Sonication processes can be carried out by the use of a probe-type ultrasonic homogenizer or an ultrasonic bath. Although, both techniques apply ultrasound to the sample, there are significant differences in effectiveness, efficiency and process capabilities. Probe-type ultrasonicators excel ultrasonic bath drastically when it comes to ultrasound intensity, amplitude, uniform processing, and reproducibility.

The desired effects from the ultrasonication of liquids – including homogenization, dispersing, deagglomeration, milling, emulsification, extraction, lysis, disintegration and sonochemical effects – are caused by cavitation. By introducing high power ultrasound into a liquid medium, the sound waves are transmitted in the fluid and create alternating high-pressure (compression) and low-pressure (rarefaction) cycles, with rates depending on the frequency. During the low-pressure cycle, high-intensity ultrasonic waves create small vacuum bubbles or voids in the liquid. When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high-pressure cycle. This phenomenon is termed cavitation. During the implosion very high temperatures (approx. 5,000K) and pressures (approx. 2,000atm) are reached locally. The implosion of the cavitation bubble also results in liquid jets of up to 280m/s velocity. [Suslick 1998]

Moholkar et al. (2000) found that the bubbles in the region of highest cavitation intensity underwent a transient motion, while the bubbles in the region of lowest cavitation intensity underwent stable/ oscillatory motion. The transient collapse of the bubbles that gives rise to local temperature and pressure maxima is at the root of the observed effects of ultrasound on chemical systems.
The intensity of ultrasonication is a function of the energy input and the sonotrode surface area. For a given energy input applies: the larger the surface area of the sonotrode, the lower the intensity of ultrasound.
Ultrasound waves can be generated by different types of ultrasonic systems. In the following, the differences between the sonication using an ultrasonic bath, ultrasonic probe device in an open vessel and ultrasonic probe device with flow cell chamber will be compared.

Comparison of the cavitational hot spot distribution

For ultrasonic applications, ultrasonic probes (sonotrodes / horns) and ultrasonic baths are used. “Among these two methods of ultrasonication, the probe sonication is more effective and powerful than the ultrasonic bath in the application of nanoparticles dispersion; the ultrasonic bath device can provide a weak ultrasonication with approximately 20-40 W/L and a very non-uniform distribution while the ultrasonic probe device can provide 20,000 W/L into the fluid. Thus, it means that an ultrasonic probe device excels the ultrasonic bath device by the factor of 1000.” (cf. Asadi et al., 2019)

Ultrasonic bath

In an ultrasonic bath, cavitation occurs non-conformable and uncontrollably distributed through the tank. The sonication effect is of low intensity and unevenly spread. The repeatability and scalability of the process is very poor.
The picture below shows the results of a foil testing in an ultrasonic tank. Therefore, a thin aluminum or tin foil is placed at the bottom of a water filled ultrasonic tank. After sonication, single erosion marks are visible. Those single perforated spots and holes in the foil indicate the cavitational hot spots. Due to the low energy and the uneven distribution of the ultrasound within the tank, the erosion marks occur only spot-wise. Hence, ultrasonic baths are mostly used for cleaning applications.

The figures below show the uneven distribution of cavitational hot spots in an ultrasonic bath. In Fig. 2, a bath with a bottom area of 20×10 cm has been used.

For the measurements shown in Fig. 3, an ultrasonic bath with a bottom space of 12x10cm has been used.

Both measurements reveals that the distribution of the ultrasonic irradiation field in the ultrasonic tanks is very uneven.
The study of ultrasonic irradiation at various locations in the bath shows significant spatial variations in the cavitation intensity in the ultrasonic bath.

Fig. 4 below compares the efficiency of an ultrasonic bath and an ultrasonic probe device exemplified by the decolorization of azo dye Methyl Violet.

Dhanalakshmi et al. found in their study that probe-type ultrasonic devices have a high localized intensity compared to tank-type and hence, greater localized effect as depicted in Fig. 4. This means a higher intensity and efficiency of the sonication process.
An ultrasonic setup as shown in picture 4, allows for full control over the most important parameters – amplitude, pressure, temperature, viscosity, concentration, reactor volume.

Ultrasonic Probe Device in an open beaker

When samples are sonicated using an ultrasonic probe device, the intense sonication zone is directly beneath the sonotrode/ probe. The ultrasonic irradiation distance is limited to a certain area of the sonotrode’s tip. (see pic.1)
Ultrasonic processes in open beakers are mostly used for feasibility testing and for sample preparation of smaller volumes.

Ultrasonic probe device in continuous flow mode

The most sophisticated sonication results are achieved by a continuous processing in a closed flow-through mode. All material is processed by the same ultrasound intensity as the flow path and residence time in the ultrasonic reactor chamber is controlled.

Pic. 4: 1kW ultrasonic system UIP1000hd with flow cell and pump

The process results of ultrasonic liquid processing for a given parameter configuration are a function of the energy per processed volume. The function changes with alterations in individual parameters. Furthermore, the actual power output and intensity per surface area of the sonotrode of an ultrasonic unit depends on the parameters.

The cavitational impact of ultrasonic processing depends on the surface intensity which is described by amplitude (A), pressure (p), the reactor volume (VR), the temperature (T), viscosity (η) and others. The plus and minus signs indicate a positive or negative influence of the specific parameter on the sonication intensity.

By controlling the most important parameter of the sonication process the process is fully repeatable and the results achieved can be scaled completely linear. Different types of sonotrodes and ultrasonic flow cell reactors allow for the adaption to specific process requirements.

Summary

Whilst a ultrasonic bath provides a weak sonication with approx. 20-40 W/L and a very non-uniform distribution, ultrasonic probe-type devices can easily couple approx. 20.000 W/L into the processed medium. This means that an ultrasonic probe-type device excels an ultrasonic bath by factor of 1000 (1000x higher energy input per volume) due to a focused and uniform ultrasonic power input. The full control over the most important sonication parameters ensures completely reproducible results and the linear scalability of the process results.