Examination of the concentration dependence of
acoustical phenomenon in water based suspensions
ATTILA LŐRINCZ – MIKLÓS
NEMÉNYI
University
of West Hungary, Faculty of Agricultural and Food sciences, Mosonmagyaróvár;
Institute
of Agricultural, Food and Environmental Engineering
There is no cavitation in the ultrasound field
until the amplitude of the acoustic pressure exceeds a certain level, the
cavitation threshold (Fry 1978). Cavitation threshold is proportional with the
frequency of ultrasound, with the hydrostatic pressure in the liquid, and with
the viscosity of the sample and it is inversely proportional with the gas content and temperature of the sample
(Suslick 1988 cit. Ter Haar 1988). There are two types of cavitation that are
stable and transient cavitation
(Suslick 1988 cit. Frizzel 1988). Basically two reactions take place when ultrasound and a media
interact with each other. One of them is the absorption the other one is the
scattering, which changes e.g., the speed of propagation of the sound in the
subject media (Fry 1978 cit. Hill et al. 1978). Due to the absorption, the
intensity of ultrasound decreases exponentially with distance and the
absorption coefficient primarily depends on the speed of propagation of the
sound in the subject media, on the wave type, on the material situated in the
ultrasound field and on the frequency. The absorption always characterizes a
media, a structure or an environment that determines the parameters of
propagation (Kurtuff 1991). When absorption coefficients were measured in oxo-
and és methemoglobin, it was observed that the absorption is proportional with
the concentration of hemoglobin in the concentration range between 0 and 15
[g/100ml] (Carstensen and Schwann 1959). It was clearly established that the profile of the ultrasound
propagation speed depends on the concentration profile of the suspension
(Wedlock et al. 1993). Effects of the size and concentration of the suspended
particles on the propagation speed of ultrasound was examined in water based
suspensions. It was established that the speed of sound largely depended on the
particle size and concentration (Sayan and Ulrich 2002). In vitro cavitation
threshold measurements were carried out in human blood. In the fresh blood that
contained every blood component, the amplitude of the acoustic pressure belonging
to the cavitation threshold was higher than in diluted blood (Deng et al.
1996). Due to cavitation caused by ultrasound, acoustic streaming was formed in
the liquid (Saad and Williams 1985). Acoustic streaming is a movement of the
liquid that is caused by intensive ultrasound (Mitome 1998). Mixing of liquid
was experienced in the ultrasound field due to acoustic streaming (Watmough et
al. 1990). An acoustic reflector placed opposite to the transducer causes a
standing wave to be formed. In a standing wave the materials whose density are
lower and higher than of the liquid drift to propagation cluster planes
(pressure antinodes), and pressure nodes, respectively (Suslick 1988 cit. Ter
Haar 1988). The ultrasonic separation is used in analytical biotechnology
applications. This procedure is based on the fact that in a standing wave
field, where there is no cavitation,
the cells are arranged in bands distances of which are smaller than a
millimeter and they can be separated from these bands (Coakley 1997). Yeast (Saccharomyces
cerevisiae) and rubber particles were manipulated in a standing wave
ultrasound field at frequencies of 1 and 3 [MHz]. The particles formed bands in
pressure nodes whose distance from each other was equal to half of the
wavelength. In the direction of the radiation the bands formed column like
structures. Stability of the bands, the conditions under which they are broken
and the formation of the acoustic streaming were investigated in (Hawkes et al
1998). Stability of the banded columns formed by the effect of the standing
wave, and the appearance of the
cavitation were examined by detecting the formation of the general
cavitation sound (Gould 1992). Effectiveness of the cell separation of Escherichia
coli bacteria and Saccharomyces cerevisiae yeast cells from a yeast
suspension was examined at frequencies of 1 and 3 [MHz] (Hawkes et al. 1997).
As a result of the standing wave, the bands and the bubbles were separated into
different layers. In their experiments, the authors placed an absorber opposite
to the transducer for avoiding the formation of the standing wave, but the
layer of air located opposite to the transducer resulted in an almost total
reflection and as a consequence of this, an almost perfect standing wave was
formed (Chrunch and Miller 1983).