# 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.

## Mechanism and principles

The formation of droplets during ultrasonic atomization remains complex and not fully understood, though several theories attempt to explain it. One leading 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. The average diameter estimation was obtained 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 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 scepticism, 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, with the development in the scope of ultrasound generation (piezoelectricity), transfer (sonotrode materials), and control (horn analyzers).

### Atomization of aqueous solutions

First commercial application of ultrasonic atomization effect was nebulizers. Ultrasonic nebulizers made their first appearance in 1949, initially designed as humidifiers. Medical professionals quickly recognized their potential for delivering therapeutic aerosols suitable for inhalation, leading to the incorporation of medications into the nebulization process. 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

### Aqueous solutions containing metal derivatives

In the late 20th century, scientists exploring nanoparticle synthesis via spray pyrolysis began to testsee ultrasonic atomization as a promising technique for precursor droplet formation such as noble metal based salts solutions. Known as ultrasonic spray pyrolysis (USP), this technique allowed for finer control over particle size as it strongly depends on the frequency, 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 like lithium-ion batteries. 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.

### Liquid metals

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.
In 2017 Żrodowski received a Ministry of Science and Higher Education grant named "Diamentowy grant" during which he studied laser powder bed fusion of zirconium-based bulk metallic glasses. This has resulted in the research and development of ultrasonic atomization of metals for additive manufacturing at Warsaw University of Technology and establishing the spin-off company Amazamet. 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.The sonotrode used in the experiments was made of high-temperature resistant carbon fiber plate to atomize pure aluminum melted at a temperature of 800°C. The ultrasonic atomizer was also used to atomize magnesium alloy, and metals with melting over 1500°C such as zirconium alloy, titanium alloy and high-entropy alloy.