Observed the properties of the water beginning to shift into the flammable state. This is due to particle oscillation the water is being subjected to by the ultrasonic unit. Water acts as catalyst at this stage.
Ultrasonic power supply (generator) converts DC voltage to high
frequency 25 kHz (25,000 cylces per second) electrical energy. This electrical energy is
transmitted to the transducer within the handpiece, where it is changed to mechanical
vibrations. The vibrations from the transducer are intensified by the probe (horn),
creating pressure waves in the liquid. This action forms millions of microscopic bubbles
(cavities) which expand during the negative pressure excursion, and implode violently
during the positive excursion. It is this phenomenon, referred to as cavitation, which
produces the powerful shearing action at the probe tip, and causes the molecules in the
liquid to become intensely agitated.
FREQUENCY AND AMPLITUDE
The radiating-wave frequencies most commonly used in ultrasonic cleaning, 18-120 kHz,
lie just above the audible frequency range. In any sonic system,the harmonics of the
fundamental frequency, together with vibrations originating at the container walls and
liquid surface, produce audible sound. Thus, an operating system that is fundamentally
ultrasonic will nonetheless by audible, and low frequency (20-kHz) systems will
generally be noisier than higher-frequency (40-kHz) systems.
Moreover, ultrasonic intensity is an integral function of the frequency and amplitude of a
radiating wave; therefore, a 20-kHz radiating wave will be approximately twice the
intensity of a 40-kHz wave for any given average power output, and consequently the
cavitation intensity resulting from a 20-kHz wave will be proportionately greater than that
resulting from a 40-kHz wave.
The cavitation phenomenon will, of course, occur less frequently at 20 kHz, but this is
not thought to have a significant bearing on effectiveness. However, the longer
wavelengths of low-frequency ultrasonic systems result in substantially different
standing-wave patters throughout the liquid medium.
The standing or stationary waves produced by ultrasonics in liquid media result from
the simultaneous transmission of the surface-reflected wave motion and the wave
motion originating at the transducer radiating surface. The fixed points of minimum
amplitude are called nodes, and the points of maximum amplitude are called loops.
The distance between the nodes and loops of the 20-kHz standing wave (2 in.) will be
approximately twice that of the 40-kHz wave. Because cavitation takes place primarily at
the loops, the distance between cavitation sites will thus be larger with 20-kHz than with
40-kHz radiation, and the 20-kHz waves will also have larger dead zones (i.e., zones with
little or no cavitation activity).
It is for this reason that work resulting from 20-kHz radiation is likely to be less
homogeneous and less consistent, even though this frequency produces more intense
cavitation. Much of the inhomogeneity in ultrasonic fields can, however, be reduced or
wholly eliminated through the use of sweep frequencies, or radiating waves with a
multitude of different frequencies. By this means, several overlapping standing waves
can be generated at the same time, thereby eliminating much of the dead zone.
The amplitude of the radiating wave is directly proportional to the electrical energy that
is applied to the transducer. In order for cavitation to be produced in a liquid medium,
the amplitude of the radiating wave must have a certain minimum value, which is usually
rated in terms of electrical input power to the transducer. No cavitation can occur below
this threshold value, and the use of electrical power over and above the minimum level
results not in more intense cavitation activity but rather in an increase in the overall
quantity of cavitation bubbles. The minimum power requirement for the production of
cavitation varies greatly with the colligative properties and temperature of the liquid and
with the nature and concentration of dissolved substances.
CAVITATION
If a sound wave is impressed upon a liquid and the intensity is increased, a point will be
reached where cavitation occurs. Cavitation is the formation of a gas bubble in the liquid
during the rarefaction cycle. When the compression cycle occurs the gas bubble
collapses. During the collapse tremendous pressures are produced. The pressure may
be of the order of several thousand atmospheres. Thousands of these small bubbles are
formed in a small volume of the liquid. It is quite generally agreed that it is cavitation
that produces most of the biological, detergent, mechanical, and chemical effects in the
application of high intensity sound to various mediums.
The intensity with which cavitation takes place in a liquid medium varies greatly with the
colligative properties of that medium, which include vapor pressure, surface tension,
viscosity, and density, as well as any other property that is related to the number of
atoms, ions, or molecules in the medium. In ultrasonic cleaning applications, the surface
tension and the vapor pressure characteristics of the cleaning fluid play the most
significant roles in determining cavitation intensity and, hence, cleaning effectiveness.
The energy required to form a cavitation bubble in a liquid is proportional to both
surface tension and vapor pressure. Thus, the higher the surface tension of a liquid, the
greater will be the energy that is required to produce a cavitation bubble, and,
consequently, the greater will be the shock-wave energy that is produced when the
bubble collapses. In pure water, for example, whose surface tension is about 72
dyne/cm, cavitation is produced only with great difficulty at ambient temperatures.
It is, however, produced with facility when a surface-active agent is added to the liquid,
thus reducing the surface tension to about 30 dyne/cm. In the same manner, when the
vapor pressure of a liquid is low, as is the case with cold water, cavitation is difficult to
produce but becomes less and less so as temperature is increased. Every liquid, in fact,
has a characteristic/temperature relationship in which cavitation exhibits maximum
activity within a fairly narrow temperature range.
THERMAL EFFECTS OF ULTRASONICS
There is considerable temperature rise in the ultrasonic field in a liquid. A rise of several
degrees per minute can be obtained. The generation in heat is due to dissipation of the
sound by absorption in the liquid. The generation of head by the action of ultrasonics
obscures the effects which can be attributed to sound alone because many chemical
and biological phenomena observed when ultrasonics are applied are also obtained by
the application of heat. The practical value of heating by ultrasonics remains to be seen.
