Ultrasonic manipulation of particles has significant potential, particularly in microfluidic devices for controlling and trapping biological cells. One approach to particle manipulation employs planar standing waves to move particles to pressure nodes. Devices with axial dimensions less than a wavelength are well-suited to use in microfluidic systems. The nodal planes in such devices can be positioned at any axial position within the device by appropriate choice of materials and dimensions.
Resonators that excite near quarter wave modes within a fluid can be used to force particles onto a surface or to position particles near a surface to enhance particulate concentration prior to sensing. However the position of the node is sensitive to a large number of parameters, some of which are difficult to determine without specific experimental data. This paper describes applications of quarter wave resonators and discusses the design of devices that provide the required nodal position for different applications.