(102c) Optical Characterization of Viscosity-Dependent Residence Time Distribution in the Static Micromixer Statmix6

The investigations address the influence of viscosity and flow rate on the residence time behaviour of a split and recombine mixer. Therefore, the flow of water and water/glycerol mixtures (up to 50 weight percent) were studied in the flow rate range between 50 and 3000 ìL/min. The residence time behaviour of the static chip micro mixer Statmix6 (IPHT Jena) was characterized by a pulse tracer experiment applying a micro photometric measurement with two flow-through detectors. RTD functions for all flow rates and viscosities were calculated using two different models. The mean hydrodynamic residence time was in the range of 0.75 to 50 s. An aqueous solution of Cochenille red A (E 124, absorbance 2.5 at an optical path length of 1 mm) was used for tracing. The frequency of photometric measurements (illumination at a wave length of 505 nm) was adapted to the flow rates up to a maximum value of 10 kHz.

The residence time behaviour shows a characteristic shift with viscosity and flow rate. The change in RTD function can not be explained by reconsidering the Reynolds number exclusively. At lower flow rates, the shift of tailing with flow rate is nearly independent on viscosity. In a transition region, the tailing is reduced strongly, but the characteristic flow rate of transition is a function of viscosity.

The RTD behaviour of the micro mixer was compared with the behaviour of a PTFE tube (1 mm inner diameter) under same experimental conditions and with similar mean residence times (about 0.5 to 40 s). In the tube, a reduction of tailing with increasing flow rate was observed, too. But, the change of RTD function with flow rate and their viscosity-dependence differs considerably of the functions found in the static micro mixer.

The experimental results can be interpreted by changes in the structure of local convection mechanism inside the micro device and in the tube. Such changes in mechanism seem to exist in case of the complex geometry of micro mixer as well as in case of the simple tube. But the characteristic transitions in the shape of RTD-function are strongly influenced by the structure of device and by the viscosity of the liquid.