Pumping with acoustic waves

Figure 3.51 Pumping with flexural-wave device. Data velocity measured from video recorded motion of polystyrene spheres vs amplitude of wave motion. Solid line theoretical values based on acoustic streaming theory. (Reprimed with permission. See Ref. [74). 1989...

Figure 10.9. Continuous system used by Su et al. to implement various anaiyticai methods inciuding a fiow-injection manifoid, a gas-diffusion unit and a buik acoustic impedance sensor. A acceptor, BAWIS buik acoustic wave impedance sensor, C — carrier, GDC — gas diffusion ceii, iV — injection vaive, PP — peristaitic pump, R — reagent, RC — reaction coii, SL — sampie ioop, 1/1/— waste, WB — water bath. (Reproduced with permission of Eisevier, Ref. [98].)...

Z. Guttenberg, H. Muller, H. Habermuller, A. Geisbauer, J. Pipper, J. Felbel, M. Kielpinski, J. Scriba, and A. Wixforth, Planar chip device for PCR and hybridization with surface acoustic wave pump. Lab on A Chip, vol. 5, no. 3, pp. 308-317, 2005. 

An example of one of TSA/TSL s R D funded MEMS based project is the Sandia National Laboratories (SNL) MicroHound project. This is based on the SNL Micro Chem Lab on a Chip , illustrated in Figure 1. The original prototype system from SNL was developed for high vapour pressure, chemical weapons (CW) detection, which utilized a MEMS GC separator, with miniature surface acoustic wave (SAW s) based sensors. The system included an inlet, coated pre-concentrators, detectors, and pumps. To make this useful for trace explosives detection, the addition of an alternate front-end sample collection/macro-preconcentrator and MEMS based coated-preconcentrator is necessary, along with the option to utilize or exclude the MEMS GC separator followed by detection by either, or both, SAW s and miniaturized IMS detectors. 

An ultrasonic transducer is an integrated component of acoustic pumps. The transducers that generate ultrasonic energy with megahertz frequency for ultrasonic pumps make use of the piezoelectric effect. A piezoelectric layer is the vital component of the ultrasonic transducer and provides the oscillatory motion that ultimately produces the surface acoustic waves (a good review on IDT and SAW can be found elsewhere [3]). 

Acoustic waves can potentially damage biological samples (e.g., cause cell lysis). Due to this reason, careful control of the SAW frequency and device optimization are necessary. Understanding the complicated mechanisms governing the fluid-structure interactions will help in the optimization of ultrasonic pumps dealing with biological samples. 

By the interaction of sufficiently energetic acoustic waves of suitable frequency with the surface of a liquid, vortices are formed which produce an aerosol. The diameter of the aerosol droplets depends on the frequency and the physical properties of the liquid. For water, aerosol droplets formed at a frequency of 1 MHz have a diameter of ca. 4 pm. The energy can be focused with the aid of a liquid lens on the surface of the sample solution, or the sample liquid can be pumped continuously over the transducer, which must be cooled efficiently (Fig. 22). 

An approximate analysis of FPW fluid pumping [74,75,78] indicates that acoustic streaming accounts well for the observed pumping speeds observed in the experiments with water and the marker spheres. The transport of granular particles is believed to result from pumping of the ambient air. In addition, the FPW wave intensity and relatively low frequency favor the production of sono-chemical effects [79]. 

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