Probe-Type Sonication vs. Ultrasonic Bath: An Efficiency Comparison
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
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.
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.
- fully controllable
- even distribution
- linear scale-up
- batch and in-line
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.
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.
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.
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.
Facts Worth Knowing
Ultrasonic tissue homogenizers are often referred to as probe sonicator, sonic lyser, ultrasound disruptor, ultrasonic grinder, sono-ruptor, sonifier, sonic dismembrator, cell disrupter, ultrasonic disperser or dissolver. The different terms result from the various applications that can be fulfilled by sonication.