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.
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,[2] 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[3] 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,[4] 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,[5] 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.[6]
History
The phenomenon of ultrasonic atomization was first reported by Wood and Loomis in 1927.[7] 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,[8] leading to the incorporation of medications into the nebulization process.[9] 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[10]
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.[11] 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.[12]
In 1965, Pohlman and Stamm[13] 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.[14]
Before 2016, ultrasonic atomization of metals remained largely experimental, hindered by sonotrode erosion, process instability, and difficulties in achieving spherical powders.[15] These challenges were linked to insufficient cooling of the ultrasonic horn and limited control of the inert-gas atmosphere.[16] In contrast, gas atomization was the established industrial method, producing kilogram-scale batches with relatively consistent output.[17][18]
However, gas atomization also had limitations for metal additive manufacturing: powders often contained satellite particles and gas porosity, showed broad particle size distributions with a significant fraction of coarse particles, and were less suitable for reactive alloys such as titanium, which required crucible-free techniques like Electrode Induction Melting Gas Atomization (EIGA) [19] .[20]
First compact ultrasonic metal atomizer premiere at Formnext 2017
In 2016, the Polish company 3D LAB[21] began developing a compact ultrasonic atomization system intended to address the limitations of conventional powder production methods for Additive Manufacturing and high-temperature alloys.[link]. In 2017, the company filed a patent application for the device,[22] listing Łukasz Żrodowski, Robert Rałowicz, Jakub Rozpendowski, and Katarzyna Czarnecka as inventors. Later that year, the prototype of compact ultrasonic metal atomizer, named ATO One, was publicly presented at the Formnext 2017 trade fair in Frankfurt.[23] In 2017, Łukasz Żrodowski received a “Diamentowy grant” from the Polish Ministry of Science and Higher Education[24][25] for research on laser powder bed fusion 3D printing of bulk metallic glasses, a class of advanced amorphous materials known for their mechanical strength and corrosion resistance.[26] Ultrasonic atomization was not included in the project scope.[27]
First ATO Lab installed at REMET S.A.[28] metal 3D printing lab in 2019
ATO Lab, a laboratory ultrasonic atomizer unit, was introduced in 2018 [29][30] and the first installation took place at the REMET S.A. metal 3D printing laboratory in 2019.
In same year, 3D LAB began commercial sales of its ATO systems[31] to industrial and research organizations.The ATO Lab Plus model enabled processing of materials with high-melting points,[32][33][34] using a water-cooled sonotrode, TIG arc heating, and a controlled inert gas environment. According to company reports, process enabled the production of highly spherical metal powders with low oxygen content.[35]
In 2019, following the commercial launch of ATO atomizers, Łukasz Żrodowski, in collaboration with the Warsaw University of Technology, established the spin-off company AMAZEMET sp. z o.o.[37][38] (previously Uniq). Since then, cold‑crucible melting routes[39] and other variants such as induction-based[40] approaches were investigated.
Feeding options for induction ultrasonic metal atomization
In 2023, the company 3D LAB introduced the ATO Induction Melting System (IMS) module,[41] which incorporated induction melting for low-melting-point metals. The system included two unique feed configurations: a pressure-controlled crucible, which could process heterogeneous solid metal feedstock and allow in-situ alloying, and a contact-free rod feeder. These configurations were intended to reduce evaporation losses and contamination. According to company reports, tests with aluminum,[42]copper,[43]magnesium,[44] and gold[45] produced powders with high sphericity and low oxygen content.[35]
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.[1][46] 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,[47] and metals with melting over 1500 °C such as zirconium alloy,[39]titanium alloy[48] and high-entropy alloy.[49]
In 2024, 3D LAB launched ATO Suite,[50] a platform integrating ultrasonic atomizers and auxiliary devices for metal powder production. The Suite included several atomizer models—ATO Lab Plus (for reactive and non-reactive alloys), ATO Noble (for precious metals), and ATO IMS (for low-melting-point metals)—along with auxiliary devices such as a vacuum casting furnace, sieving station, and cleaning units for feedstock and atomizer components. The system architecture, featuring modular atomizers, multiple ultrasonic frequency options, and different feed systems, allowed processing of various feedstock forms to produce powders with narrow particle size distributions for additive manufacturing. The platform was designed to support material circular economy and reuse of scrap by atomizing it into powder for additive manufacturing. It also incorporated automatic and remote-control functions and was designed for use in both laboratory and industrial environments.
Key innovations, including the water-cooled ultrasonic sonotrode and advanced inert-gas handling, are protected by patents granted to the 3D LAB in the United States,[51] China,[52] Japan[53] and South Korea.[54]
