Ultrasonic atomization

Ultrasonic atomization

Ultrasonic atomization is a process in which a liquid, in contact with a surface vibrating at ultrasonic frequencies, forms standing capillary waves that lead to the ejection of fine droplets. As the amplitude of these waves increases, the wave crests can reach a critical height where the cohesive forces of the liquid are overcome by the surface tension, leading to the ejection of small droplets from the wave tips.

Principles of Ultrasonic Atomization

The formation of droplets under MHz-range ultrasound remains complex and not fully understood, though several theories attempt to explain it. One main theory, the capillary wave hypothesis by Lang, suggests that droplets form at the peaks of capillary waves on the liquid surface. Lang developed a formula that relates droplet size to capillary wavelength, he average diameter was using a constant that was later adjusted by Yasuda to better predict smaller droplet sizes in the micrometer range. This prediction aligns well with observations from laser diffraction, though other methods have detected even finer droplets that Lang's model does not account for. An alternative theory, proposed by Sollner, is the cavitation hypothesis. This theory links droplet formation to cavitation—when bubbles in the liquid rapidly form and collapse, creating shockwaves that break apart the liquid surface into droplets. Sollner’s findings suggest cavitation is essential for dispersing liquids and shares similarities with emulsion formation. A combined theory was later proposed by Bograslavski and Eknadiosyants, suggesting that both mechanisms work together: shockwaves from cavitation enhance the breaking of capillary wave crests, leading to droplet formation. However, this combined theory faces some skepticism, as cavitation requires high power at MHz frequencies, which some researchers argue may be too high to support this mechanism effectively in practice.

History

The phenomenon of ultrasonic atomization was first reported by Wood and Loomis in 1927. They observed that a fine mist was produced from the liquid surface when a liquid layer was subjected to high-frequency sound waves. Wood and Loomis's work hinted at a variety of applications for ultrasonics, many of which became realities in later decades. Their findings on cavitation, for example, are crucial to ultrasonic cleaning, sonochemistry, and even medical therapies.

Medical and healthcare

Ultrasonic nebulizers made their first appearance in 1949, initially designed as humidifiers. Medical professionals quickly recognized their potential for delivering therapeutic aerosols, leading to the incorporation of medications into the nebulization process. The technology behind ultrasonic nebulizers involves the use of piezoelectric crystals that vibrate at high frequencies (1 to 3 MHz). This vibration generates ultrasonic waves that break liquid medications into fine aerosol particles suitable for inhalation. The 1950s marked a period of increased interest in aerosol therapy, with ultrasonic nebulizers gaining traction due to their silent operation and ability to produce fine mist. Over the years, ultrasonic nebulizers have undergone significant design improvements, becoming smaller, lighter, and more efficient. These advancements have made them more user-friendly and suitable for home use, particularly for patients with chronic respiratory conditions. Ultrasonic nebulizers have been utilized for various respiratory diseases, including asthma and cystic fibrosis. Their ability to deliver medications directly to the lungs has made them a valuable tool in managing these conditions.

Nanotechnology

In the late 20th century, scientists exploring nanoparticle synthesis began to see ultrasonic atomization as a promising technique. By generating fine, controlled mists, ultrasonic atomization could produce uniform precursor droplets that would later be used in a high-temperature environment to form nanoparticles. Known as ultrasonic spray pyrolysis (USP), this technique emerged as an alternative to traditional particle production methods, which were often less precise. USP technology allowed for fine control over particle size and distribution, making it particularly suited for nanomaterials used in electronic devices, solar cells, and batteries. By the 1980s and 1990s, ultrasonic atomization was gaining ground as researchers demonstrated its utility in producing complex oxides and other materials essential for energy storage and electronics.

Through the 1990s and early 2000s, ultrasonic atomization continued to evolve within nanotechnology as researchers refined the process to achieve tighter control over particle morphology, a critical requirement in applications like fuel cells and lithium-ion batteries. Ultrasonic atomization became particularly significant in the development of thin-film technologies, which rely on uniform, precisely deposited layers to function efficiently. By the early 2000s, this method was integral to industries seeking uniform coatings and nanoparticle films, demonstrating the impact of ultrasonic atomization on industrial manufacturing.

Ultrasonic atomizer up to 350 °C

Additive manufactoring

In 1965, Pohlman and Stamm published a book, which marked a contribution to the field of ultrasonic atomization by identifying and describing the parameters influencing the process such as viscosity, capillary wavelength, surface tension and amplitude. One of the key chapters in the book, titled "5.1 Vernebelung geschmolzener Metalle," detailed the first experiments on the high temperature ultrasonic atomization in which molten metals were used. They discussed its potential technical applications as well as limitations stating that the transition from successful laboratory experiments to a usable technical plant has not yet been found due to issues with conciliation wettability and sonotrode durability. They were able to atomize lead at 350 °C and showcased the damage to the sonotrode induced by cavitation. In 1967, Lierke and Grießhammer published their work in which they were able to ultrasonically atomize metal with melting points up to 700 °C.

RePowder - up to 1300 °C atomization system

In 2017 Żrodowski received a Ministry of Science and Higher Education grant named “Diamentowy grant” during which he studied laser powder bed fusion of bulk metallic glasses. This has resulted in the development of ultrasonic atomization of metals for additive manufacturing at Warsaw University of Technology and establishing the spin-off company “Amazemet Sp. z o.o.”. In 2024 a joint article lead by Dmitry Eskin and Iakovos Tzanakis in which new insights into the mechanism of ultrasonic atomization were described stating that the cavitation during the process plays a critical role in the ultrasonic atomization which was also filmed for the first time using high-speed imaging.

Futher application

Ultrasonic atomization is observed in applications ranging from consumer products like air humidifiers, ultrasonic toothbrushes, medical like, microencapsulation to industrial uses, such as ultrasonic nozzles for spray drying and the production of metal powders for additive manufacturing and even analytical usage as plasma–mass spectrometry (inductively coupled plasma mass spectrometry).

Studied materials

List of materials that have been successfully atomized using ultrasonic atomization:

• water
• ethanol - water mixtures
• rice wine
• Wood's metal
• zinc alloy
• magnesium alloy
• zirconium alloy
• titanium alloy
• high-entropy alloy

See also

• nebulizer
• metered-dose inhaler
• metal powder
• induction plasma
• atomization