Ultrasonic cleaners use ultrasound and cleaning agents to remove contaminants from the object’s surface. They lean on the ability of sound waves to propagate in a medium and vibrate its molecules, allowing them to access and clean every nook and cranny of the object’s surface.
How Do Ultrasonic Cleaners Work?
At the most basic, ultrasonic cleaners consist of:[1-2]
- One or more metal chambers filled with water or cleaning liquid. Items for cleaning are immersed in the liquid-filled chamber before generating the ultrasound. Most cleaners have a removable stainless steel mesh basket where items are placed and immersed in the solution without touching the chamber surface. Some have hinges or slots in the chamber where a metal tray or beakers can be suspended without touching the chamber surface.
- An ultrasonic generator converts the electrical energy from the power outlet into the ultrasonic frequency so that it can be transduced into ultrasound.
- An ultrasonic transducer converts alternating electrical energy into mechanical waves that ultimately cause the liquid molecules to vibrate.
- A heater is assembled with the generator and transducer to heat the solution in the chamber, promoting the cleaning and rinsing reactions.
Ultrasonic cleaning technology is considered superior to other cleaning methods. It can remove unwanted particles and destroy contaminants such as viral particles, bacteria, and fungal spores on the item’s surface. The device can clean durable objects like surgical tools, laboratory ware, and delicate items such as jewelry, circuit boards, and silicon wafers.
What Does Ultrasound Do?
Ultrasound is a high-frequency sound wave above 18 kHz, which is inaudible to the human ears. In ultrasonic cleaners, generated ultrasound induces cavitation in the cleaning liquid.
When a sound wave is generated, it is transmitted through a medium, temporarily displacing molecules of the sound-conducting medium. As the wave passes, the molecules compress and enter the compression stage. They decompress and enter the rarefaction phase as the wave progresses through the medium. These phases occur in alternation until the wave dissipates.
At the compression area, the pressure of the medium is positive and becomes negative during rarefaction. The negative pressure increases as the amplitude increases, leading to a formation of a powerful burst of the jet stream made from vacuum bubbles at the rarefaction area, termed cavitation.
The pressure becomes positive when the molecules enter the compression phase, causing the cavitation bubbles to expand and implode. This implosion results in a staggering increase in the temperature that could generate a shock wave at the implosion site.
How Does Cavitation Assist Cleaning?
1. Enhanced dissolution
Ultrasound-induced cavitation decreases the time required for dissolving contaminants. When the cleaning solution dissolves the contaminants to saturation, it forms a layer between the undissolved contaminants and the fresh solution. The cavitation jet stream can dislocate the layer of saturated solution, reducing the time contaminants come into contact with the fresh cleaning solution.
2. Chemical bond interruptions
Soluble and insoluble molecules in contaminants are held together by non-covalent bonds such as ionic, adhesive, and cohesive forces. Cavitation from ultrasound can interfere with these loose chemical bonds, dissociating these molecules so that each can come into contact with the cleaning solutions.
3. Expedited cleaning reactions
After cavitation and implosions, the pressure and temperature of the cleaning solution are remarkably high. As a result, chemical reactions at the implosion site are accelerated, including contaminant dissociation.
Types of Ultrasonic Cleaners
Ultrasonic cleaners can be categorized based on the cavitation properties as follows:
- Ultrasonic cleaning refers to cleaning devices that produce ultrasound waves lower than 500 kHz. Most devices use waves between 20 to 350 kHz, which make transient cavitation bubbles. These bubbles implode many times before the ultrasonic waves dissipate, leading to a considerable increase in pressure and temperature followed by shockwaves that erode the item’s surface.
- Megasonic cleaning uses higher frequency ultrasound waves in the cleaning process. The frequencies range from 500 to 2000 kHz, producing smaller cavitation bubbles. Since the cavitation bubbles from megasonic cleaning are smaller, they tend to last through several cycles and exert less energy when imploded.
Apart from the frequency, ultrasonic cleaners are also classified based on construction:
- Single-tank ultrasonic cleaners are table-top ultrasonic cleaners containing one chamber for ultrasonic cleaning. Cleaned items are rinsed and dried in separate containers or other equipment. Single-tank ultrasonic cleaners are frequently used in small to medium laboratories, clinics, and jeweler’s stores.
- Multiple-tank ultrasonic cleaners consist of three separate chambers that clean, rinse, and dry items after cleaning. Some may contain additional chambers for pre-cleaning. These cleaners are heavy-duty and used in high-throughput production lines.
- An ultrasonic rod transducer is a portable ultrasonic device shaped into a rod. It generates ultrasonic waves that emanate from all directions when the rod is immersed in the liquid medium. They are suitable for cleaning cylindrical and hollow-shaped objects.
How to Operate Ultrasonic Cleaners
- Choose appropriate cleaning and rinsing solutions.
- Prepare the solution following the manufacturer’s instructions and fill the chamber to at least the minimum operating level.
- Cover the chamber and turn on the degas function. If no dedicated degas function exists, degas the cleaning solution by heating and letting the machine generate the ultrasonic wave without any item for 5 to 10 minutes.
- Items can be suspended on a wire or placed in a mesh basket. Gradually lower the basket into the chamber filled with the cleaning solution. Items can also be placed in a beaker or tray filled with a cleaning solution. The container is hinged onto the chamber wall or placed in a designated spot.
- Place the cover and turn on the ultrasonic function, heater, and timer. Repeat this step two to three times, if necessary.
- When the items are cleaned, turn off the ultrasonic cleaner and heater. Slowly remove the items and containers from the chamber, rinse the cleaned item in another container, and dry them in open-air or in a drying oven. In the case of multi-tank cleaners, transfer the basket containing the cleaned items to the subsequent chambers for rinsing and drying.
Do’s and Don’ts of Using Ultrasonic Cleaners
Ultrasonic cleaners are easy-to-use and do not require specialized training. However, workers should be informed of potential electrical and chemical hazards, safety measures, and precautions that could circumvent any harm to the operator and device.
To prevent electrical shock:
- Do not immerse ultrasonic cleaners in water or cleaning solution.
- Avoid spills over the control panel.
- Unplug from the power outlet before filling or emptying the chamber.
To prevent damage to items and the cleaner, or harm to the operator:
- Place a closed cover on top of the chamber before initiating the ultrasonic function.
- Make sure that the cleaning and rinsing solutions are compatible with the device. Generally, they are inflammable water-based solutions that are mildly acidic or alkaline.
- Do not use strong acids or bases dilutions as cleaning or rinsing solutions.
- Never touch the exterior of the cleaner or solution when cleaning. Use a thermometer to test the temperature, or drop pieces of thin aluminum foil to test the working and uniformity of the ultrasound. Punctured and wrinkled aluminum foils indicate a working ultrasonic system.
- Regularly exchange the cleaning and rinsing solutions after use. Do not skip the degassing step, and ensure the amount meets the minimum operating level before ultrasonic cleaning.
Factors to Consider When Choosing Your Ultrasonic Cleaners
Apart from the capacity, here are the factors to consider when choosing an ultrasonic cleaner:
1. Ultrasonic vs. megasonic cleaning?
Ultrasonic cleaning is generally harsher than megasonic cleaning. The former often generates shockwaves that erode the items’ surface. Therefore, if the target items have delicate parts on the surface, megasonic cleaning is a better choice.
2. Temperature, ultrasonic power, and frequency setting?
High cleaning temperature, ultrasonic power, and low vibrating frequency add to the cavitation intensity. Most ultrasonic cleaners operate at a temperature of 70-80°C and a specific ultrasonic frequency and power. This condition is optimum when water is used as the cleaning medium, and it applies to most durable objects.
Nonetheless, the default condition can be too rough for objects with different materials or contaminants. Consider choosing an ultrasonic cleaning device with adjustable temperature and frequency if it is expected to clean various materials.
3. Cleaning and rinsing solution
The cleaning and rinsing solution are essential to the efficiency of ultrasonic cleaning. They must be compatible with the cleaning device so that they don’t react with the device surface but the contaminations on the item’s surface.
A good cleaning and rinsing solution should be inflammable and free from bleaches or inorganic acids. It shouldn’t be too vicious when diluted to the working concentration so that the effect of cavitation can be maximized.
Ultrasonic cleaning technology is regarded as a standard cleaning technology. It uses ultrasound and compatible cleaning solutions to produce cavitation bubbles, thoroughly removing contaminants from the object’s surface.
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- Fuch, FJ. “Chapter 2.2 The Fundamental Theory and Application of Ultrasonics for Cleaning” Handbook for Critical Cleaning, edited by Barbara Kanegsberg and Edward Kanegsberg, CRC Press, 2001