Micro-flow analysis by molecular tagging velocimetry and planar Raman-scattering

: Figure 1: Result of a tagging mesurement, averaged data

In this project, a modified “Molecular Tagging Velocimetry” (MTV) with an examination by the “method of the optical flow” and planar spontaneous Raman-scattering are utilized to analyze microfluidic flows. For the determination of velocity vector fields of the flow, it was tagged by structured illumination of a fluorescence dye; the pattern written this way was detected time resolved by a camera. The tagging is possible by a photo chemical change in the dye molecule. Initially the fluorescence ability is deactivated by an additional functional group in the molecule, thus this dyes are called “caged dyes”. The bond can be broken by exposure of UV light so that the original unaltered dye is present again. The UV excitation is done spatially structured by imaging a mask with a XeF-excimer laser; a well defined pattern within the dye loaded flow is obtained. The dye can now be excited with an Ar(+)-laser to fluoresce and the spatial fluorescence pattern can be read time resolved by a camera.

The image series grabbed this way were evaluated by a special algorithm in regard to the flow velocities. Given that the dye diffuses in the fluid and for this reason the written pattern are washed-out in a time series classical correlation algorithms as used for PIV are only suitable to a limited extent. Instead a specially adapted variant of the method of the optical flow is used.

: Figure 2: Raman image of a mixture of ethanol and water. Ethanol flows from the bottom from several inflows into water. The Raman filter was optimized for the detection of Ethanol.

For the optimization of microfluidic mixers and reactors, it is very important to know the chronological sequence of the mixture generation or reaction. The determination of concentration fields in micro mixers uses the fact that different species of molecules are clearly distinguishable from another by means of their characteristic Raman spectra. In this “spectral fingerprint” it is normally possible to find identifying features for the differentiation in the form of particular Raman bands which are characteristic for a single species. Narrow band filters allow to spectrally separate the Raman stray light of the relevant band and detect it unaffected from the Raman stray light of other species. The local stray light intensities obtained this way are a direct measure for the density distributions of the examinated species.

The combination of these measurement principles provides important information for the optimization of flow phenomena in microfluidic systems.

Sponsorship:

Deutschen Forschungsgemeinschaft im Rahmen des Schwerpunktprogrammes 1147 "Bildgebende Messverfahren zur Strömungsanalyse"

Co-operation partners:

Universität Heidelberg, Prof. Dr. K. Jähne
Universität Münster, Prof. Dr. C. Denz
Institut für Mikroverfahrenstechnik des Forschungszentrums Karlsruhe

Further information:

Roetmann K, Garbe CS, Beushausen V, 2005, „2D-Molecular Tagging Velocimetry zur Analyse Mikrofluidischer Strömungen“, Tagungsband Lasermethoden in der Strömungsmesstechnik, 26/1 – 26/10, ISBN 3-9805613-2-1

Roetmann K, Schmunk W, Garbe CS, Beushausen V, 2006, „Analyse Mikrofluidischer Strömungen mit Molecular Tagging Velocimetry und Planarer Ramanstreuung“, Tagungsband Lasermethoden in der Strömungsmesstechnik, 31/1 – 31/8, ISBN 3-9805613-3-X

Roetmann K, Garbe CS, Schmunk W, Beushausen V, 2006, “Micro-Flow Analysis by Molecular Tagging Velocimetry and Planar Raman-Scattering”, Proc. of 12th International Symposium on Flow Visualization

Roetmann K, Schmunk W, Garbe CS, Beushausen V, 2007, “Micro-Flow Analysis by Molecular Tagging Velocimetry and Planar Raman-Scattering”, Experiments in Fluids, DOI: 10.1007/s00348-007-0420-1

Garbe CS, Roetmann K, Beushausen V, Jähne B, 2007, “An optical flow MTV based technique for measuring microfluidic flow in the presence of diffusion and Taylor dispersion”, Experiments in Fluids, DOI: 10.1007/s00348-007-0435-7