A NEW TECHNIQUE FOR VISUALIZATION OF ACOUSTIC PARTICLE AGGLOMERATION

A new visualization technique is presented for microscopic particle trajectories displaying interaction and agglomeration phenomena in a high intensity acoustic field. The experiments are carried out in a small-scale observation chamber using a CCD camera in conjunction with a high resolution video system. A homogeneous acoustic velocity field is generated by two square, flat-membrane loudspeakers which comprise two opposing walls in the observation chamber. Glass microspheres (diameters 8.1 and 22.1 mum) and arbitrarily shaped quartz particles (diameter<50 mum) are used for the observation of interaction and agglomeration trajectories under the influence of an intense acoustic velocity field (1.2-0.53 m/s @400-900 Hz). The new technique allows the observation of the particles' general motion as well as acoustically induced oscillations. The direction of propagation of a particle can be extracted by following its trajectory in a 2D laser lightsheeet. Image processing of the digitized data allows the reconstruction of particle trajectories for time spans up to 0.5 s. From the images, the particle size can be estimated based on measurements of the acoustic entrainment factor. Most importantly, with the new experimental technique it is possible to resolve particle interaction and agglomeration processes caused by the acoustic field. The recorded digitized images show a number of different interaction phenomena as well as one distinct pattern that resembles the shape of a tuning fork (thus called the tuning fork agglomeration). The latter appears to be the predominant agglomeration mechanism leading to rapid particle approach and multiple, subsequent particle interactions at high frequencies and large acoustic velocities.