Acoustofluidics 9: Modelling and applications of planar resonant devices for acoustic particle manipulation

Abstract

5This article introduces the design, construction and applications of planar resonant devices for particle and cell manipulation. These systems rely on the pistonic action of a piezoelectric layer to generate a one dimensional axial variation in acoustic pressure through a system of acoustically tuned layers. The resulting acoustic standing wave is dominated by planar variations in pressure causing particles to migrate to planar pressure nodes (or antinodes depending on particle and fluid properties). The consequences of 10 lateral variations in the fields are discussed, and rules for designing resonators with high energy density within the appropriate layer for a given drive voltage presented.  IntroductionThere is a need to manipulate micron-scale particles and cells in many areas of physics, analytical chemistry and the biosciences. Techniques such as filtration, centrifugation and sedimentation are well-established in macro-scale applications, but in microfluidic 15 systems the use of other approaches including optical, magnetic, dielectrophoretic and acoustic forces are of interest. Acoustic radiation forces, typically at ultrasonic frequencies in the hundreds of kHz to tens of MHz region, have wavelengths that are well matched to microfluidic channel scales, yet are capable of generating potential wells with significantly larger length scales. The technology is also relatively straightforward to integrate within microfluidic systems. Acoustic radiation forces 20Acoustic radiation forces can be generated on particles by both travelling and standing acoustic fields. Those in standing wave fields (of primary interest in the applications considered here) are generated by the nonlinear interaction between the acoustic field scattered by the particle and the standing wave field itself. The time averaged radiation force () F r on a small (in comparison with a wavelength) spherical particle of volume V located at r within a stationary acoustic field was shown by Gor'kov 1 to relate to the gradients of the time averaged kinetic and potential energy densities ( kin E and pot E respectively) within the field:The kinetic energy density gradient (a function of the acoustic velocity field within the standing wave) is weighted by a function of the densities ( on them that tends to move them to the acoustic pressure node, and the acoustic velocity antinode. In a planar resonator these are colocated. Generating the required acoustic fieldMany approaches to generating the required acoustic wave field have been reported in the literature. These include the use of near-field effects 2 or focussed ultrasound 3, 4 to trap particles and cells, the excitation of lateral standing waves in which the predominant energy 35 gradients run parallel to the face of the excitation transducer 5,6 , and the generation of cylindrical resonances to focus 7 or arrange

A thin-reflector microfluidic resonator for continuous-flow concentration of microorganisms: a new approach to water quality analysis using acoustofluidics

“…65 To achieve the desired performance, we adopted two key strategies: mechanically stabilising the glass thin reflector layer; and preventing bacterial adhesion to the reflector surface by modifying it with covalently attached coatings.…”

“…31 Investigation of the streaming patterns in the thin reflector device will be the subject of a subsequent paper. 65 This design results in a pressure node located near the mid-plane of the fluid layer, and pressure maxima near the channel walls ( Figure 1a). HW resonators represent the most widely adopted device configuration in continuous-flow acoustofluidics.…”

“…The reflector and fluid layers thickness are much less than the acoustic wavelength, causing particles in the fluid medium to be attracted in the direction of the reflector-air interface. 65 However, physical contact between the processed biological sample and the reflector surface (typically glass) may result in the occurrence of sample adhesion to the surface, and potentially bio-fouling after long-term operation. Ultrasound-induced deposition of cells has application in immunologically based detection; 33 its occurrence is generally undesired in continuous-flow concentrators.…”

Acoustofluidics 23: acoustic manipulation combined with other force fields

“…This series of tutorial articles has explored the theory underlying acoustic radiation forces 1 and acoustically-induced streaming 2 , has described experimental techniques for evaluating these effects 3,4 and has discussed a variety of applications [5][6][7][8] based on both bulk acoustic waves 9 and surface acoustic waves 10 . It has become apparent that the acoustic phenomena employed have a potentially large scale of action 11 , can trap and manipulate relatively large particles and agglomerates 8 , are appropriate for biological use 12 , and are suitable for integration with many microfluidic fabrication techniques 13 .…”

“…In traditional planar resonators 9 the transducer is usually placed under the channel, with corresponding primary radiation forces out of the plane of the substrate. The in-plane designs of Peterson et al 14,15 , along with devices actuated by surface acoustic waves 10,16 , coupling wedges 17 or flexural waves 18 have enabled the acoustic excitation to be placed some distance from the channel, allowing easier integration with other technologies.…”