R.J. Townsend

Performance of a quarter-wavelength particle concentrator

A series of devices have been investigated which use acoustic radiation forces to concentrate micron sized particles. These multi-layered resonators use a quarter-wavelength resonance in order to position an acoustic pressure node close to the top surface of a fluid layer such that particles migrate towards this surface. As flow-through devices, it is then possible to collect a concentrate of particulates by drawing off the particle stream and separating it from the clarified fluid and so can operate continuously as opposed to batch processes such as centrifugation. The methods of construction are described which include a micro-fabricated, wet-etched device and a modular device fabricated using a micro-mill. These use silicon and macor, a machinable glass ceramic, as a carrier layer between the transducer and fluid channel, respectively. Simulations using an acoustic impedance transfer model are used to determine the influence of various design parameters on the acoustic energy density within the fluid layer and the nodal position. Concentration tests have shown up to 4.4-, 6.0- and 3.2-fold increases in concentration for 9, 3 and 1 microm diameter polystyrene particles, respectively. The effect of voltage and fluid flow rates on concentration performance is investigated and helps demonstrate the various factors which determine the increase in concentration possible.

An ultrasonic MEMS particle separator with thick film piezoelectric actuation

An ultrasonic resonator has been microfabricated from layers of silicon and Pyrex. A fluid channel of approximately 200?m in depth is etched into the Pyrex and allows particles within the fluid to be moved by acoustic radiation forces into the pressure nodal planes of the ultrasonic standing wave. Depending on the required application this can be used to generate a particle-free fluid sample, to concentrate particles prior to analysis, or to move particles to a surface within the resonator to aid analysis. In previously published work this resonator has been driven using a thickness mode bulk piezoceramic. While this has provided reasonable performance, the adhesion of the piezoceramic plate to the silicon has proved both the least repeatable and the least reliable element of the fabrication process. It has also been a factor in the long-term failure of test devices. To overcome these issues, multilayer thickfilm printed actuators have been developed to replace the bulk piezoceramic. Thick-film processing offers an effective means of depositing active materials onto substrates, and the technique is compatible with the microfabrication process, allowing multiple actuators to be printed onto a wafer comprising multiple devices. A variety of structures has been tested on ceramic substrates and shown to provide acceptable acoustic outputs when compared with bulk transducers mounted on identical substrates. A two layer actuator provides a good performance without excessive complexity and this configuration has been used on the resonator. Further acoustic and flow modeling of the device is described, and this has been used both to improve the channel geometry and to select better operating conditions for the system. It is shown that the thick-film actuated device working at the new operating conditions provides significantly improved performance when compared with the bulk piezoceramic device, and in particular is able to offer a five-fold reduction in concentration for 1?m latex particles, which had previously proved difficult to manipulate successfully.

The design and modeling of a lateral acoustic particle manipulator exhibiting quarter‐wave operation

Prior work [Petersson et al., 2004] has demonstrated the operation of a half‐wave acoustic particle manipulator, whose forces act in the plane of a silicon substrate. In a half‐wave device particles are directed to the centre of a channel. Devices acting in plane have manufacturing advantages, and lend themselves to many microfluidic applications. We demonstrate, for the first time, such a device with a quarter‐wave mode that is able to manipulate particles to the side of a channel in addition to a near half wave mode. The design utilises resonant “islands” to create the necessary pressure release boundary condition. The device is conventionally milled in brass, permitting cheaper and quicker fabrication than in silicon. Finite element modelling is presented to elucidate the operation of both half and quarter‐wave devices. In contrast to essentially one‐dimensional planar devices, the two‐dimensional distribution of the velocity and pressure fields result in particles being constrained to a line within th...

An integrated multimodal acoustic particle manipulator and optical evanescent field waveguide

A new acoustic-optical-microfluidic system is presented for the manipulation of bead-tagged DNA molecules. Acoustic radiation forces are used to manipulate microspheres into and away from the evanescent field of a laser coupled waveguide that is integrated into the reflector of the acoustic chamber. With suitable fluorophores the presence of the target DNA can be detected with a fluorescence microscope enabling large populations of beads to be examined simultaneously. The integrated waveguide and multimodal acoustic chamber are presented here, with results showing that the microspheres can be successfully detected as they are brought into the evanescent field using a quarter-wave acoustic configuration. It is also shown that by measuring the time of flight of a microsphere between the half- and quarterwave nodal planes the bead size can be determined, providing a means of multiplexing the detection (detecting a range of different target DNA sequences).