused L (2) waveguide exhibited a higher captured fraction (eta) and lower propagation loss when compared to conventional two-dimensional (2-D) focused L (2) waveguides. Because the proposed 3-D focused L (2) waveguides can be generated in planar PDMS microfluidic devices, such optofluidic waveguides can be integrated with precise alignment together with other in-plane microfluidic and optical components to achieve micro-total analysis systems (mu-TAS).