Development on a Non-Contact Mixing Device for Micro-Liquids
Authors: Katou Hajime , Miyake Ryou 
Published: February 2009

We have developed a new device that can mix a micro-liquid without contact (i.e., one without a paddle or a screw). Essentially, non-contact mixing does not cause any cross-contamination or carryover, therefore it should be applied to a chemical analyzer, where high accuracy is needed. In the field of chemical analysis, especially for medical diagnostics using blood, decreasing the volume of samples and reagents is very important. Chemical analysis at low sample and reagent volumes will bring several merits: 1) Low sample volume will reduce indisposition in patients. 2) Low sample volume will allow analysis in babies or infants, from whom large samples can’t be collected the supply of. 3) Low reagent volume will reduce the cost of testing. 4) Low reagent volume will reduce exhausting liquids after tests. In our laboratory, we have found that a liquid in a vessel can flow when a proper wave on a free surface is generated. Using this phenomenon, we developed a non-contact mixing device for micro-liquids. To generate a wave on a free surface, we used an ultrasound. The free surface is pushed out when the ultrasound propagating in the liquid reaches the free surface. This effect is due to the radiation pressure caused by an ultrasound. Our developed mixing device consists of only two mechanical components: a vessel and a sound source. The vessel used in our demonstration was rectangular. A cross section of the vessel was 3.8 × 5.6 mm, with a depth of 20 mm and walls 0.6 mm thick. Thus, this vessel can be filled with about 400 μ L of liquid. Actually, because a portion is needed to hold the vessel, we used less than 12 mm of the depth (250 μ L liquid). The frequency of the ultrasound we used was 1.6 MHz, and the sound source for emitting the ultrasound was made of PZT. To obtain its effective power, the PZT thickness resonance was used. Therefore, we made the PZT plate 1.1 mm thick. The sound source was arranged outside the vessel, and it emitted ultrasound toward the free surface in the vessel. Emitted ultrasound permeates through the wall of a vessel and reaches the free surface of a liquid. When it is pulsatile, the ultrasound reaching the free surface generates a wave. In the liquid under the wavy free surface, a circulating flow occurs. The intensity of the flow depends on the amplitude and frequency of the surface. From our theoretical and experimental study, we found that the best pulsating frequency was 20 Hz for our vessel. We measured the velocity of the circulating flow under this condition by using PIV. The results were that a maximum velocity of 300 mm/sec was observed. In the next step, we applied our device to mixing a real sample and reagent. A serum of a horse was used as the sample. In general, there is a difference in refractive index between the sample and reagent. By using the Schlieren visualization method, we observed the mixing process between the sample and reagent, and evaluated the mixing time needed for them to be fully homogeneous. Our results demonstrated that 250 μ L of liquid can be mixed within 1.8 sec.