Ultrasonic equipment usage has spiked in recent years in many different fields of research including chemical, biological and engineering. In recent years, they have gone from relative obscurity to mainstream as more and more researchers realize their potential. There are a variety reasons for the increased use of the different types of sonication equipment that you may find beneficial for your own experiments. In this article you will learn how the various types of sonication equipment are being used in research today, learn how they work and learn how they can be useful for your research.

One of the reasons ultrasonic equipment was rarely used in the past was because it was an unknown technique to many so in this article we’ll discuss the principles behind the technique and detail the two most common methods- ultrasonic baths and ultrasonic probes (also known as sonicators).

Principles of Sonochemistry

Ultrasonic devices produce acoustic cavitation bubbles into fluidic media. The ultrasonic waves cause oscillations which compress and stretch the molecules within the media, so that a negative pressure is built up. This causes the molecules to exceed the molecular distance required to keep the liquid medium intact and causes the liquid to break down and create voids. These voids are high in energy and collapse to induce a large amount of localized energy (5000 K, 2000 atm) into the media.

Some key parameters that can affect your sonication experiment are frequency, amplitude, temperature and pressure. Lower frequencies produce a higher amount of cavitation bubbles, whereas they are limited at higher frequencies. Amplitude is directly related to the energy input, so the higher the amplitude, the greater the amount of cavitation bubbles. For many samples, the amplitude needs to reach a threshold for the viscosity of the media to be distorted. Lower temperatures provide a greater amount of cavitation bubbles, because higher temperatures promote molecular diffusion away from the ultrasonic source. Lastly, an external pressure on the sonication device pushes the molecules closer together making it harder to produce cavitational voids.

For most experiments, water is the preferred solvent of choice due to its low viscosity and surface tension. However, in some cases apolar liquids such as organics solvents can be used providing that the surface tension and viscosity are low.

Ultrasonic Baths

If you are looking to break up aggregates and flocculations or disperse media e.g. nanotubes, then an ultrasonic bath is the method of choice. They can also be used to clean equipment. Ultrasonic baths produce low, indirect energy waves. Because they are indirect the ultrasonic waves have to pass both the water in the water bath and the container that the sample is in. This also causes and uneven energy distribution.

The low energy can breakdown the intermolecular interactions between aggregated particles so that the long range repulsion forces in the sample can reform, therefore dispersing the particles. But the energy is not high enough to disrupt and breakdown the molecules themselves. A similar approach is used in ultrasonic cleaning where the ultrasonic waves breakdown the interactions of dirt and bacteria on a surface, cleaning it. They have specific uses which are limited but can be used with molecules that are sensitive to changes in environment.

Ultrasonic Probes

Sonication probes provide the highest energy of all sonication devices because it is in direct contact with the media. The energy produced can be up to 100 times greater than ultrasonic baths. There are four main components to an ultrasonic probe- the generator, which is the source of electrical energy; an ultrasonic convertor, which converts the energy into ultrasonic vibrations; standard and booster horns which amplify the signal; and the probe itself (generally a titanium alloy), where the energy is introduced into the sample via the probe tip.

The length of the probe is important, as is the tip. The probe length allows for amplification of the ultrasonic waves into the sample (the longer the probe, the greater the amplification). The tip provides a high surface area where the waves leave the probe. Over time, the tip becomes degraded and pitted causing the probe to be less effective. This is a 3-stage process- shiny tip >>> dull tip >>> pitted tip. Many probe tips are screwed in, so they can be replaced when this happens. Whilst very useful and resistant to thermal degradation, sonication probes can also contaminate samples with metal ions. Probes made of silica glass and other metal alloys are available to reduce this factor. However, glass probes do not amplify the ultrasonic waves as effectively as metal probes.

There is an important term known as the ‘dead zone’ for sonication probes. It refers to the area of the sample far away from the probe, which doesn’t have induced cavitations. To minimise this, small cups to hold the media are generally used so that the distance from the media to the probe is reduced. However, the probe needs sufficient space away from the container otherwise it can break.

Another approach is a flow-through cell. Flow-through cells have one or more inlets and an outlet. This causes the solution to flow (by a pump), allowing for a greater surface area to be exposed to the probe. Flow-through cells are the most effective approach, but also the most complex.

Because of the high temperature and energies produced, sonication probes are best used with robust molecules. However, molecules such as proteins can be used e.g. in the production of Air Filled Emulsions, but they can be susceptible to denaturing if they are exposed to the energy for too long. In these cases, a flow-through cell makes the process less damaging.

Sonicators are now used over a wide range of applications including cell extraction, templating reactions, food formulations, homogenization, emulsions, nanodispersions, semi-conductors, protein self-assembly, breaking up cell lysis, bio-fuels, transesterification, polymer processing and the shortening of the reaction time for organic chemistry reactions, to name a few.

Ultrasonic probes are also scalable, as many sizes up to the pilot and industrial scale can be purchased. The energy to volume ratio can always be made relative to smaller scale probes due to the adjustability of the amplitude.