Cavitazione ultrasonica nei liquidi

Ultrasonic waves of high intensity ultrasound generate acoustic cavitation in liquids. Cavitation causes extreme effects locally, such as liquid jets of up to 1000km/hr, pressures of up to 2000 atm and temperatures of up to 5000 Kelvin. These ultrasonically-generated forces are used for numerous liquid processing applications such as homogenization, dispersing, emulsification, extraction, cell disruption, as well as the intensification of chemical reactions.

The Working Principle of Ultrasonic Cavitation

Quando si sonicano i liquidi ad alta intensità, le onde sonore che si propagano nel mezzo liquido danno luogo a cicli alternati di alta pressione (compressione) e bassa pressione (rarefazione), con velocità che dipendono dalla frequenza. Durante il ciclo di bassa pressione, le onde ultrasoniche ad alta intensità creano piccole bolle o vuoti nel liquido. Quando le bolle raggiungono un volume tale da non poter più assorbire energia, collassano violentemente durante un ciclo ad alta pressione. Questo fenomeno è definito cavitazione. Durante l'implosione si raggiungono localmente temperature (circa 5.000K) e pressioni (circa 2.000atm) molto elevate. L'implosione della bolla di cavitazione provoca anche getti di liquido con velocità fino a 280 m/s.

Acoustic cavitation (generated by power ultrasound) creates locally extreme conditions, so-called sonomechanical and sonochemical effects. Due to these effects, sonication promotes chemical reactions leading to higher yields, faster reaction speed, new pathways, and improved overall efficiency.

Key Applications of Ultrasonicators using Acoustic Cavitation

Probe-type ultrasonicators, also known as ultrasonic probes, efficiently generate intense acoustic cavitation in liquids. Therefore, they are widely used in various applications across different industries. Some of the most important applications of acoustic cavitation generated by probe-type ultrasonicators include:
 

1. Omogeneizzazione: Ultrasonic probes can generate intense cavitation, which is characterised as an energy-dense field of vibration and shear forces. These forces provide excellent mixing, blending and particle size reduction. Ultrasonic homogenization produces uniformly mixed suspensions. Therefore, sonication is used to produce homogeneous colloidal suspension with narrow distribution curves.
2. Nanoparticle Dispersion: Ultrasonicators are employed for the dispersion, deagglomeration and wet-milling of nanoparticles. Low-frequency ultrasound waves can generate impactful cavitation, which breaks down agglomerates and reduces particle size. In particular the high shear of the liquid jets accelerates particles in the liquid, which collide with each other (interparticulate collision) so that the particles consequently break and erode. This results in uniform and stable distribution of particles preventing sedimentation. This is crucial in various fields, including nanotechnology, materials science, and pharmaceuticals.
3. Emulsification and Mixing: Probe-type ultrasonicators are used to create emulsions and mix liquids. The ultrasonic energy causes cavitation, the formation and collapse of microscopic bubbles, which generates intense local shear forces. This process aids in emulsifying immiscible liquids, producing stable and finely dispersed emulsions.
4. Extraction: Due to cavitational shear forces, ultrasonicators are highly efficient in disrupting cellular structures and to improve mass transfer between solid and liquid. Therefore, ultrasonic extraction is widely used to release intracellular material such as bioactive compounds for the production of high-quality botanical extracts.
5. Degassing and Deaeration: Probe-type ultrasonicators are employed to remove gas bubbles or dissolved gases from liquids. The application of ultrasonic cavitation promotes the coalescence of gas bubbles so that they grow and float to the top of the liquid. Ultrasonic cavitation makes degasification a quick and efficient procedure. This is valuable in various industries, such as in paints, hydraulic fluids, or food and beverage processing, where the presence of gases can negatively impact product quality and stability.
6. Sonocatalysis: Ultrasonic probes can be used for sonocatalysis, a process that combines acoustic cavitation with catalysts to enhance chemical reactions. The cavitation generated by ultrasonic waves improves mass transfer, increases reaction rates, and promotes the production of free radicals, leading to more efficient and selective chemical transformations.
7. Sample Preparation: Probe-type ultrasonicators are commonly used in laboratories for sample preparation. They are used to homogenize, disaggregate, and extract biological samples, such as cells, tissues, and viruses. The ultrasonic energy generated by the probe disrupts the cell membranes, releasing cellular contents and facilitating further analysis.
8. Disintegration and Cell Disruption: Probe-type ultrasonicators are utilized to disintegrate and disrupt cells and tissues for various purposes, such as extraction of intracellular components, microbial inactivation, or sample preparation for analysis. The high-intensity ultrasonic waves and the thereby generated cavitation cause mechanical stress and shear forces, resulting in the disintegration of cell structures. In biological research and medical diagnostics, probe-type ultrasonicators are used for cell lysis, the process of breaking open cells to release their intracellular components. Ultrasonic energy disrupts cell walls, membranes, and organelles, enabling the extraction of proteins, DNA, RNA, and other cellular constituents.

 
These are some of the key applications of probe-type ultrasonicators, but the technology has an even wider range of other uses, including sonochemistry, particle size reduction (wet-milling), bottom-up particle synthesis, and sono-synthesis of chemical substances and materials in various industries such as pharmaceuticals, food processing, biotechnology, and environmental sciences.

Una sequenza ad alta velocità (da a a f) di fotogrammi che illustrano l'esfoliazione sono-meccanica di un fiocco di grafite in acqua utilizzando l'UP200S, un ultrasuonatore da 200 W con sonotrodo da 3 mm. Arrows show the place of splitting particles with cavitation bubbles penetrating the split.
© Tyurnina et al. 2020

Video of Acoustic Cavitation in Liquid

The following video demonstrates acoustic cavitation at the cascatrode of the ultrasonicator UIP1000hdT in a water-filled glass column. The glass column is illuminated from the bottom by red light in order to improve the visualization of the cavitation bubbles.

La tabella seguente fornisce un'indicazione della capacità di lavorazione approssimativa dei nostri ultrasonori: