and Yu equation (15). The estimated values are close to those obtained from
experiments.
The behaviour of a bed of SiO2 nanoparticles under the effect of application of high
intensity (150 dB) acoustic fields of different frequencies and without any fluidizing
gas is reported in Fig.6. Analysis of the pictures shows that sound of appropriate
frequencies is able to promote an apparent self-fluidization of a relatively thin portion
of the upper part of the bed. The range of active frequencies is rather narrow, from
40 to 60 Hz, and smaller than that appropriate to promote a fluidization stage of the
powders under sound assisted aeration. In any case relatively large aggregates of
particles are formed. Visual observation of the bed also showed that the low as well
the high frequencies of sound did not produce self fluidization: for f=30 Hz all the bed
moves as a piston while for f=120 Hz an apparent static bed is found.