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Article Summary
The article by Xu Zhu and Eun Sok Kim is about the generation of microfluidic motion using acoustic waves, from piezoelectric zinc oxide (ZnO) film. Majorly, the authors use the principle that a self-focusing acoustic transducer can form constructive wave interference in a liquid like an optical Fresnel lens, so they enable only some areas on the ZnO to generate the acoustic waves. The other areas with the potential of producing acoustic waves are the half-wave-band which the authors establish that on their own they can create acoustic waves with peaks proportional to their numbers. They also show that upon the generation of Radio Frequency (r.f) signals on the transducer, acoustic waves can be created via a piezoelectric action on the ZnO film whereby the effect is seen by the way a liquid is propagated in an air-liquid boundary without the involvement of any heat; making the setup a perfect utility in the transportation of temperature sensitive fluids. Similarly, when a fabricated traducer on a silicon wafer is used, there is usually a rotation of fluorescent microspheres in the set liquid with the capacity of moving up to 2 microliters of a liquid.
However, the authors emphasize that to achieve microfluidic motion; it is important to consider the thickness of ZnO. Ideally, it should be10mm since it should be an odd number of a multiple of the wavelength when halved (2n+ l) λ /2). The ZnO film is also supposed to be loosely focused on an air-liquid interface to avoid the discharge of the liquid due to pressure.

In addition, electrodes in the transducer should be set in such a way that the acoustic waves can interfere in a constructive manner in order to have a focal point (f) at some distance above the boundary of air and liquid infinite delays with the radius (r n) satisfying the function in which λw is the liquid’s acoustic wavelength and n is 1,3,5,7…
In conclusion, it is clear from the article that by loosely focusing acoustic waves on a piezoelectric film of ZnO, microfluidic motion can be achieved without any noticeable increase of temperatures. Therefore, the mechanism developed by the authors can be applied in systems which have to mix or transport temperature sensitive fluids.
Work Cited
Zhu, Xu, and Eun Sok Kim. “Microfluidic motion generation with acoustic waves.” Sensors and Actuators A: Physical 66.1-3 (1998): 355-360.