Acoustic force distribution in resonators for ultrasonic particle separation

Abstract

The effectiveness of particle -liquid separation by ultrasonic radiation forces depends on the acoustic energy density distribution in the standing-wavefield. The energy distribution in an ultrasonic particle-separation device was analyzed to assist continued optimization and design efforts. Measurements of the energy-density distribution in the liquid using a microscope-based imaging system were compared to laser inteferometer measurements of the velocity-amplitude distribution on the transducer and rejlector sur- introductionThe capabilities of ultrasonic radiation forces for manipulating suspended particles have engendered a broad spectrum of experimental investigations. Ultrasonic aggregation has been exploited to improve the sensitivity of medical latex agglutination tests for detecting immunogenic agents (Grundy et al., 1994; Gualano et al., 19951, to fuse different cell types in combination with electric-field pulses (Vienken et al., 19851, and to enhance the recovery of two cell populations by phase partitioning (Allman and Coakley, 1994). The potential for continuous separation of particles from fluid suspensions was demonstrated by Tolt and Feke (1993). Continuous cell separation has been achieved for mammalian cells Correspondence concerning this article should be addressrd to S. M. Woodside. (Doblhoff-Dier et al., 1994; Trampler et al., 19941, yeast (Hawkes and Coakley, 1996), and bacteria (Hawkes et al., 1997). Fractionation according to particle size (Mandralis et al., 1994), continuous fractionation of mixed-particulate suspensions based on the compressibility of the solid phase (Gupta et al., 19951, and the selective retention of generally larger viable mammalian cells over nonviable cells and cell debris (Gaida et al., 1996) have been reported.Particle separation by ultrasonic forces exhibits innate advantages relative to conventional methods, particularly in the area of biotechnology. Cross-flow membrane filtration and spin-filter separators suffer from fouling; continuous centrifuges are susceptible to mechanical failure; conventional sedimentation systems require long holdup times. For ultrasonic separation, no physical barrier is required, there are no 1976September 1998 Vol. 44, No. 9 AIChE Journal moving mechanical parts, holdup times are low, and chemical flocculants are not necessary. Trampler et al. (1994) demonstrated the advantages of ultrasonic separation, running a continuous mammalian-cell perfusion culture for more than one month at cell concentrations up to 6X107/mL, with no decline in separation performance. In continuous separation systems, particles collect at the nodal planes of the standing-wave field due to the axial primary radiation force (PRF). Fluid flow is generally parallel to the nodal planes. The transverse PFR aggregates the particles within the nodal planes and retains them in the field against the flow-induced drag. The retention performance of these devices is therefore particularly dependent on the magnitude of the transverse PRF. The axial and transverse pri...

Kinetics of ultrasonically induced coalescence within oil/water emulsions: Modeling and experimental studies

“…The standing wave-field inside an acoustic chamber can possess non-uniform energy density in the lateral directions due to several reasons. Woodside et al (1997Woodside et al ( , 1998 performed experimental studies to measure the energy density and energy density gradients in different regions of an acoustic resonator of geometry similar to that used in this work. They obtained energy density values by fitting measured particle velocities in the acoustic field to analytical velocity expressions due to axial and lateral acoustic radiation forces.…”

Investigation of two-dimensional acoustic resonant modes in a particle separator

“…This will typically move solid phase particles to the pressure node(s) and form a layer of concentrated particles. However, lateral movement of particles is frequently observed as variations within the acoustic field exist with the particles moving within the nodal plane to areas of high acoustic kinetic energy [1]. The lateral movement of particles can be attributed to lateral variations within the acoustic field which give rise to lateral acoustic radiation forces.…”

Diagnostic particle agglutination using ultrasound: a new technology to rejuvenate old microbiological methods

“…A ®xed, constant ultrasonic standing wave system can be generated in one of two ways. The ®rst involves propagation of wavefronts from a piezoelectric transducer (usually planar) [19] through a medium such as water and re¯ection of the acoustic wave from a suitable re¯ector [19,20] forming a coincident wavefront which interacts with the incident wave. The re¯ector may be a water±air interface or a solid surface [19].…”