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]

Probe-type sonicator UP100H vs ultrasonic bath: Probe-type sonicators excel with focused ultrasound transmission and reproducible results

Probe-type sonicator vs ultrasonic bath – Explore why probe-type sonicators excel in efficiency and reliability

 

In this video, we compare the extraction power of an ultrasonic bath - also known as an ultrasonic cleaner - with that of a Hielscher UP100H ultrasonic probe.

Mushroom Extraction - Bath vs Probe Ultrasonication - Side-by-Side Comparison

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Cavitation bubbles can be differentiated in stable and transient bubbles. (Click to enlarge!)

Fig. 1: Creation of stable and transient cavitation bubbles. (a) displacement, (b) transient cavitation, (c) stable cavitation, (d) pressure
[adapted from 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.
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)

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

Learn why ultrasonic probes outperform ultrasonic cleaning tanks and bath-type sonicators.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 ultrasonic hot spot occur very unevenly. (Click to enlarge!)

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.
 

Ultrasonic probe-type devices vs ultrasonic tanks. Hielscher Ultrasonics demonstrates the differences in acoustic cavitation fields

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.

Uneven cavitation in an ultrasonic bath (Click to enlarge!)

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.

Comparison probe tank sonication

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.

Sonicator UP200St with sonotrode S26d7D for the batch-type homogenization of eggnog

Probe-type sonicator UP200St with sonotrode S26d7D for the batch-type homogenization of samples

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An ultrasonic probe (sonotrode) is a titanium rod that transmits ultrasound waves into liquids. As result, in the liquid occurs acoustic cavitation, which provides the mechanical shear forces for ultarsonic processing.

Pic 1: Sonotrode transmitting power ultrasound into liquid. The fogging beneath the sonotrode surface indicates the cavitational hot spot area.

Advantages Probe-Sonication:

  • intense
  • focused
  • fully controllable
  • even distribution
  • reproducible
  • linear scale-up
  • batch and in-line

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.

Ultrasonic recirculation set: UIP1000hdT with flow cell, tank and pump

Ultrasonic recirculation set: UIP1000hdT with flow cell, tank 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.

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: 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.

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This video shows the 200 watts ultrasonic cuphorn for dispersing, homogenizing, extracting or degassing of lab samples.

Ultrasonic Cuphorn (200 Watts)

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Literature/References

  • Asadi, Amin; Pourfattah, Farzad; Miklós Szilágyi, Imre; Afrand, Masoud; Zyla, Gawel; Seon Ahn, Ho; Wongwises, Somchai; Minh Nguyen, Hoang; Arabkoohsar, Ahmad; Mahian, Omid (2019): Effect of sonication characteristics on stability, thermophysical properties, and heat transfer of nanofluids: A comprehensive review. Ultrasonics Sonochemistry 2019.
  • Moholkar, V. S.; Sable, S. P.; Pandit, A. B. (2000): Mapping the cavitation intensity in an ultrasonic bath using the acoustic emission. In: AIChE J. 2000, Vol.46/ No.4, 684-694.
  • Nascentes, C. C.; Korn, M.; Sousa, C. S.; Arruda, M. A. Z. (2001): Use of Ultrasonic Baths for Analytical Applications: A New Approach for Optimisation Conditions. In: J. Braz. Chem. Soc. 2001, Vol.12/ No.1, 57-63.
  • Santos, H. M.; Lodeiro, C., Capelo-Martinez, J.-L. (2009): The Power of Ultrasound. In: Ultrasound in Chemistry: Analytical Application. (ed. by J.-L. Capelo-Martinez). Wiley-VCH: Weinheim, 2009. 1-16.
  • Suslick, K. S. (1998): Kirk-Othmer Encyclopedia of Chemical Technology; 4th Ed. J. Wiley & Sons: New York, 1998, Vol. 26, 517-541.



Frequently Asked Questions About Ultrasonic Probes (FAQs)

What is an ultrasonic probe sonicator?

An ultrasonic probe sonicator is a device that uses high-frequency sound waves to disrupt or mix samples. It consists of a probe that, when immersed into a liquid, generates ultrasonic vibrations, leading to cavitation and the desired sample processing effects.

What is the principle of probe sonication?

Probe sonication works on the principle of ultrasonic cavitation. When the probe vibrates in the sample, it creates microscopic bubbles that rapidly expand and collapse. This process generates intense shear forces and heat, disrupting cells or mixing components at a microscopic level.

Is an ultrasonic cleaner the same as a sonicator?

No, they are not the same. An ultrasonic cleaner uses very mild ultrasonic waves in a bath to clean items, mainly through vibration and very littly cavitation. A sonicator, specifically an ultrasonic probe sonicator, is designed for direct, intensive ultrasonic treatment of samples, focusing on disruption or homogenization.

What is the use of an ultrasonic probe?

An ultrasonic probe is primarily used for sample preparation tasks such as cell disruption, homogenization, emulsification, and dispersion of particles in a variety of research and industrial applications across chemistry, biology, and materials science.

What is the difference between probe sonicator and cup-horn?

A probe sonicator directly immerses the probe into the sample for intense sonication. A cup-horn sonicator, on the other hand, does not immerse the probe but uses an indirect method where the sample is placed in a container within a water bath that transmits the ultrasonic energy.

Why use a probe sonicator?

A probe sonicator is used for its ability to deliver direct, high-intensity ultrasonic energy to a sample, achieving efficient disruption, homogenization, or emulsification. It is particularly valuable for tough-to-process samples or when precise control over the process is required.

What are the advantages of a probe sonicator?

Advantages encompass efficient and rapid sample processing, versatility in applications, precise control over the sonication parameters, and the ability to process a wide range of sample sizes and types, from small-volume laboratory samples to larger industrial batches or flow-rates.

How do you use an ultrasonic probe sonicator?

Using an ultrasonic probe sonicator involves selecting the appropriate probe size and sonication parameters, immersing the probe tip into the sample, and then activating the sonicator for the desired time and power settings to achieve effective sample processing.

What is the difference between sonication and ultrasonication?

Sonication refers to the general use of sound waves for processing materials, which can include a range of frequencies. Ultrasonication specifies the use of ultrasonic frequencies (typically above 20 kHz), focusing on applications that require high-energy sound waves for sample processing. However, most people actually refer to ultrasonicators, when they use the word sonicator.

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