Blog Purpose

The CTG Technical Blog is intended as a source of information on subjects related to industrial and precision cleaning technology. The writer of the blog, John Fuchs, has 40+ years of experience covering the entire gamut of cleaning. Mr. Fuchs has extensive knowledge of ultrasonic cleaning technology having been employed by Blackstone-Ney ultrasonics and its predecessors since 1968. The blog is also intended to serve as a forum for discussion of subjects related to cleaning technology. Questions directed to the blog will be responded to either in the blog (if the topic has general interest) or directly by email. Emails with questions about the current blog should be entered in the comments section below. Off-topic questions related to cleaning may be sent to

Capillary Flow

May 18th, 2015

In the process of parts cleaning, capillary flow can be our friend or our enemy – sometimes both at the same time.  So let’s take a minute or two to understand this interesting phenomenon.

Capillary flow is a property that is exhibited when liquids spontaneously penetrate narrow passageways.  The classic example of a capillary (narrow passageway) is a small internal diameter tube.  The following illustration demonstrates how three different liquids might respond in a capillary space.

Liquids climb to different heights in the same diameter capillary space depending on their properties.  The size and contact angle of the meniscus where the liquid meets the capillary wall also changes based on the properties of the liquid.

Liquids climb to different heights in the same diameter tube depending on their properties. The size and contact angle of the meniscus where the liquid meets the capillary wall will also change based on the properties of the liquid.

The flow of liquid into a capillary space is spontaneous and occurs without pressure, vacuum or any other outside influence.  The height (or depth) of penetration can, of course, be affected by pressure or vacuum but without them, the degree of penetration in similar capillary spaces is related most directly to the surface tension and specific gravity of the liquid.  The meniscus formed at the point where the liquid contacts the surface of the capillary space is analogous to the contact angle measurement for surface tension.   Contact angle was discussed in previous blogs as both an indicator of cleanliness (if the liquid is held constant) or as a measure of surface tension of a liquid (if the contact surface is maintained constant).

Although we often think of capillaries as tubes, the same phenomenon will occur wherever there are closely spaced surfaces of any geometry including flat plates or more complex shapes like screw threads in contact with a threaded fastener or tapped hole.

Effect of varying capillary spacing

Varying the capillary spacing using the same liquid and surface conditions will produce different results in capillary flow. Closer spacing results in deeper penetration.

The degree of capillary penetration is also directly related to the dimension of the capillary space or gap.  A larger capillary gap results in less penetration.  Once a specific spacing is exceeded (depending on the surface activity of the capillary walls and the surface tension of the liquid), there will be no noticeable spontaneous penetration into a capillary space although the meniscus effect will still be evident where the liquid and surfaces forming the capillary space meet.

Note – Capillary flow is also commonly referred to as “wicking” as it is this effect which causes liquids to rise in the wick of an oil lamp or candle.  Liquid may also “wick” in porous solids.

In cleaning, capillary flow results in difficulties in both rinsing and drying.  Once liquid has entered a capillary space (by overcoming gravity) the only way to remove it is evaporation or use of mechanical means such as high pressure air or centrifugal force.  The liquid will not just “drain” out.

The purpose of this blog is not to suggest ways to overcome the effects of capillary flow but, rather, to make the reader aware that the effect exists any time small passageways are encountered.  The result may be inconsequential in some instances but in others is a means for real concern.

-  FJF  -


February 17th, 2015

We have all heard of servos (short for servomechanism) but exactly what characterizes a “servo” is often a cloudy topic.

Steam engine governor developed by James Watt.

Steam engine governor developed by James Watt.

Most sources are in agreement that the centrifugal ball governor invented by James Watt and used to control the speed of steam engines was the first application of a powered servo mechanism.  This eloquently simple device utilized centrifugal force to raise or lower a set of balls that were attached to a shaft driven by the engine being controlled.  As the speed of the engine increased, the balls would move higher and higher as centrifugal force increased.  The height if the balls controlled a valve which delivered more or less steam to the engine as required to maintain the desired ball height, hence the speed of the engine.

In the cleaning industry, servos are often used to control temperature and motion.  So exactly what is a “servo?”  The characteristic of a servo which differentiates it from other control schemes is that it is interactive.  Consider the following illustrations.  In both cases, the task is to move an item (in this case a box, for example) from position “A” to position “B.”  This is to be accomplished with a conveyor belt with a means to detect position.  In a conventional control system, limit switches are used to detect when the conveyor belt is in position “A” or position “B.”  The limit switches control a fixed speed motor by turning the motor on or off.  A flange attached to the conveyor belt activates the limit switches.

In a conventional control system, position is sensed with limits are met. Between limits, there is no feedback on position.

In a conventional control system, position is sensed with limits are met. Between limits, there is no feedback on position.

Although the speed of the motor may be variable, the speed of the belt does does not change during the transfer from position “A” to position “B” except for starting and stopping.  Considerations in a conventional control system are

  • Acceleration must be limited to a speed that will not damage or cause slipping of the load on the conveyor belt.
  • The speed must be limited to prevent “overshoot” when the desired position is achieved.
  • Conditions are totally unknown between the start point and the end point.

Now the servo system.

In the case of the servo, a position sensor detects the position of the conveyor belt throughout its travel. An amplifier controls the speed of the motor based on comparing the feedback from the position sensor with the desired target positions

In the case of the servo, a position sensor detects the position of the conveyor belt throughout its travel. An amplifier controls the speed of the motor based on comparing the feedback from the position sensor with the desired target positions

A servo system includes a position feedback device which reports the position of the conveyor constantly.  This feedback device can be in the form or a potentiometer, an optical counter or any of a number of suitable devices.  The information from this indicator is received by a control interface (often called an “amplifier” by convention) which compares it to the desired value at any given time.  The control interface interprets the feedback and applies an appropriate corrective signal to drive the controlled device to the target.  The control interface can be customized to react in specialized ways.  For example, if the feedback indicates that the target is some distance away, it might increase the speed of travel and then ramp down as the target is approached.  As a result, the time required to move from “A” to “B” in our example above can be greatly reduced as shown in the graphs above each illustration.

Servo systems can also be used to help control temperature as described in the blog on temperature controls.

Servo controls, although usually more costly than more conventional controls have the benefit of being able to provide precise control in a programmable way.

-  FJF  -

Is it Viscosity or is it Surface Tension??

February 13th, 2015

Viscosity and surface tension are properties that are often intuitively linked to one another.  Because these properties are of primary importance in cleaning it will be worth the while to understand them a bit better.

Surface tension is the property of a liquid that exists at the interface of the liquid with another media (usually air) to hold the surface intact much like the “skin” of a balloon.  Surface tension is created as molecules or atoms at the surface of the liquid have a stronger attraction for one another than they do for the particles or atoms of the adjoining media.  The higher the surface tension, the more stable the barrier between the two that prevents their mixing.  As you have probably heard, a blob of liquid in an environment with no gravity will form a perfectly round ball as it seeks the lowest energy state.  “Lowest energy” meaning the least stretching of the “skin” formed by surface tension.  The surface tension of a liquid nearly always decreases as the temperature of the liquid is increased.  Soaps and other chemical additives also reduce surface tension.  In fact, surface tension is a good measure of the effectiveness of many cleaning agents.  One means of measuring surface tension is by determining the contact angle where a drop of a liquid meets a surface.  If the activity of the surface is known, the contact angle measures surface tension. If the surface tension of the liquid is known, the contact angle indicates the activity of the surface.

Viscosity is a measure of a liquid’s resistance to motion.  Viscosity is weird, taking on one of five general characteristics.  Some materials increase in viscosity as motion is increased while others decrease in viscosity as motion is increased.  These are known as non-Newtonian liquids and include things like quicksand and a mixture of corn starch and water.  It is sufficient to know for our purpose here that viscosity is the resistance to motion of a liquid.  The viscosity of tooth paste is greater than the viscosity of motor oil while the viscosity of motor oil is greater that that of water.  In most cases except for a few “engineered” fluids like particular kinds of motor oil, viscosity decreases as temperature is increased – - sort of the “molasses in January” thing.

So why the confusion?  In general, we think of very viscous liquids as having high surface tension and intuitively link the two.  In fact, a very viscous liquid like wood glue has very low surface tension to be able to penetrate and bond to the irregular wood surface.  Liquid mercury, on the other hand has relatively low viscosity and extremely high surface tension.  The table below shows the viscosity and surface tension of some familiar liquids.

Viscosity and Surface Tension Table

The important “take away” here is that one can not assume that just because a liquid has low viscosity that it also has low surface tension or vice versa.  The two properties are different and not related in the way we would intuitively think.  Liquids with high surface tension and/or high viscosity can be difficult to remove from a surface.  Liquids with low surface tension and low viscosity are good cleaners.  Increasing temperature moves both properties in the direction in favor of successful cleaning.

Things to ponder - Is the viscosity of “silly putty” high or low?  What about its surface tension?  How about mayonnaise? (answers are available on the internet)

-  FJF  -

Considerations for Water Makeup

February 10th, 2015

Adding water to cleaning and rinsing tanks is inevitable in a cleaning process.  Most modern machines include a means to sense liquid level and add more water or other appropriate liquid as necessary.  It would seem simple enough, but there are a lot of factors to consider in managing liquid makeup.

The first thing to determine is where the cleaning or rinsing solution is going.  In general, water is lost in one of two ways, either through evaporation or by being carried out on the parts being cleaned or rinsed.  In the case of evaporation, the loss is primarily water as it evaporates leaving chemistry or contaminants behind.  In the case of carry-out from a cleaning tank, both water and chemistry are depleted from the tank over time.  Of course, in the real world, losses from a cleaning or rinsing tank are usually due to a combination of the above.

Liquid lost to evaporation requires only water makeup.  Liquid lost to carry-out contains chemistry which must be replaced along with makeup water.

Liquid lost to evaporation requires only water makeup. Liquid lost to carry-out contains chemistry which must be replaced along with makeup water.

To replace water lost to evaporation, it is only necessary to replace water.  A level sensor senses low liquid level and opens a valve allowing fresh water to enter the tank until the proper operating level is achieved.  In the case of chemical and water loss due to carry-out, both the water and the chemical component need to be replaced.  This can be accomplished by monitoring the chemical concentration using one of the means we have discussed in previous blogs and adding chemistry automatically or manually as needed.  Another option is to replenish using a mixture of water and chemistry.  This can be accomplished using chemical dispensing devices.  The important thing to understand is that the chemical concentration of the replenishing liquid usually doesn’t need to be as high as the target concentration in the tank.  This is because part of the losses are probably due to water evaporation.  When setting a system of this type, one should consider monitoring chemical concentration over time to determine the proper makeup concentration to maintain the desired chemical concentration in the tank.

If water or water/chemical is added in large quantity there may be other process ramifications.  For example, adding a large quantity of cold water may significantly reduce the temperature of the bath.  This, of course, could have a detrimental impact on cleaning if the process is temperature dependent.  One solution would be to use hot water for makeup.  A second consideration in ultrasonic tanks is that the addition of fresh solution that has not been degassed will add entrained gas into the system which may have a detrimental effect on ultrasonic cavitation.  For these reasons, level controls should be adjusted so that any one addition of makeup liquid is as small as practical and doesn’t exceed 2 to 5% of the overall tank capacity.  In a case where a large amount of makeup solution is necessary, such as when a tank is drained and re-filled, production should be halted until the solution temperature recovers and, in the case of an ultrasonic tank, the solution has been fully degassed.

Finally, in the case of overflow rinses where water is being added continuously, flows should be limited to the minimum that will give the desired result.  Heaters should be sized to assure that liquid temperature is maintained at the highest anticipated flow rate.

-  FJF  -


“Oil” is oil. Right?

February 5th, 2015


I don’t think most of us would consider using motor oil to moisten our dry skin.  By the same token, we wouldn’t use body oil in our car’s engine.  The notion seems to persist in the cleaning industry, however, that oil is oil.  We often neglect to differentiate between oils when it comes to developing or recommending a cleaning process or chemistry.  In fact, there are probably thousands of different oils each with its own set of characteristics which can have a great impact on the process and chemistry required to remove them.

Oils can be distillates of crude oil, man-made or synthetic oil, animal oil, or vegetable oil or mixtures of any of these.  Additives including wetting agents, stabilizers, emulsifiers and a host of others are added to give the oil the properties required for a particular application.  As an example, let’s look at a two classifications of oil, their general characteristics and what impact those characteristics might have on removing them from a substrate.

Lubricants -

Lubricants, in general, are oils designed to penetrate into capillary spaces and remain in place for a long period of time.  They are not water soluble and have relatively low surface tension to allow them to penetrate and wet the surfaces being lubricated.  However, their surface tension needs to be high enough to prevent their continued migration too far beyond the location requiring lubrication.  They also have low volatility to slow evaporation so that they will continue to lubricate over the long term.   In some cases they must be able to resist high temperatures.  Viscosity is tailored depending on the application.  Because of their relatively low surface tension, lubricating oils are best removed by a chemistry that will emulsify them into solution.  Finding a chemistry that will preferentially wet the substrate (“splitting” the oil) is not generally an option.

Coolants -

Coolants and some types of cutting oils are usually emulsions of oil and water.  A surfactant is used to assist in the emulsification process.  The primary purpose of a coolant is to cool the cutting tools and substrates during a machining process.  In general, the lubricating function, although important, is a secondary characteristic.  Coolants are usually delivered in large quantity to completely flood the area where machining is taking place.  The oil component of a coolant usually leaves a thin coating of lubricant on the surface while the water component cools the surfaces.  Because of their relatively low surface tension, coolants are often best removed using a chemistry that “splits” the oil from the substrate.  The benefit to using a “splitting” chemistry is that the chemistry is not consumed or depleted during the cleaning process.  In some cases, the recovered oil can even be re-used.

The list of industrial oil classifications goes on to include penetrating and/or water-displacing oil, hydraulic oil, rust inhibitors and many others such as the oil used for dampening in shock absorbers.  Add to this the classifications of oil used in non-industrial applications such as skin care and pharmaceuticals and the list becomes almost limitless.

The “take away” here is that removing oil from parts is far beyond a case of “no brainer” solutions.  Just because a process is effective in one oil removal application does not mean that is going to work in any other even if the two appear to be similar.  Knowing the type of oil being removed and its inherent characteristics is critical to success.

-  FJF  -

More for Less with Accumulators

January 13th, 2015

As a prolog to the following let me just say, as I may have said before in this blog, that being an engineer is both a blessing and a curse.  A blessing because you intuitively understand how things work (or at least think you do) that other people see as magic, and a curse because it is not possible to turn off being an engineer, which can be frustrating.  Also, interesting engineering is sometimes found in the least expected places.  This one, which I feel has a potential use in cleaning system applications, I first discovered at Epcot Center in Disney World.  Should I be surprised?

In the center of the Innovations area of Epcot Center, there is a huge fountain which performs in synchronization with music.  Multiple jets of water spurt up into the air at least 150 feet with microsecond timing and cascade back down over the edge of the fountain.  Looking at the fountain (as an engineer) I was at first impressed with the capacity of pumps that must be required to pump all that water.  During a later trip when the fountain was drained for maintenance and the workings were revealed, it became clear to me that the pumping requirement wasn’t as huge as I had at first imagined.  The “trick” is that each nozzle is fed by an “accumulator” consisting of a pressure canister.  During the time that the nozzles are not functioning, the accumulators are filled with water which pressurizes the air within them.  Then, at the proper moment, the accumulated water, pressurized by the trapped air, is released to the nozzle.  In short, the pumping capacity required is only a fraction of what I had first thought since it is water under pressure from the accumulators that actually propels the jets and not a pump.  Since each jet is on for only a short period of time, there is time for refilling and pressurizing the accumulators between bursts.  In fact, this is not unlike a toilet which accumulates water in a tank which is released during flushing in a large volume to achieve the desired result.

Subsequently, it has occurred to me that this same concept might be used in cleaning systems in cases where a relatively large volume of water or water under high pressure (as in a deluge rinse or spraying application) is required for only a short period of time.  Instead of having a pump that can, for example, deliver 10 gallons per minute at a pressure of 100 pounds per square inch, why not have a pump that can deliver 1 gallon per minute at 100 pounds per square inch (at lower cost) and accumulate it for use at the appropriate time propelled by the pressure of air from an accumulator?

Accumulator System

In the lower of the two illustrations above, a short burst of high pressure liquid can be supplied using a lower capacity pump thereby reducing the pump capacity and short term energy requirement as pressure is built up in the accumulator(s) during the time that the nozzle is not in use.

The pragmatist might counter that as the air in the accumulator expands, the available pressure decreases.  Although true, many applications will tolerate reducing pressure.  If not, there are a couple of options.

  • Size the accumulator(s) so that the pressure will be adequate even despite the pressure drop at the end of the anticipated requirement.  This can be accomplished by increasing the size or number of devices.
  • Pressurize above the required pressure and use a pressure reducing valve to control a constant pressure to the nozzle.

Again, I believe this arrangement has potential use in cleaning system applications where significant amounts of liquid are required for only a short period of time.

-  FJF  -

De-Ionized Water – Things I Did(n’t) Know – Part 2

January 8th, 2015

Since DI water quality is critical to many cleaning processes, it is important to understand its behavior.  In too many cases, the quest for numbers results in high costs in the use of DI water.  In short, delivering super-high resistivity water does not always have a significant impact on the resistivity of de-ionized water in the ultimate application.

DI water systems can supply water with resistance ranging from 1 megohm to greater than 18 megohms depending on the configuration of the system.  Systems producing higher resistivity water are more costly to both purchase and maintain.  The rating of a system is based on the water quality that will be delivered directly at the outlet of the system.  The road from there is a bumpy one as the pure DI water is looking to gobble up ions wherever they can be found – and they can be found everywhere.  In most cases, the quality of water delivered to the process is much more dependent on what happens as the water navigates itself to the point of use than on its quality at the outlet of the de-ionizing system.  The following links to provide some insight into the actual use conditions for DI water and what water quality can be maintained in some typical applications.

Although extremely illuminating, the above are a bit lengthy so let me just summarize them quickly.

  1. High quality DI water (over 15 megohm resistivity) seldom exists other than within the confines of the plumbing as it exits the de-ionizing equipment.  Once the water is exposed to the open environment (including external plumbing), the resistivity immediately starts to decrease.
  2. Maintaining anything above 1 megohm resistivity in a tank or any container open  to the atmosphere is near impossible no matter what quality of water is used at the inlet.  Increasing the inlet water from a purity of 1 megohm to 18 megohms will not necessarily result in higher resistivity in the process tank, at least over the long term (longer than 15 minutes).
  3. Testing for DI water quality other than directly at the site of use is very difficult.  Once the water is collected and transported, the resistivity will likely have dropped significantly (usually by an order of magnitude).  Any verification testing should be done directly at the point of use.  Most DI water systems incorporate a sensor that reads resistivity within the plumbing at the outlet of the system but one can not assume that the quality of water will be maintained into the process tank.
  4. In most cases, the quality of DI water used in an application is considerably lower than it is measured at the source.  In most rinsing applications, use of water with a resistivity above 500k ohms does not contribute to its effectiveness.  A de-ionized water feed of over 2 megohm resistivity only ads cost to the process.
  5. De-ionized water should not be sprayed in air.  Spraying only increases the surface area available for contaminants to enter the liquid.

In summary, it is important that the process engineer understand the behavior of DI water in designing a cleaning process.  Similarly, the equipment supplier needs to understand what it takes to achieve and maintain high quality DI water under the intended conditions of use.  Certain measures may be taken to minimize absorption of ions.  They include – -

  1. Minimizing the length of plumbing between the DI water source and the point of use.  This plumbing should also be selected specifically for DI water use.
  2. The surface area of the water that comes into contact with air should be minimized.  Using narrow, deep tanks is better than using wide shallow tanks.
  3. DI water inlets should be located below the liquid level of tanks to prevent splashing as the DI water enters the tank.

 -  FJF  -

De-Ionized Water – Things I Did(n’t) Know – Part 1

January 6th, 2015

De-ionized water is commonly used in both industrial and precision cleaning systems, primarily for rinsing.  De-ionized water is a little like the speed of light or absolute zero in that you can never quite TOTALLY remove all the ions from the water (although we can get close) and, once a certain level of de-ionization is achieved, it is hard to maintain or measure (like some of those particles they talk about in quantum physics).  In short, de-ionized water is pretty special stuff!

The reasons why we use de-ionized (commonly called DI) water are the exact same reasons why it is so difficult to make and maintain. Water without ions is so “hungry” for ions that it will do almost anything to get them.  In many ways it can be compared to a strong acid and, in fact, will attack even the most chemical-resistant materials including the high grade stainless steels commonly used in the construction of cleaning tanks and plumbing.  This “hunger” for ions makes it great for rinsing cleaned parts as it will reach into even the most inaccessible places in an attempt to satisfy its ionic hunger by devouring residues from the cleaning process.

Ions are removed from water by passing the water through a column or columns containing resins which strongly attract ions.  This process was described in general in a previous blog.  Once the ions have been removed, the process of replenishing them starts immediately as the de-ionized water flows through the plumbing lines to the point of use (cleaning system).  There is no way to completely stop de-ionized water from re-acquiring ions, but there are ways to reduce the inevitable.  One way is to use plumbing materials that minimize the availability of ions.  Most plumbing carrying de-ionized water is constructed using grades of plastic that minimize the availability of ions for this reason.  Since the re-ionization is also time related, DI water supply systems are often of the recirculating loop design with one or more de-ionizing devices located in the loop as shown in the illustration below.

Plumbing loop illustration

In the top example above, the entire delivery system is stagnant “dead leg” unless DI water is being drawn from the system. In the bottom illustration, a recirculating system reduces the “dead leg” to just that plumbing between the main supply pipe and the tap even though no water is being used.  Re-circulation assures that high quality DI water is available from all taps all the time.

Flow velocities in the recirculating in the loop are typically from 3 to 5 feet per second to minimize the exposure time of the DI water to the plumbing before again being de-ionized.

Note – Another benefit of a re-circulating supply system is that continuous high flow through the de-ionizing beds prevents “channeling” of the beds as described in a previous blog thereby maximizing their life and efficiency.

DI water is supplied through spigots connected directly to the loop with minimum plumbing runs to minimize stagnation.  In critical applications, the “loop” may even enter and exit the cleaning system to assure the shortest possible plumbing run to the point of use.  Even with these precautions, there are applications which require a final de-ionization unit (usually a mixed-bed resin column) immediately at or following the supply tap.

-  FJF  -

Dryer Configurations – Benefits of Upflow?

December 12th, 2014

Drying is almost always the longest step in a cleaning process.  Because of this, anything that will speed up the drying process is of special interest to the process engineer.  Hot air drying is, by far, the most common means of drying used in the industrial parts cleaning arena.  Major variables in hot air drying are temperature, airflow and relative humidity.  Increasing temperature or airflow or reducing relative humidity (often by adding heat) all contribute to the speed at which water will evaporate from a surface.  Another factor that has a significant effect on the speed of evaporation of a given volume of water is the amount of surface area interface of the liquid with air available for the evaporation to take place.  A larger surface area interface results in faster evaporation.  In most cases, water tends to drain to the bottom of a part due to gravity resulting in drops of water at the bottom of the part.  A water droplet tends to pull into a spherical shape as a result of surface tension and, as such, presents the minimum amount of surface area per unit of volume.  This, obviously, is not the best condition for drying by evaporation.

Upflow vs. Downflow Hot Air Drying -

The direction of airflow in a dryer, if it has sufficient velocity, can help increase the surface area interface available for evaporation.  If airflow is from the top to the bottom of a dryer (which is most common), the airflow works along with gravity to encourage the formation of drops of water at the bottom of a part.

Downflow Drying vs. Upflow

Airflow in a downward direction encourages the formation of water droplets at the bottom of a part. Reversing the direction of airflow may increase the surface area of the droplet by blowing it up the sides of the part and thereby increasing the surface area available for evaporation.


Changing the airflow direction to from bottom to top can (and again this depends on air velocity), reverse the process of drop formation by pushing the water up so that it is distributed over more of the surface of the part.  This, in turn, increases the surface area of the liquid/air interface available for evaporation.  Increased surface area may result in an increased speed of evaporation and faster drying

It can’t be stressed enough that upflow will only be of benefit if the velocity of airflow is sufficient to overcome the effects of gravity and surface tension.  It could be argued that increasing the air velocity in a downflow scenario could increase the likelihood water droplets clinging to the bottom of a part will be physically removed (blown away).

To the best of my knowledge, the upflow concept is theoretical (as are many things in the cleaning world).  I don’t know that there have been any A-B tests conducted under controlled and documented conditions.  If readers are aware of any, I would enjoy seeing the results along with any analysis of what might have contributed to the outcome.  Specifically, what air flow was required to make upflow beneficial.


-  FJF  -



Sound Physics – Nodes and Antinodes – Part II

December 10th, 2014

When a sound wave encounters an object or discontinuity in the sound conducting medium, at least part of the sound is reflected.  The way the sound reflects depends in large part on the rigidity of the reflecting surface.  If the object is hard, like a substantial piece of metal, the wave reflects as shown below.

When a sound wave reflects from a hard surface, the phase of the reflected wave is reversed in phase.

When a sound wave reflects from a hard surface, the phase of the reflected wave is reversed in phase.

The hardness of the material prevents displacement.  Notice that the surface does not yield to the arrival of the compression of the sound wave.  As a result, there is a pressure generated at the interface in lieu of displacement.  This results in a displacement node (no displacement) at the interface.  When the sound wave encounters a surface that is softer, like a piece of plastic or rubber, the softer material allows displacement at the interface and the sound wave reflects as shown below.

When a sound wave reflects from a soft surface, the phase of the reflected wave is the same as the incoming wave.

When a sound wave reflects from a soft surface, the phase of the reflected wave is the same as the incoming wave.

Notice that in this case, the surface yields to the arrival of the compression of the sound wave.  As a result, there is displacement generated at the interface in lieu of pressure.  This results in a pressure node (no change in pressure) at the interface.  This is very much like the discussion of resonance of a tube with and open end vs. a tube with a closed end that appeared in an earlier blog.

So, what does this mean when it comes to cleaning?  In the case of a hard surface, because of the reverse phase reflection, the sound wave reflecting on itself looks like the following.

When a sound wave is reflected from a hard surface, a pressure anti-node (maximum pressure deviation) is formed at the reflecting surface.

When a sound wave is reflected from a hard surface, a pressure anti-node (maximum pressure deviation) is formed at the reflecting surface.  Click on animation to start in a new window.

The formation of a displacement node where the sound wave intersects the reflecting surface results in a pressure anti-node (maximum pressure variation). Since cavitation and implosion are a result of changing pressure, the pressure anti-node at the interface provides an ideal site for cavitation and implosion at the surface for ultrasonic cleaning.  When a sound wave reflects from a soft surface the result is as shown below.

When a sound wave reflects from a soft surface, a pressure anti-node (relatively constant pressure) is created at the interface.

When a sound wave reflects from a soft surface, a pressure anti-node (relatively constant pressure) is created at the reflecting surface.  Click on the animation to re-start in a new window.

In this case, a pressure node (area of minimum pressure variation) forms at the surface and, as a result, cavitation and implosion do not occur adjacent to the surface.  In essence, a large portion of the sound energy is absorbed as the surface yields to the incoming compression with the reflected portion forming cavitation bubbles at some distance from the surface where  they have minimal benefit to the cleaning process.

Although the models shown above are very simplistic, they illustrate the physics behind the variations in cleanability of surfaces we observe that can be associated with their physical properties.

-  FJF  -