Application of Power Ultrasound using Ultrasonic Horns

Ultrasonic horns or probes are used widely for manifold liquid processing applications including homogenization, dispersing, wet-milling, emulsification, extraction, disintegration, dissolving, and de-aeration. Learn the basics about ultrasonic horns, ultrasonic probes and their applications.

Ultrasonic Horn vs Ultrasonic Probe

Often, the term ultrasonic horn and probe are used interchangeably and refer to the ultrasonic rod that transmits the ultrasound waves into the liquid. Other terms that are used for the ultrasonic probe are acoustic horn, sonotrode, acoustic waveguide, or ultrasonic finger. However, technically there is a difference between an ultrasonic horn and an ultrasonic probe.
Both, horn and probe, refer to parts of the so-called probe-type ultrasonicator. The ultrasonic horn is the metal part of the ultrasonic transducer, which gets excited through piezoelectrically generated vibrations. The ultrasonic horn vibrates at a certain frequency, e.g. 20kHz, which means 20,000 vibrations per second. Titanium is the preferred material for the fabrication of ultrasonic horns due to its excellent acoustic transmission properties, its robust fatigue strength, and surface hardness.

The ultrasonic probe is also called sonotrode or ultrasonic finger. It is a metal rod, most often made from titanium, and threaded to the ultrasonic horn. The ultrasonic probe is an essential part of the ultrasonic processor, that transmits the ultrasound waves into the sonicated medium. Ultrasonic probes / sonotrodes are in various shapes (e.g. conical, tipped, tapered, or as Cascatrode) available. Whilst titanium is the most commonly used material for ultrasonic probes, there are also sonotrode made from stainless steel, ceramic, glass and other materials available.

Since the ultrasonic horn and probe are under constant compression or tension during sonication, the material selection of horn and probe are crucial. High-quality titanium alloy (grade 5) is considered the most reliable, durable and effective metal to withstand stress, to sustain high amplitudes over long periods of time, and to transmit the acoustical and mechanical properties.

High-performance ultrasonicators mostly work in the frequency range of 20-30kHz. At 20 kHz, the ultrasonic probe is typically a one-half wavelength long resonant rod, which is constantly expanding and contracting 20,000 times per second. The expansion and contraction movements are transmitted as high-power ultrasound into the process medium, i.e. fluid or slurry, in order to fulfil applications such as

• ultrasonic high-shear mixing
• ultrasonic wet-milling
• ultrasonic dispersion of nano-particles
• ultrasonic nano-emulsification
• ultrasonic extraction
• ultrasonic disintegration
• ultrasonic cell disruption and lysis
• ultrasonic degassing and de-aeration
• sono-chemistry (sono-synthesis, sono-catalysis)

How does Power Ultrasound Work? – The Working Principle of Acoustic Cavitation

For high-performance ultrasonic application such as homogenization, particle size reduction, disintegration or nano-dispersions, high-intensity, low-frequency ultrasound is generated by an ultrasound transducer and transmitted via ultrasonic horn and probe (sonotrode) into a liquid. High-power ultrasound is considered ultrasound in the range of 16-30kHz. The ultrasound probe expands and contracts e.g., at 20kHz, thereby transmitting respectively 20,000 vibrations per second into the medium. When the ultrasonic waves travel through the liquid, alternating high-pressure (compression) / low-pressure (rarefaction / expansion) cycles create minute cavities (vacuum bubbles), which grow over several pressure cycles. During the compression phase of the liquid and bubbles, the pressure is positive, while the rarefaction phase produces a vacuum (negative pressure.) During the compression-expansion cycles, the cavities in the liquid grow until they reach a size, at which they cannot absorb further energy. At this point, they implode violently. The implosion of those cavities results in various highly energetic effects, which are known as the phenomenon of acoustic / ultrasonic cavitation. Acoustic cavitation is characterized by manifold highly energetic effects, which impact liquids, solid/liquid systems as well as gas/liquid systems. The energy-dense zone or cavitational zone is known as so-called hot-spot zone, which is most energy-dense in the close vicinity of the ultrasonic probe and declines with increasing distance from the sonotrode. The main characteristics of ultrasonic cavitation include locally occurring very high temperatures and pressures and respective differentials, turbulences, and liquid streaming. During the implosion of ultrasonic cavities in ultrasonic hot-spots, temperatures of up to 5000 Kelvin, pressures of up to 200 atmospheres and liquid jets with up to 1000km/h can be measured. These outstanding energy-intense conditions contribute to sonomechanical and sonochemical effects that intensify processes and chemical reactions in various ways.
The main impact of ultrasonication on liquids and slurries are the following:

• High-shear: Ultrasonic high-shear forces disrupt liquids and liquid-solid systems causing intense agitation, homogenization and mass transfer.
• Impact: Liquid jets and streaming generated by ultrasonic cavitation accelerate solids in liquids, which leads subsequently to interparticluar collision. When particles collide at very high speeds, they erode, shatter and get milled and dispersed finely, often down to nano-size. For biological matter such as plant materials, the high velocity liquid jets and alternating pressure cycles disrupt the cell walls and release the intracellular material. This results in highly efficient extraction of bioactive compounds and the homogeneous mixing of biological matter.
• Agitation: Ultrasonication causes intense turbulences, shear forces and micro-movement in the liquid or slurry. Thereby, sonication always intensifies mass transfer and accelerates thereby reactions and processes.

Ultrasonic homogenizers and high-shear mixers are used in almost any processing industry, that works with liquids or slurries. The intense ultrasonic cavitational forces create intense agitation, shear, particle breakage and mass transfer. Thereby, liquids are homogenized, dispersed, emulsified, extracted, dissolved and/or chemical reactions are initiated. Overall, ultrasonication is a process intensifying method that increases yield, improves conversion rates and makes processes more efficient.
Common ultrasonic applications in the industry are spread across many branches of food & pharma, fine-chemistry, energy & petrochemistry, recycling, biorefineries, etc. and include the following:

• ultrasonic biodiesel synthesis
• ultrasonic homogenization of fruit juices
• ultrasonic production of vaccines
• ultrasonic Li-ion battery recycling
• ultrasonic synthesis of nano-materials
• ultrasonic formulation of pharmaceuticals
• ultrasonic nano-emulsification of CBD
• ultrasonic extraction of botanicals
• ultrasonic sample preparation in laboratories
• ultrasonic degasification of liquids
• ultrasonic desulphurization of crude
• and many more …

Ultrasonic Horns and Probes for High-Performance Applications

Hielscher Ultrasonics is long-time experiences manufacturer and distributor of high-power ultrasonicators, which are worldwide used for heavy-duty applications in many industries.
With ultrasonic processors in all sizes from 50 watts to 16kW per device, probes at various sizes and shapes, ultrasonic reactors with different volumes and geometries, Hielscher Ultrasonics has the right equipment to configure the ideal ultrasonic setup for your application.
The table below gives you an indication of the approximate processing capacity of our ultrasonicators: