Acoustic trapping

H.M. Hertz, Standing-wave acoustic trap for nonintrusive positioning of microparticles, Journal of Applied Physics, 78(8), 4845 849 (1995). 

The intention of the device presented by Lilliehorn et al. was to utilize the miniaturized transducers in combination with the laminar flow conditions in microfluidic systems to create a dynamic arraying device holding several individually controlled transducers as envisioned in Figure 44.21. Here, an array of 16 transducers are loaded with different functionalized beads in a first step. The beads are acoustically trapped in the positions given by the transducers. In a second step, samples are supplied through the orthogonal channels and the response is, for example, read by fluorescence. The transducers are then deactivated and the beads are flushed out of the device and the whole procedure is repeated again for a new set of beads and samples. 

An interesting application of the acoustic trap is to use it for enrichment of particles/cells from a dilute sample. The origin may be a rare event experiment or a sample where everything but... 

FIGURE 44.25 Principle of cell enrichment using the acoustic trapping, (a) The tranducer is activated, (b) the sample is supplied, (c) the particles/cells in the sample liquid are trapped at the transducer, and (d) the switching flow is switched and the transducer is deactivated. The trapped particles/cells are released to the exit down left. 

Evander M, Nilsson J (2012) Acoustofluidics 20 applications in acoustic trapping. Lab Chip 12 4667 676... 

The major piezoelectric applications are sensors (pickups, keyboards, microphones, etc.), electromechanical transducers (actuators, vibrators, etc ), signal devices, and surface acoustic wave devices (resonators, traps, filters, etc ). Typical materials are ZnO, AIN, PbTiOg, LiTaOg, and Pb(Zr.Ti)03 (PZT). 

In early type stars, the bottom boundary penetrates into convective core (Osaki 1975). Accordingly, convective motion of eddies excites sound waves, as in the case of acoustic noise emmision from incompressible turbulence, shown by Lighthill (1978). Since the frequency of excited waves is higher than the Brunt-Vaisala frequency at the photosphere, the waves are not trapped, but running outward (cf. Unno et al 1979). 

Retrieval was accomplished with an acoustic release (Helle Engineering) located between the bottom trap and a concrete anchor. All trap surfaces... 

Fig. 6.34 (a) Trapped-energy filter, (b) Surface acoustic wave (SAW) filter. 

Collection of Settling Particles in Sediment Traps. Sediment traps (Plexiglas tubes with a height-to-diameter ratio of 10 1) were exposed in the deepest part of the lake at 15- and 28-m depths two identical tubes were exposed at each depth (23). The line carrying the traps was moored by using an acoustic release device at the bottom (24). The particulate material in the traps was collected approximately every 3 weeks for 15 months and was subsequently freeze-dried until analysis. Sedimentation rates were quantified by weighing the dry material. The material from both tubes at the same depth was mixed for analysis after weight determination. 

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