TECHNICAL
ASPECTS
ULTRASONIC
by 0.
OF
CLEANING
A. ANTONY*
To a great extent, the cleaning fluid used determines the effectiveness of an ultrasonic
cleaner. This article describes experiments for identifying cavitation and determining its
distribution so that the workpiece can be put in the right position in the tank. Viscosity,
surface tension and vapour pressure of a liquid decide whether it has good cavitating properties, and it is important to choose the right solvent for each cleaning task. Greasy, dustcovered and oxidized surfaces are each cleaned best with a particular range of solvents
U
ltrasonic cleaning is based on cavitation, a phenomenon
produced in most liquids when an ultrasonic wave is made
to travel through them .lv2 When an ultrasonic wave passes
through a liquid, the hydrostatic pressure fluctuates, and if
the wave is of sufficient intensity, the pressure can become
negative in the rarefaction half of the cycle. This gives rise
to the hollows or cavities from which the phenomenon takes
the name of cavitation.
All liquids contain a certain amount of gas, which exists
either as dissolved gas or as microscopic gas bubbles
attached to minute solid particles. During the rarefaction
the bubbles grow and absorb more gas, which comes out of
solution in the water and passes into them. When compression begins, the bubbles may either shrink steadily, if
their internal pressure balances out the external pressure, or
may implode violently, generating local pressures as high as
1,000 atm as they do so. Such pressure waves, coming from
numerous tiny imploding bubbles, penetrate the smallest
crevices; their scrubbing action helps the solvent to wet the
submerged surface and dissolve dirt from it which would
otherwise adhere too firmly to be dissolved off. Therefore
the efficiency of a cleaning fluid depends on its properties as
a solvent and its ability to support cavitation.
In most ultrasonic cleaning tanks the ultrasonic vibrations
are transmitted from transducers cemented to the bottom of
the tank. A certain amount of energy is lost by reflections
at the cement-to-tank and tank-to-liquid interfaces, and
from this point of view aluminium, polystyrene or even
glass would be more suitable materials than stainless steel
to make the tank from. However, stainless steel is usually
chosen in spite of its greater reflecting properties because of
*Thorn-A.E.I.
194
Radio Valves and Tubes Ltd.
u~~AsoN~cs/October-December
1963
its ruggedness and lack of rusting; it also resists most of the
solvents used for cleaning.
Cleaning systems today use frequencies of 20-60 kc/s
because cavitation is more efficiently produced in this range
than at 1 MC/S, which was used at first. At 1 MC/S a more
intense wave was needed to produce cavitation and the
ultrasonic field was highly directional.
PRODUCTION OF STABLE SONIC WAVES IN A LIQUID
Various types of sonic field can exist in the liquid in a
container, depending upon the dimensions of the transducer
and the shape and material of the tank. The tank material
influences the field because different materials have
different reflectivities.
A stable standing wave inside the liquid is the best condition for cavitation, as the pressure variations in the
medium are then a maximum. Thus, minimum energy is
required for cavitation or, vice versa, maximum cavitation
occurs for a particular energy.
The column of liquid inside the container can be compared to a closed pipe where resonance is produced when
the ends are the nodes and the length of the closed pipe is
an integral number of half wavelengths. The wavelengths
of the sonic wave can be calculated for the particular
temperature of operation if the velocity of sound in that
liquid is known. Since velocity is substantially constant over
a large range of frequencies the same value may be used
from 20 kc/s to 60 kc/s. Thus in an ultrasonic tank the
theoretical ideal level of liquid would be an integral multiple
of h/2 for the particular liquid and frequency. This will not
always be strictly true in practice for maximum cavitation,
owing to complex cavitation regions produced in small tanks
by energy which is reflected from the sides of the tank and
interferes with the main beam.
WIRE
FRAME,?
0.00047
’ ALUMINIIJM
1. Pattern
of punctures
observed on a foil after 2 min
exposure to a 40 kc/s ultrasonic
field in perchlorethylene
at 28°C.
Frame was held vertical in the
middle of the tank. Half inch
(1.28 cm) separations
are equivalent to .I/2 for 40kc/s waves
travelling at 1027 m/s. Punctures
were about a quarter of the size
of pinheads
Fig.
IN
I
FOIL
/
REGIONS WHERI
PUNCTURES WERE
OBSERVED_
Fig. 2. Pattern on foil after
30 set exposure
to 40 kc/s
ultrasonic field in water. Foil
surface is eroded at areas
separated
by $ in (1.91 cm)
which corresponds
to A/2
for 40 kc/s ultrasonic waves
in water
Fig. 3. Aluminium foil 003047 in thick after 15 set exposed to
a 40 kc/s ultrasonic field in water. The theoretical half wavelength is 0.72 in and the ruptures appeared with approximately
0.8 in separation
between centres. Individual punctures can be
observed near the areas of rupture
ENERGY DISTRIBUTION
Depths at which cavitation is greatest should, theoretically,
be separated at half wavelengths for standing waves. Thus
for water v = 1484 m/s and for a 40 kc/s transducer system
the wavelength should be 1.46 in. The depths of maximum
cavitation should thus be spaced out at O-72 in intervals.
Jn perchloroethylene v is 1027 m/s at 28°C. This gives us
a figure of l-02 in for h, or approximately 4 in separation.
The method of plotting the levels of cavitation is to dip
into the liquid a OWI47 in thick strip of aluminium foil
fixed on a wire frame and to observe the effect produced
on the foil when the sonic energy is applied. The unit is
switched on for a known time with the foil in the liquid.
The foil is then taken out and a series of punctures is
observed at approximately 4 in separations. This is shown
diagrammatically in Fig. 1. These punctures are due to the
minute gas bubbles imploding and releasing sufficient energy
to puncture the aluminium foil; values of a few tens of
atmospheres have been quoted as common figures for this
implosion energy. A measure of the energy is made by
counting the number of punctures in each experiment and
the number obtained with water, taken as 100. Since the
puncture pattern is found to be uniform, the average of
three exposures of the foil in the liquid is considered to be
sufficient to assess the magnitude of the cavitation energy.
This method of energy plotting has been used throughout
the investigation for the identification of energy levels.
Similar tests done with water as the liquid show that the
foil is torn and frittered away at approximately O-8 in
separations, as is shown in Fig. 2 and 3. The foil rupture
unR.ksoNIcs/&tober-December
1963
195
was evident after as little as 15 set exposure. This is
probably due to the higher surface tension of water (see
later section). Simple reasoning will show that maximum
cavitation regions will give maximum cleaning, and this can
be verified by two experiments.
In the first experiment the 0*00047 in thick aluminium
foil was sprayed with a thin layer of calcium carbonate
coating and dried. The foil was then fixed onto the frame
and immersed in water contained in the ultrasonic tank.
The unit was switched on and left for a few seconds. On
close examination it was found that coating was removed
from the foil in proportion to the number of punches.
Regions of greatest removal were separated at half wavelengths of sound waves in water.
In the second experiment a nickel wire 1 mm in diameter
was taken and straightened. Measurements of thickness
were made accurately with the aid of a micrometer. The
wire was dipped in an etch consisting of 10% formic acid,
10% hydrogen peroxide and 80% water and placed in the
ultrasonic tank with an aluminium foil alongside it. After
sonic energy had been applied for a known time, the thickness of the wire was measured at various points. It was
found that at the depths corresponding to the maximum
energy shown by the aluminium foil, the wire diameter was
Spin less than the other parts of the wire.
It can thus be assumed that the positions of the punctures
on the foil can be taken as a good guide to the cavitation
regions. This verifies the theoretical postulate that maximum energy levels are at X/2 intervals.
It must, however, be emphasized that in small tanks the
field is not always a standing wave. One may get diffuse
fields which give rise to complex cavitation regions as well
as patches of cavitation. In actual practice the punches on
the foil obtained near corners of the tank have shown
slightly different patterns to the uniformly spaced h/2
separations.
the same vapour pressure were used in an ultrasonic tank,
and the aluminium foil method of energy plotting was
carried out. It was found that the amount of pitting had
a direct relation to the surface tension of the liquid. This
is shown in Fig. 4.
0
20
I
I
I
I
I
30
40
50
60
70
Surface
tension,
dynlcm
Fig. 4. Cavitation plotted against surface tension for different
liquids. Units of cavitation on the Y axis are referred to water
as 100
Surface tension,
dynJcm
Amy1 alcohol
22
Toluene
30
Phenol
Aniline
:‘:
Water
12
CAVITATION IDENTIFICATION
Regions of cavitation can be detected by observing a beam
of light passed through the liquid being irradiated by sonic
energy. Regions of negative pressure have different transparency to light. It is necessary to use a pulsed beam of
light, e.g. with a Kerr cell, to observe them.
LIQUID CHARACTERISTICS WHICH AFFECT CAVITATION
Three basic characteristics physically affect the cavitation
energy available for cleaning, namely, surface tension,
viscosity and vapour pressure.
04
Viscosity,
I.0
poise
Fig. 5. Cavitation plotted against viscosity for water and
glycerol. Units of cavitation energy are referred to water as 100
Surface tension
Inside a bubble of radius R in a liquid the pressure is
where P = pressure inside bubble -_
P, = hydrostatic pressure at that depth in the liquid
u = surface tension of liquid.
The internal pressure of a bubble depends largely upon
the surface tension when the bubble is small. When the
bubble collapses the potential energy of the bubble surface
is concentrated into a very small volume, and thus the
higher the surface tension the higher the energy of collapse.
This was borne out experimentally in the following
experiment :
Liquids of different surface tension and approximately
196
rrLTRAsoNIcs/October-December
1963
Viscosity
The more viscous a liquid, the higher the sonic energy
required to produce cavitation. This is borne out by the
results from glycerol and from low viscosity materials like
water and alcohol. A given sonic energy produces far less
cavitation in glycerol.
Fig. 5 shows the amount of cavitation energy obtained in
glycerol compared with water; water cavitation energy is
taken as 100. The straight line drawn between water and
glycerol is purely tentative owing to lack of sufficient data.
It is known definitely that the higher the viscosity the lower
is the cavitation energy, but it is yet to be determined what
law it follows with increase of viscosity.
Table 1. CHARACTERISTICS OF TWO VERY DIFFERENT SOLVENTS
Surfa;;n~msion,
--
-_
Viscosity,
poise
Vapour pressure,
mmHg
----
Water
72
0.01
17.0 at 20°C
Glycerol
63.4
13.4
<l
at 20°C
From the above table it is clear that there is not a great
deal of difference in surface tension between water and
glycerol, but a thousandfold difference in the viscosity.
Allowing 10-15 % decrease in cavitation energy for the
lower surface tension of glycerol, we must attribute the rest
of the difference in the cavitation energy to the greater
viscosity.
100
1
0
90
n
y
ao-
6
.z
5 705
5 60k6 50 sy 40._
j; 305
8 20 1
j
10
8
”
”
8
”
“1
300 240 ZOO170 IS0 I20 100 72 60 44 30 20 16 IO
Size
of
mesh
Fig. 6. Transmission of cavitation energy through a stainless steel mesh
Vapour pressure .
It has been found experimentally that liquids of higher
vapour pressure give lower cavitation energies available for
cleaning if other quantities like surface tension and viscosity
are approximately the same. This may be explained as
follows. At a particular frequency, but for liquids of
different vapour pressure, the amount of vapour released
by the liquid during the negative half of the cycle depends
upon the vapour pressure of the liquid: the pressure inside
the liquid is less for liquids of lower vapour pressure. The
implosion energy of a bubble of lower internal pressure
would be more than that of a bubble of higher internal
pressure. This probably accounts for the falling off of the
cavitation energy for liquids of higher vapour pressures, but
of the same surface tension and viscosity.
Thus the three principal characteristics of a liquid which
contribute to good cavitation are high surface tension, low
vapour pressure and low viscosity.
moisture and form acidic solutions. The other obvious
factor is to choose a solvent which does not attack the
workpiece chemically.
GAS DISSOLVED IN THE LIQUID
Ultrasonic cleaning would be impossible if the liquid contained no gases at all and if the vapour pressure was also
very low. In practice most liquids under normal conditions
contain gases dissolved in them and this factor is not of
undue concern in the choice of solvent.
SCATTERING BY GAUZE; POSITION OF WORKPIECE;
CONTAINERS
Sonic energy may be reflected or scattered or transmitted
by a rigid body placed in the liquid. Scattering of the sonic
waves by gauze was studied by the pitting method of energy
plotting to give an indication of the amount of energy
scattered by the gauze. Pieces of stainless steel gauze of
different mesh were immersed in the liquid and the cavitation
energy transmitted was plotted for each mesh. The distribution is shown in Fig. 6. A large part of sonic energy
seems to be scattered by gauze between 10 and 170 mesh.
The most satisfactory position for cleaning of the workpiece is to suspend it in the liquid without any containers.
This is not always possible, especially if very small components are being cleaned, and the use of trays or containers
becomes essential.
From the experiment conducted above, it is seen that
gauze of mesh finer than 240 and coarser than 10 is suitable
for transmission of sonic energy. Trays may thus be made
out of 240 or 300 mesh gauze. The material best suited for
this is stainless steel because of its resistance to most solvents. Almost complete transmission of sonic energy may be
obtained with trays made out of a solid material which
offers low impedance to the sonic waves: such materials are
glass, aluminium and polystyrene. Since it is not always
practicable to make trays out of these materials, stainless
steel sheets 0.003 in thick may be used, as with thin stainless
steel only a small fraction of the energy is reflected. Suitable
glass beakers may be used with the same solvent in them as
in the tank; or a small quantity of the solvent may be used
in the beaker and water used in the tank to act as the
coupling agent.
TIME OF CLEANING
Though it is difficult to estimate the time of cleaning, it has
been found that in a period of 2-4 min the bulk of the
contaminants has been removed. Bell Laboratories Monograph No. 3143 reports that 99% of the contaminant is
removed in 120 set with the most suitable solvent for that
particular contaminant.
It may be said that in general
2-4 min is the optimum period for cleaning. If sufficient
cleaning is not achieved by this time it may be necessary to
change the solvent or the other characteristics of the system,
e.g. sonic output power or filtering of the solvent,
OTHER LIQUID CHARACTERISTICS FOR CHOICE AS SOLVENT
Other liquid characteristics to be looked for in good
cleaning liquid are efficient wetting, good solvent properties
for the particular type of contaminant to be removed,
stability in ultrasonic fields and safety to the component and
the tank material. Stability in ultrasonic fields is important
as it is known that certain solvents, e.g. trichlorethylene,
tend to be unstable and break down in the presence of
FLOW RATE OF SOLVENT IN THE TANK
The flow rate of water in a tank with recirculating arrangements, which are sometimes used to filter away contaminants removed from the workpiece, affects the cavitation
energy produced. The higher the flow rate, the less the
cavitation. This is due to the turbulence caused in the
C
ULTRASONICS/October-December
I%3
197
cavitation regions which obstructs the formation of negative
pressures. Bell Laboratories report in their monograph
that for flow rates of up to 1% of the volume of liquid in the
tank per minute no effect is produced on the cavitation
energy. The present author has found that cavitation
energies are not affected even up to 10% flow rate for his
particular tank. No general figure can be recommended as
this is purely a feature of the tank dimensions and flow
conditions.
DISCUSSION ON THE GENERAL CHARACTERISTICS OF SOME
SOLVENTS
The uses of some solvents and their characteristics are given
in Tables 2 and 3.
____-Removing
Solvent
__-
light coatings
of grease
--
Perchlorethylene
Tk;;kJ;eth ylene
1 : 1 : 1 Trichlorethane
Carbon tetrachloride
Removing
adhering
non-greasy
contaminants
on small components (e.g. carbon, dust pressed
on to metal)
Water/alcohol mixture; water
92-88 “/,, alcohol 8-12 “/,
Components
Formic
acid/distilled
water
mixture plus hydrogen peroxide
slightly oxidized
Table 3. CHARACTERISTICSOF SOME SOLVENTS
-Surface tension,
dyn/cm
Trichlorethylene
32
32
1 :l :l
Carbon
Trichlor. tetraethane chloride
0+)09
Acetone
__
28
25
23.7
----
--------____
Viscosity, poise
-
Perchlorethylene
---_--~
0+)055
0.0061
0409
17 at
20°C
100 at
60°C
CONCLUSIONS
60 at
20°C
319 at
60°C
100 at
20°C
500 at
60°C
91 at
20°C
450 at
60°C
184 at
20°C
1.14 atm
at 60°C
DEGREASING SOLVENTS
Trichlorethylene, perchlorethylene, acetone and 1 : 1 : 1 trichlorethane can all be used as general degreasing agents
quite satisfactorily. The solvents dissolve the grease into
solution. The cavitating properties of the chlorinated
solvents are better than those of acetone, which has comparatively high vapour pressure. Perchlorethylene appears
to have the best cavitating properties and is claimed to be
more stable than trichlorethylene in the presence of moisture.
Chemically there seems to be little to choose between these
solvents as far as their cleaning goes. Their cavitation
energies are far less than that of water owing to their lower
surface tensions. It can be said that the solvent most
suitable for ultrasonic degreasing is perchlorethylene. Trichlorethane is probably more suited to degreasing in large
industrial installations.
198
Various combinations of hydrogen peroxide, formic acid
and distilled water have been recommended by Koontz in
Bell Laboratories’ Monograph No. 3143 for slight descaling.
For oxidized nickel wire the appropriate solution gives
extremely good cleaning in an ultrasonic field. The procedure was to immerse the wires in the ultrasonically
agitated bath for about 2 min and again in distilled water
to remove residues from the wires. They were later washed
in running de-ionized water until the conductivity fell to a
low level. 0.5 x 10m6mho can be taken as a “low level.”
It is to be noted that the de-ionized water should initially
have this conductivity so that it will fall back to this value
after the wires have been washed in it. When this is done
with enough care it is possible to get a finish where no
interference films due to the slight oxidation are visible on
examination under a microscope. The surface was found to
be lightly pitted on giving this treatment. In the light of the
success obtained for nickel, the author assumes that the
proportions recommended by Koontz for other metals will
give equally satisfactory results for these metals under
ultrasonic conditions. It may be added that, after one
month’s storage in open air, these wires had kept their
shiny appearance.
00031
---__________
Vapour pressure,
mmHg
Where degreasing is not necessary, distilled water may be
used as a solvent for removing dust adhering to and dirt
pressed onto the component and because of the high surface
tension of water, good cavitation is available. A mixture
of 8-12 % alcohol and 92-88 ‘A water has proved very satisfactory for the same purpose. The small’ percentage of
alcohol probably adds to the cleaning effect, as its wetting
properties are better than those of water and it brings down
the surface tension. Components cleaned in this mixture
were found to be much cleaner than those cleaned in plain
water.
OXIDIZED COMPONENTS
Table 2. USESOF SOLVENTS
Use
SOLVENTS FOR CLEANING OFF DUST ADHERING TO
COMPONENTS
uLTRAsoNICS/October-December
1963
The solvent most suitable for degreasing is perchlorethylene,
even though a liquid with similar chemical properties but
with a high surface tension would be preferable.
For general cleaning of dust-contaminated components
a mixture of 8-12% alcohol and. 88-92% water has been
found very satisfactory.
For removing oxides and slight descaling of special
metals like nickel, molybdenum, tungsten, copper and
stainless steel, various combinations of a solution of
hydrogen peroxide, formic acid and distilled water in an
ultrasonic field are probably the best method known, apart
from heat treatment in a hydrogen atmosphere.
If containers are used, the most suitable material to use
is 300 mesh stainless steel gauze made up in the form of
a tray.
REFERENCES
1. HUETER and BOLT, “Sonics,” Wiley (1955).
Wiley (1938).
2. BERGMANN, “Ultrasonics,”
3. Bell Laboratories Monograph No. 3143.