what is a Ultrasonic homogenizer?
Ultrasonic homogenizers, sometimes referred to by the popular brand names Sonicator or Sonifier, disrupt tissues and cells through cavitation and ultrasonic waves. They consist of three parts:
- A generator which provides and controls the power
- A transducer (also referred to as a converter) which contains piezoelectric crystals that convert the electric energy to very high-frequency mechanical motion. The converter also amplifies this signal.
- A titanium probe or “horn” which rapidly vibrates in a longitudinal direction, transmitting the ultrasonic energy to the sample.
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 are powerful tools to mix and homogenize solid-liquid and liquid-liquid suspensions. Ultrasonic homogenization is a mechanical process to reduce particles in a liquid so that they become uniformly small and evenly distributed.
Impact of Ultrasonic Homogenizers
When ultrasonic homogeizers are are used for applications such as mixing, dispersing and emulsifying, the objective is to reduce small particles or droplets in a liquid or slurry to improve uniformity and stability of the mixture. These particles (disperse phase) can be either solids or liquids. A reduction in the mean diameter of the particles increases the number of individual particles. This leads to a reduction of the average particle distance and increases the particle surface area. The graphic shows the correlation between individual particle diameter and total surface area. Surface area and average particle distance can influence the rheology of a liquid. If there is a difference in specific gravity between the particles and the liquid, the homogeneity of the mixture can influence the stability of the dispersion. If the particle size is similar for the majority of the particles, the tendency to agglomerate during settling or rising reduces, because the similar particles have a similar speed of rising or settling.
Advantages of Ultrasonic Homogenizers
When compared to conventional homogenization methods, ultrasonic homogenizers excel with the following qualities: Ultrasonic homogenizers produce small particles / droplets and a narrow distribution curve. Ultrasonic homogenizers can handle high solid concentrations. Ultrasonic homogenizers prepare stable suspensions, dispersions and emulsions. Ultrasonic homogenizers are precisely controllable as the important process parameters (e.g. amplitude, power, time, temperature and pressure) can be influenced and adjusted. Ultrasonic homogenizers are very effective, energy-efficient, user-friendly and safe to operate.
Ultrasonic homogenizers are very efficient for the reduction of soft and hard particles. The homogenization is based on cavitation. When liquids are exposed to intense ultrasonication, sound waves propagate through the liquid causing alternating high-pressure and low-pressure cycles (approx. 20000 cycles/sec.). During the low-pressure cycle, high-intensity small vacuum bubbles are created in the liquid, as the liquid vapor pressure is attained.
When the bubbles reach a certain size, they collapse violently during a high-pressure cycle. During this implosion very high pressures and high speed liquid jets are generated locally. The resulting currents and turbulences disrupt particle agglomerates and lead to violent collisions between individual particles.
One major advantage of ultrasonic homogenizers is the low number of wetted and moving parts. This reduces frictional wear and cleaning time. There are only two wetted parts: The sonotrode and the flow cell. Both have simple geometries and no small or hidden orifices. Another advantage is the exact control over the operational parameters influencing the cavitation.
Benefits & Drawbacks
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.
Tips for Using Ultrasonic Homogenizers
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.
Things to Consider when Purchasing an Ultrasonic Homogenizer
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.