Probe-Type Sonication vs. Ultrasonic Bath: An Efficiency Comparison

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.

The desired effects of sonicating liquids – including homogenization, dispersing, deagglomeration, milling, emulsification, extraction, lysis, disintegration, sonochemistry – are caused by acoustic 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]

Fig. 1: Creation of stable and transient cavitation bubbles. (a) displacement, (b) transient cavitation, (c) stable cavitation, (d) pressure
[phỏng theo Santos et al. 2009]

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.
Cường độ của ultrasonication là một chức năng của đầu vào năng lượng và diện tích bề mặt sonotrode. Đối với một đầu vào năng lượng nhất định được áp dụng: diện tích bề mặt của sonotrode càng lớn, cường độ siêu âm càng thấp.
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)

Comparison of Cavitational Hot Spot Distribution

In the realm of ultrasonic applications, both ultrasonic probes (sonotrodes/horns) and ultrasonic baths play pivotal roles. However, when it comes to nanoparticle dispersion, probe sonication significantly outperforms ultrasonic baths. According to Asadi et al. (2019), ultrasonic baths typically generate a weaker ultrasonication of about 20-40 W/L with a highly non-uniform distribution. In stark contrast, ultrasonic probe devices can deliver an astonishing 20,000 Watts per liter into the fluid, showcasing an effectiveness that surpasses ultrasonic baths by a factor of 1000. This marked difference highlights the superior capability of probe sonication in achieving efficient and uniform nanoparticle dispersion.

Ultrasonic Baths

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.
 

In an ultrasonic bath or tank, the hot spot of acoustic cavitation occurs very unevenly.

 
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.
 

Fig.2 shows the spatial distribution of the ultrasonic field in the ultrasonic bath:
(a) using 1 L of water in the bath and (b) using the total volume of 2 L of water in the bath.
[Nascentes et al., 2010]

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

Fig. 3 shows the spatial distribution of the ultrasonic field in an ultrasonic bath:
(a) using 1 L of water in the bath and (b) using the total volume of 1.3 L of water in the bath.
[Nascentes et al., 2001]

 
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.

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

Fig. 4: Probe-type sonicators deploy localized very high energy intensity in comparison to the low ultrasound density of ultrasonic tanks and baths.

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 figure 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, such as amplitude, pressure, temperature, viscosity, concentration, reactor volume.

The Advantages of Probe-Type Sonicators

Ultrasonic probes or sonotrodes are designed to concentrate ultrasonic energy into a focused area, typically at the tip of the probe. This focused energy transmission allows for precise and efficient treatment of samples. As the probe design ensures that a significant portion of the ultrasonic energy is directed towards the sample, the energy transfer is significantly enhanced when compared to ultrasonic baths. This focused transmission of ultrasound power is particularly advantageous for applications requiring precise control over sonication parameters, such as cell disruption, nano-dispersion, nanoparticle synthesis, emulsification, and botanical extraction.
Therefore, probe-type sonicators offer distinct advantages over ultrasonic baths in terms of precision, control, flexibility, efficiency, and scalability, making them indispensable tools for a wide range of scientific and industrial applications.

Probe-type Sonicators for Open Beaker Processing

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 tip. (see pic.1)
Ultrasonic processes in open beakers are mostly used for feasibility testing and for sample preparation of smaller volumes.

Probe-type Sonicators with Flow Cell for Inline Processing

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.

Bộ tuần hoàn siêu âm: UIP1000hdT với tế bào dòng chảy, bể chứa và bơm

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.

Tác động xâm thực của xử lý siêu âm phụ thuộc vào cường độ bề mặt được mô tả bởi biên độ (A), áp suất (p), thể tích lò phản ứng (VR), nhiệt độ (T), độ nhớt (η) và các loại khác. Các dấu cộng và trừ cho thấy ảnh hưởng tích cực hoặc tiêu cực của thông số cụ thể đến cường độ sonication.

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: Probe-Type Sonicator vs Ultrasonic Bath

Whilst an ultrasonic bath provides a weak sonication with approx. 20 Watts per liter, only and a very non-uniform distribution, probe-type sonicators can easily couple approx. 20000 Watts per liter into the processed medium. This means that an ultrasonic probe-type sonicator 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.