What Is Ultrasonic Cavitation
Ultrasonic Cavitation is the momentary creation of vacuum "tears" commonly
referred to as "bubbles" in the fluid which immediately and violently implode to
produce millions of microscopic jets of liquid which
gently scrubs the parts which are submerged in the tank. In addition,
local temperatures near this activity has been shown to be as high as 10,000
degrees Celsius, and the pressure produced may be as high as 10,000 PSI.
These tears or cavities are created tens of thousands of times each second to
gently remove contaminants without damage, as long as the ultrasonic frequency
selected is correct for the cleaning application. At 80kHz, cavities are
generated 80,000 times each second.
The photograph at the right depicts an imploding cavity, and clearly
demonstrates the process. Notice that the top of the cavity is folding
inward and producing a jet of liquid which can be seen down the center of the
cavity. Millions of times each minute, the surface of any parts submerged
in the bath are being attacked by these implosions, although the cleaning action
produced is rather gentle.
Although these cavities are produced by the millions, the distribution of
these cavities is determined by the ultrasonic frequency in operation.
Every ultrasonic cleaning system produces a cleaning action that is distributed
as a series of equidistant bands of activity. These bands are known as
"standing waves", and cleaning action between standing waves is only a fraction
of the energy which is produced at a standing wave location. This is why
selection of the appropriate ultrasonic frequency is so important to developing
an effective cleaning process. The frequency selected must produce a
distribution of cavitation which ensures that the entire part is successfully
cleaned. More information can be found in the Technical Info drop-down
menu above under "Frequency
Sound waves are composed of 2 actions; and expansion cycle during which the
liquid molecules are being pulled apart, and a compression cycle, during which
the molecules are being compressed. If the expansion cycle of the wave has
enough energy to overcome the forces which hold the molecules of liquid
together, a cavity is produced. Immediately following the expansion cycle,
the compression cycle follows, rapidly compressing the cavities created.
The photograph shown above indicates that the compression cycle has begun to
implode the cavity.
What Affects Ultrasonic Cavitation
The ultrasonic frequency in use determines how often cavities are produced
per unit of time, the size of the cavity, the distribution of cleaning action,
and the force behind cavitational implosion. Since an 80kHz system has
much shorter compression/expansion cycles, cavities that are produced are
smaller since there is less time to increase the size of the cavity during the
expansion cycle. When compression cycles follow, the duration of
compression is also shorter than lower frequency systems, yielding a more gentle
Movement of the cleaning fluid during ultrasonic cavitation will drastically
reduce ultrasonic cavitation until the circulation is reduced to some degree.
The smaller the ultrasonic tank, the higher the ultrasonic frequency, or higher
the watt/gallon density of the system, the faster the system can recover from
fluid motion. This is the primary reason why filtration systems are
typically not operated when parts are being ultrasonically cleaned. More
information on this phenomenon can be found in the Technical Info drop-down
The amount of dissolved gasses within the fluid will also affect ultrasonic
cavitation. Dissolved gasses are a compressible medium which act as a
"shock absorber" to ultrasonic energy being emitted. Although cavitation
will be present, its power is reduced. Fluids which have been degassed
produce 25-50% more cleaning effect than fluids containing dissolved gasses.
Fortunately, the ultrasonic system will automatically degas the cleaning fluid,
the speed of which will be determined by the volume of liquid in the tank, and
the watt/gallon power density of the system. More information regarding
degassing can be found in the Technical Info under
Degassing of Fluid.
Basket designs can also have an enormous affect on the cavitation generated.
Although baskets for use in an ultrasonic cleaning system are typically
manufactured of wire mesh, the size of the mesh can drastically affect
ultrasonic cavitation. Generally, the tighter the mesh, the more
destructive the basket will be for ultrasonic cleaning. Ultrasonic waves
are distorted by these designs which drastically reduces the scrubbing action
produced. Baskets which are manufactured of plastics should never be used
in an ultrasonic cleaning system unless absolutely required. Most plastics
completely absorbs the energy being emitted by the transducers, preventing
cavitation on the opposite side of the plastic wall. If plastics are
required, keep the material as thin as possible, and the design of the basket
should be as "open" as possible. In most cases, plastics are required to
prevent part damage. If this is the case, one must consider to only soften
the contact points between the materials being cleaned and the basket or parts
Temperature of the cleaning fluid will also affect ultrasonic cavitation.
As temperatures increase, the cavities immediately fill with liquid vapor which
cushions the implosive action which cleans the parts. Cavitation will
degrease at temperatures above 65% of the boiling point of the fluid.
However, since the effectiveness of some cleaning agents can overcome this loss
at higher temperatures, it is not un-common to find ultrasonic cleaning systems
operating at well over 65% of the boiling point of the fluid. More
information regarding temperature and its effect on ultrasonic cleaning can be
found in the Technical Info drop-down menu above under
The cleaning agents selected for a given cleaning application will change the
distribution of ultrasonic energy, and the power of cavitational implosive
force. While some cleaning agents will tend to even out the energy
distribution and enhance implosive force, some agents will reduce or completely
eliminate effective ultrasonic cavitation. The cleaning agents selected
must consider how the agent will affect ultrasonic cavitation in the tank.
For additional information,
Zenith Mfg. & Chemical Corp.
85 Oak St.
Norwood, NJ 07648-0412