A NEW TECHNIQUE FOR VISUALIZATION OF ACOUSTIC PARTICLE AGGLOMERATION

A new visualization technique is presented for microscopic particle tr ajectories displaying interaction and agglomeration phenomena in a hig h intensity acoustic field. The experiments are carried out in a small -scale observation chamber using a CCD camera in conjunction with a hi gh resolution video system. A homogeneous acoustic velocity field is g enerated by two square, flat-membrane loudspeakers which comprise two opposing walls in the observation chamber. Glass microspheres (diamete rs 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 fiel d (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 osci llations. The direction of propagation of a particle can be extracted by following its trajectory in a 2D laser lightsheeet. Image processin g of the digitized data allows the reconstruction of particle trajecto ries for time spans up to 0.5 s. From the images, the particle size ca n be estimated based on measurements of the acoustic entrainment facto r. Most importantly, with the new experimental technique it is possibl e to resolve particle interaction and agglomeration processes caused b y the acoustic field. The recorded digitized images show a number of d ifferent interaction phenomena as well as one distinct pattern that re sembles the shape of a tuning fork (thus called the tuning fork agglom eration). The latter appears to be the predominant agglomeration mecha nism leading to rapid particle approach and multiple, subsequent parti cle interactions at high frequencies and large acoustic velocities.