Simple quantitative optical method for monitoring the extent of mixing applied to a novel microfluidic mixer

A novel scheme that allows the quantification of the extent of diffusive mixing between two fluids is introduced. The method, which relies upon an increase in the fluorescence intensity of fluorescein at basic pH, allows quantification of the extent of interdiffusion without requiring a full three dimensional mapping of the concentration of the dye. This allows for the characterization of mixers that could otherwise only be characterized by confocal microscopy, such as microfluidic mixing devices. The detection scheme is used to characterize a novel and highly efficient microfluidic mixer that is based on the principle of creating small fluid striations, and has been fabricated with polymeric laminate films. In this and many other mixers, the fluidic interfaces are oriented orthogonal to the optical axis, so that spatial variations in the intensity that change with the extent of mixing cannot be imaged. A 1.8 and 3.0-fold enhancement in the rate of mixing over that in a straight channel was seen after the first and second mixing units, respectively. This mixing detection scheme allows easy and inexpensive comparison of different types of mixers over a wide range of conditions.