Introduction

1. Mode Frequency Description Nodal Position 1 1.712 MHz Quarter wave Device Transducer face 2 2.274 MHz Quarter wave cavity Pyrex boundary 3 3.442 MHz Half wave device Cavity, between centre and pyrex 4 4.100 MHz Half wave cavity mid cavity A Dual Frequency, Ultrasonic, Microengineered Particle Manipulator N. Harris1*, Y. Shen2, M. Hill2, S. Beeby1 , N. White1 1ECS, Southampton University, UK; 2SES, Southampton University, UK; nrh@ecs.soton.ac.uk • Introduction • Ultrasonic standing waves can be used to generate forces on particles within a fluid. Recent work has concentrated on developing devices that manipulate the particles so that they are concentrated near the centre of the cavity. It is also possible to design a device that concentrates the particles at the wall of a cavity. This paper describes a device that has the capability of operating in several modes to allow concentration of particles at either the cavity wall or the centre on the cavity. • Figure 1: A Cross-section of an acoustic particle manipulator • Figure 1 shows a cross-section of a typical device • Modelling • The device is modelled as a multilayer structure as shown in figure 2. • Figure 2: Multilayer structure used for modelling • Modelling carried out using Matlab • Pyrex thickness of 1700um • Silicon thickness of 525um • Cavity depth varied from 160um to 195um • Optimum depth to see all four modes is 175um • Figure 4 shows the device mounted on a manifold • Figure 4: Manipulator mounted on an aluminium manifold • Model Results • The 4 modes of interest are are predicted at • 1.8MHz, 2.3MHz, 3.5MHz and 4.2MHz • Quarter wave modes are less efficient than half wave modes • Figure 5 shows the relative performance • Figure 5: Energy frequency product against frequency • Experimental Results • 4 modes were found at the frequencies shown in table 1 • Particle Concentration • A microscope was used to focus on the nodal planes • Particles of yeast were seen to concentrate in nodal planes at different frequencies • The frequencies were as in table 1 • Figure 6 shows a sequence describing the concentrating effect for mode 1 • Over a period of 20 seconds, particles can be seen to be in focus, indicating movementinto the focal plane • The other modes showed similar results Figure 5: Pressure profiles of the 4 main modes Figure 7: Showing particles moving to the pyrex boundary Conclusions A Multimodal micro machined acoustic particle concentrator has been modelled, constructed, and the model verified experimentally, allowing particles to be positionedeither within the cavity or driven to a cavity face. Such devices can be constructed using standard micro machining techniques. Acknowledgements This work has been supported by EPSRC grant number GR/R13333/01, DSTL,and Porvair PLC.