Homogenization occurs both directly through the ultrasonic forces as well as through cavitation - the rapid formation and collapse of bubbles. This combination of forces make ultrasonic homogenizers excellent for cell disruption and particle size reduction.
In the video below (note: the video has sound), you can see the powerful disruption of the water below the tip. The “foggy” appearance in the water is microscopic bubbles forming and collapsing.
Note that ultrasonic homogenizers may deposit trace amounts of titanium in the sample. While this does not have a practical effect in the vast majority of applications, if you are concerned that trace amounts of titanium in your sample may affect your downstream procedures, you may want to consider another homogenizer.
Ultrasonic homogenizers excel when your sample is mostly liquid or consists of small particles in a liquid and you desire thorough homogenization. These include particle size reduction, cell disruption, creating emulsions, and dispersion of nanoparticles or other small particles. Additionally, certain applications requiring very high energy over a small area, such as DNA shearing, may be performed using an ultrasonic homogenizer.
Ultrasonic homogenizers are unsuitable for applications where your sample is mostly solid and / or not immersed in a liquid buffer. For example, homogenization of large, tough tissue samples is generally only possible if the tissue is macerated prior to processing. Additionally, ultrasonic processing for life science applications where intact cells or organelles are desired.
Ultrasonic homogenization generates a very large amount of heat. For heat-sensitive applications, such as RNA extraction, an ultrasonic homogenizer may not be appropriate. If one is to be used, ensure that you have a way of cooling your sample and use a pulsed homogenization program to allow heat to dissipate between pulses.
Ultrasonic homogenizers are very powerful but also create a large amount of heat and noise - a consequence of them using ultrasonic energy. Because of this, it is highly recommended that you use ear protection during homogenization and / or place them in an insulated enclosure to reduce the noise levels. Most ultrasonic homogenizers have such enclosures available as optional accessories. Also, if heating is a concern, samples should be homogenized on ice or cooled in another manner during processing. Many units have specialized horns which use a flow water around the sample to help cool the samples during processing. These are offered as optional extras.
To maximize the life of your horn, clean it and dry it after each use. Failure to properly maintain the horn can lead to pitting (localized corrosion which creates small holes in the titanium) and this pitting will decrease the effectiveness of the horn.
Ultrasonic homogenizers must be used with a liquid sample. They are not designed to be used for dry homogenizing applications. Immerse the horn into the sample before turning on the instrument, and ensure the horn does not touch the sides of the container during operation.
When considering the purchase of an ultrasonic homogenizer, take a careful look at the horns which are available for that unit. Ensure that they meet your sample size needs. Also, some have special horns, such as cup horns, which indirectly homogenize samples in sealed microtubes and which help keep samples cool, or flow-through horns, which allow a semi-continuous stream of sample to be processed and which are often water-jacketed for improved cooling.
If you are going to be homogenizing samples which contain a gritty solid, this may lead to increased wear and pitting of the horn tip, which will decrease the effectiveness of homogenization. To help reduce costs while maintaining efficient homogenization, see if you can get a horn that has replaceable tips or a horn which is designed to operate under these conditions.
While a higher power homogenizer determines how large of a sample it can handle, a higher-wattage unit can also be used to homogenize smaller samples more thoroughly and more quickly. Don’t just purchase the cheapest unit that meets your sample size needs unless you are confident that your homogenization should be fairly easy.