Tl. Hoffmann et Gh. Koopmann, A NEW TECHNIQUE FOR VISUALIZATION OF ACOUSTIC PARTICLE AGGLOMERATION, Review of scientific instruments, 65(5), 1994, pp. 1527-1536
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.