Acoustic Particle Concentration

FIGURE 44.4 Schematic view of an acoustic particle concentrator according to the Lund method, enabling online visual quahty control of the particles focusing, (a) Cross-section of the Lund method acoustic focusing chip, (b) Schematic top view of the Lund method acoustic focusing chip. 

Acoustic droplet ejection Acoustic levitation of droplets Acoustic particle concentration Drop on demand Droplet manipulation Droplet transport by surface acoustic waves Radiation pressure Surface acoustic waves... 

Doppler Flow Meters. Doppler flow meters sense the shift in apparent frequency of an ultrasonic beam as it is reflected from air bubbles or other acoustically reflective particles that ate moving in a Hquid flow. It is essential for operation that at least some particles ate present, but the concentration can be low and the particles as small as ca 40 p.m. CaUbration tends to be influenced by particle concentration because higher concentrations result in mote reflections taking place neat the wall, in the low velocity portion of the flow profile. One method used to minimize this effect is to have separate transmitting and receiving transducers focused to receive reflections from an intercept zone neat the center of the pipe. 

We are continuing our research with U.S. Department of Energy support to further improve our understanding of several important aspects of acoustic agglomeration such as acoustic energy absorption by hot gases and large particle concentrations,... 

In principle, the ultrasonic techniques described for solid-liquid flow measurement can be applied to measure air flow rate and particle velocity. Direct measurement of air flow rate by measuring upstream and downstream transit times has been demonstrated. But, the Doppler and cross-correlation techniques have never been applied to solid/gas flow because the attenuation of ultrasound in the air is high. Recent developments have shown that high-frequency (0.5-MHz) air-coupled transducers can be built and 0.5-MI Iz ultrasound can be transmitted through air for a distance of at least 1 in. Thus, the cross-correlation technique should be applicable to monitoring of solid/gas flow. Here, we present a new cross-correlation technique that does not require transmission of ultrasonic waves through the solid/gas flow. The new technique detects chiefly the noise that interacts with the acoustic field established within the pipe wall. Because noise may be related to particle concentration, as we discussed earlier, the noise-modulated sound field in the pipe wall may contain flow information that is related to the variation in particle concentration. Therefore, crosscorrelation of the noise modulation may yield a velocity-dependent correlation function. 

Shilton R, Tan MK, Yeo LY, Friend JR (2008) Particle concentration and mixing in microdrops driven by focused surface acoustic waves. J Appl Phys 104 014910... 

One immediate effect of increasing the particle concentration in the emulsion is that the acoustic impedance, Zg, can no longer be approximated as equal to that of the dispersion medium. Since Eq. (1) remains valid at all concentrations commonly encountered, it is important that the correct value of Zg is used, so that the correct value of the dynamic mobility is obtained from the measured ESA signal. In principle, the value of Zg for the emulsion could be a complex function of the frequency and the properties of the suspension, but the exact behavior is of little consequence for measurements with the AcoustoSizer, since it measures the value at each frequency before calculating from ESA signal. 

The Influence of Particles on the Field Resonant chambers for particle manipulation (including microfluidic chambers) are designed to provide specific acoustic characteristics. However, the chambers must work when filled with a fluid-particle mix which will have acoustic characteristics that differ from the particle-free fluid. Many applications will need to work over a range of particle concentrations that will modify the acoustic field. In cases where the particles are very different acoustically from the fluid medium, agglomerations can cause localized breakdown of... 

In collaboration with the Virology Laboratory of the Institut de Biologie Moleculaire et Cellulaire in Strasbourg, the Laboratory of Molecular Acoustics has studied the ultrasonic absorption of two small icosahedral viruses, bromegrass mosaic virus (BMV) and tomato bushy stunt virus (TBSV). Measurements carried out between 0.6 and 40 MHz showed that ultrasonic waves in the MHz range are absorbed much more by capsids than by dissociated protein. Examples of ultrasonic spectra of solutions of BMV protein capsids and of subunit-dimers are shown in Fig. 1 (at 5 mg/ml in 10 mM Na cacodylate, 1.00 M NaCl, at 23°C capsids , pH=4.80 dimers o, pH = 6.6o). Spectra obtained for BMV (A, pH = 5 00, at the same particle concentration as for the BMV capsids, otherwise similar conditions) and for the pure solvent ( — ) are also shown. 

Goddard G, Kaduchak G (2005) Ultrasonic particle concentration in a line-driven cyUndrical tube. J Acoust Soc Am... 

Response to Electric and Acoustic Fields. If the stabilization of a suspension is primarily due to electrostatic repulsion, measurement of the zeta potential, can detect whether there is adequate electrostatic repulsion to overcome polarizabiUty attraction. A common guideline is that the dispersion should be stable if > 30 mV. In electrophoresis the appHed electric field is held constant and particle velocity is monitored using a microscope and video camera. In the electrosonic ampHtude technique the electric field is pulsed, and the sudden motion of the charged particles relative to their counterion atmospheres generates an acoustic pulse which can be related to the charge on the particles and the concentration of ions in solution (18). 

A semi-industrial pilot plant has been developed in which air-borne ultrasound has been applied to the reduction of particle emissions in coal combustion fumes [62]. The installation basically consists of an acoustic agglomeration chamber with a rectangular cross-section, driven by four high-power and highly directional acoustic transducers operating at 10 and/or 20 kHz, and an electrostatic precipitator (ESP). In the experiments, a fluidised bed coal combustor was used as fume generator with fume flow rates up to about 2000 m /h, gas temperatures of about 150 °C. and mass concentrations in the range 1-5 gm. The acoustic filter reduced fine particle emissions by about 40 %. 

The acoustic chemometric approach can also be used to monitor industrial production processes involving particles and powders and to provide a complementary tool for process operators for more efficient process control, or to monitor particle movement in a fluidized bed [7] for example. Below we illustrate the application potential by focusing on two applications process monitoring of a granulation process and monitoring of ammonia concentration. 

On-line particle sizing by ultrasonic (acoustic attenuation) spectroscopy was developed for use during batch crystallization processes.14 Crystallization of the alpha polymorph of (l) -glutamic acid from aqueous solution was monitored by continuously pumping the crystallizing solution through an on-line ultrasonic spectrometer. The method enabled measurement of the crystal size distribution and solid concentration throughout the... 

Alba, F. Crawley, G.M. Fatkin, J. Higgs, D.M.J. Kippax, P.G., Acoustic spectroscopy as a technique for the particle sizing of high concentration colloids, emulsions and suspensions Colloids and Surfaces A Physiochem. Eng. Asp. 1999, 15, 495-502. 

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