Compact Micro-Mixer Using the Switching Electroosmosis Flow

Rapid mixing is essential in the micro-fluidic systems. The purpose of this study is to develop the compact micro-mixer by switching the electroosmosis flow on the simple cross channel. Micro-mixers have been developed in a variety of ways; by the parallel and serial lamination, and the chaotic advection on the passive micro-mixers. On the active micro-mixers, the external disturbance effects such as pressure, electrohydrodynamics and acoustic are applied to enhance the mixing. However, the mixing process occurs at the mixing channel located at the downstream of junction or the mixing chamber, thus the mixing length is required to achieve a homogeneous mixing. In this study, the new concept of the mixing process, a batch processing mixing, is applied to the micro-mixer. The mixing and the transport processes are separated by switching the electroosmosis flow. On the mixing phase, the oscillating reversed flow induces the mixing at the junction of the cross channel. After the mixing process, the liquid mixture flows out through the outlet channel. The mixing is completed at the junction, thus the outlet mixing-channel is not necessary for the batch processing mixing. Experiments are carried out on the PDMS micro cross-channel chip. Channel is 200μm in width and 50μm in depth. The concentration field is measured by the fluorescence technique. The electrical field in the micro channel is generated by the high voltage power supply and is switched by the relay circuit controlled by the D/A converter. The mixing degree reaches about 90% within 0.9mm length of the outlet channel in the case of 500V applied voltage with the frequency of 1.41Hz. On the batch processing mixing, however, the reversed flow occurred at the mixing phase, the flow rate of the liquid mixture becomes small than that in the steady state voltage pattern. To increase the flow rate while keeping the same flow pattern, the electroosmotic velocity and the switching frequency are increased to keep the same Strouhal number. In the case of 900V applied voltage with 2.55Hz, the flow rate becomes 1.8 times keeping the same mixing degree. The batch processing mixing is effective for the compacting the micro-mixer.