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

Abstract

Within an acoustic standing wave particles experience acoustic radiation forces, a phenomenon which is exploited in particle or cell manipulation devices. When developing such devices, one-dimensional acoustic characteristics corresponding to the transducer(s) are typically of most importance and determine the primary radiation forces acting on the particles. However, radiation forces have also been observed to act in the lateral direction, perpendicular to the primary radiation force, forming striated patterns. These lateral forces are due to lateral variations in the acoustic field influenced by the geometry and materials used in the resonator. The ability to control them would present an advantage where their effect is either detrimental or beneficial to the particle manipulation process.The two-dimensional characteristics of an ultrasonic separator device have been modelled within a finite element analysis (FEA) package. The fluid chamber of the device, within which the standing wave is produced, has a width to height ratio of approximately 30:1 and it is across the height that a half-wavelength standing wave is produced to control particle movement. Two-dimensional modal analyses have calculated resonant frequencies which agree well with both the one-dimensional modelling of the device and experimentally measured frequencies. However, these two-dimensional analyses also reveal that these modes exhibit distinctive periodic variations in the acoustic pressure field across the width of the fluid chamber. Such variations lead to lateral radiation forces forming particle bands (striations) and are indicative of enclosure modes.The striation spacings predicted by the FEA simulations for several modes compare well with those measured experimentally for the ultrasonic particle separator device. It is also shown that device geometry and materials control enclosure modes and therefore the strength and characteristics of lateral radiation forces, suggesting the potential use of FEA in designing for the control of enclosure modes in similar particle manipulator devices.

Modelling for the robust design of layered resonators for ultrasonic particle manipulation

“…While 1D models are able to provide very useful, and in the case of nodal position accurate, predictions of device behaviour, the influences of lateral field variations are also of significance [26]. These variations are the main limitation on the performance of the concentrator described here [24].…”

Multi-modal particle manipulator to enhance bead-based bioassays

“…Hagsater et al [7] and Townsend et al [8], and this work extends that of Townsend, incorporating a finite element representation of the transducer and using the results to visualise the force distribution in three-dimensions. Even in layered resonator designs which are most suited to a one-dimensional approach, significant lateral variations in acoustic radiation forces can be observed [8], which finite element analysis could be used to model.…”

“…Even in layered resonator designs which are most suited to a one-dimensional approach, significant lateral variations in acoustic radiation forces can be observed [8], which finite element analysis could be used to model. For the axisymmetric model shown in Fig.5, the same element types were used; the additional spacer layer was also represented by PLANE42 elements.…”

Mode-switching: A new technique for electronically varying the agglomeration position in an acoustic particle manipulator

“…The estimated error associated with these values stems from the difficulty in distinguishing between a particle sedimenting very slowly and one still levitating. Further complicating measurements is the fact that there are lateral variations in the strengths of the acoustic field due to lateral acoustic modes [27][28][29]. These variations can cause particles to migrate slowly during an experiment into areas of different field strength.…”