Scholz, M.-S., Drinkwater, B. W., Llewellyn-Jones, T. M., & Trask, R. S. (2015). Counterpropagating wave acoustic particle manipulation device for the effective manufacture of composite materials. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 62(10), 1845–1855. doi:10.1109/TUFFC.2015.007116
An ultrasonic assembly device exhibiting broadband behavior and a sacrificial plastic frame is described. This device is used to assemble a variety of microscopic particles differing in size, shape, and material into simple patterns within several host fluids. When the host fluid is epoxy, the assembled materials can be cured and the composite sample extracted from the sacrificial frame. The wideband performance means that within a single device, the wavelength can be varied, leading to control of the length scale of the acoustic radiation force field. We show that glass fibers of 50 μm length and 14 μm diameter can be assembled into a series of stripes separated by hundreds of microns in a time of 0.3 s. Finite element analysis is used to understand the attributes of the device which control its wideband characteristics. The bandwidth is shown to be governed by the damping produced by a combination of the plastic frame and the relatively large volume of the fluid particle mixture. The model also reveals that the acoustic radiation forces are a maximum near the substrate of the device, which is in agreement with experimental observations. The device is extended to 8-transducers and used to assemble more complex particle distributions.