(129b) Mixing Performance of T-Shaped and Zigzag-Shaped Microreactors

Micro fluidic devices are successfully in use for several applications in chemical engineering and biotechnology. Nevertheless, there is still no breakthrough for micro process engineering because of a lack in understanding the mechanisms for local hydrodynamics and mass transfer on micro scales. Micro Particle Image Velocimetry (µ-PIV) in conjunction with Confocal Laser Scanning Microscopy (CLSM) enables the measurement of three-dimensional flow and concentration fields in micro devices for common stationary cases.

The visualization of the concentration field is carried out for the mixing of a passive tracer (Rhodamine B) as well as the reactive mixing (Calcium ions with an appropriate fluorescence tracer, i.e. "Fluo-4"). By using a confocal laser scanning microscopy (CLSM) it is possible to record image slices of a few microns thickness and a confocal pinholes removes out-of-focus emitted light and thus improves the lateral and axial spatial resolution. By taking image slices at different depths and assembling these slices together, a three-dimensional concentration field inside the mixing channel can be rendered. With the capability to visualize the tracer distribution at different cross sections a quantitative analysis of the mixing performance of different micromixer, i.e. T-shaped micromixers (typical dimensions: mixing channel width 400 micron, height 200 micron) and zigzag-shaped micromixers (typical dimensions: mixing channel width and height: 200 micron) is possible. The mixing quality can then be quantified using Danckwerts' intensity of segregation and the intensity of mixing respectively. Since these measures are insensitive to the length scales on which segregation occurs, the potential for diffusive mixing is used. This potential is a measure for the driving force for diffusive fluxes (the reciprocal value is a measure for the scale of segregation). The experimental analysis of the velocity field (2 dimensions + 2 components), i.e. the velocity components u and v, on a number of planes, e.g. 50 planes for a microchannel with a height of 200 micron) are basis for the calculation of the out-of-plane velocity w by using the continuity equation for incompressible fluids. With the knowledge of all three velocity components the 3D streamlines can be visualized. This allows a detailed characterisation of mixing performance for T-shaped and zigzag-shaped micromixers.