Characterization of viscosity dependent residence time distribution in the static micromixer Statmix6

The residence time characteristic of the static micromixer Statmix6 (IPHT Jena) was determined by a pulse trace experiment applying a microphotometric measurement set-up with two flow-through detectors. The residence time distribution (RTD) shows a characteristic shift with viscosity and flow rate. The change in RTD cannot be described by reconsidering the Reynolds number exclusively. A RTD tailing factor Θ99 was introduced for the quantitative description of parameter effects on the RTD. The tailing factor Θ99 is defined as the dimensionless time which is needed for the residence time sum function F(Θ) to reach a value of 99%. The tailing factor comprises only the elimination kinetic of the RTD without respect to the skewness. At lower flow rates, the change of the RTD tailing factor with flow rate is nearly independent on viscosity. In a transition region, the RTD tailing factor is strongly reduced, but the characteristic Re number of transition depends on viscosity. This RTD tailing factor characteristics was compared with the RTD characteristic of a straight PTFE tube (1 mm inner diameter) and a commercial interdigital micromixer (Mikroglas, Mainz) under equal conditions and comparable mean residence times (about 0.5–40 s). In the tube, a reduction of the tailing factor with increasing flow rate was observed too. But, the change of the RTD tailing factor characteristic of the tube differs considerably from the flow rate and viscosity dependence characteristic found in the static micromixer. The experimental results were interpreted by changes in the flow regime under different convection mechanism inside the microdevices. Changes in mechanism exist in case of the complex fluidic geometry of micromixers as well as in case of the simple tubes. But, the characteristic transition regions for the RTD tailing factor are strongly influenced by the structure of device and by the viscosity of the liquid feed.