Mixing principles for microstructured mixers : Active and passive mixing

Microstructured mixers have two major application fields. They are used for mixing operations in the framework of conventional chemical processing as part of ordinary industrial field equipment, i.e. use of multi-scale technology. On the other hand, these tools may be integral part of miniaturised analytical devices like lab-on-a-chip. Mixing is here part of an entire microflow process and orients on a new means of analysis. These two application fields have totally different requirements on their devices: in the first case, capacities range from 1 l/h to several m 3 /h flow for watery systems. In contrast, analytical tests typically demand for consumption of minute sample volumes, which correspond to flows in the μl/h to ml/h range. Especially at such low flow velocities, the often applied pressure-driven or electroosmotic flow mixing, the so-called passive mixing, is not adequate. Here, active mixing principles which rely on external energy are better suited and may be used over periods of several minutes. Examples for both types of mixing principles are given below.