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 see 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]
Liquid metals
First-reported metal ultrasonic atomizer
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]
Later developments of Lierke focused on making high-temperature ultrasonic atomization of metals more practical by stabilizing melt delivery and protecting the transducer from the hot zone. A patent granted in the early 1980s described feeding the liquid into velocity-nodal regions of a bending resonator to maintain an atomizable film, and proposed heating the vibrating resonator (including by induction) together with intermediate cooling sections to shield temperature-sensitive parts of the excitation system.[15]
In the early 2000s, Caccioppoli et al. reported a metal atomiser in which an alloy is induction-melted under argon and the melt is disrupted in a tubular ultrasonic resonator operated under an inert atmosphere; drawing on transducer concepts developed by Prokic[16], they described using load-tolerant “hammer”-type ultrasonic transducers designed to be less sensitive to changes in acoustic load than conventional bolt-clamped (Langevin type[17]) designs, improving stability under fluctuating melt conditions. Combined with multifrequency excitation and acoustic-activity sensing, the drive frequency can be swept around resonance, shifting vibration anti-nodes along the resonator, widening the effective atomization zone, and reducing the sensitivity of droplet size to melt flow rate.[18] Variants in which material is melted locally using highly focused energy source reducing the hot zone have been explored where an external heat source primarily generates the melt (e.g., a continuous-wave CO₂ laser producing a melt pool on a consumable substrate) while ultrasonic vibration assists melt ejection and breakup into droplets. [19]
↵↵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[20] at Warsaw University of Technology and establishing the spin-off company Amazamet. Same year in which research grant was obtained Żrodowski, Rałowicz, Rozpendowski and Czarnecka filed a patent application describing an ultrasonic metal atomizer that deliberately separates the “hot” melting zone from the ultrasonic stack: a water-cooled, non-consumable sonotrode (high thermal conductivity material acting as a heat sink) is terminated with a replaceable consumable melt tip, which is heated by an external source while the sonotrode transmits vibration to promote droplet ejection. Conceptually, the consumable-tip approach is closest to the earlier “local melt pool on a consumable substrate” concept mentioned above (and shares the same goal as Lierke’s protected-resonator ideas): keep the transducer and main sonotrode cold, and treat the melt-contacting interface as an expendable, modular element. Since then, cold‑crucible melting routes[21] and other variants such as induction-based approaches were investigated.
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][23] 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,[24] and metals with melting over 1500°C such as zirconium alloy,[25]titanium alloy[26] and high-entropy alloy.[27]
The progression of ultrasonic atomization from early physical observations to reliable processing of molten metals required the resolution of several engineering challenges that are not fully captured by theoretical models alone. Many of the technical solutions that enabled the transition of ultrasonic atomization from laboratory-scale experiments to practical and high-temperature metal processing have been disclosed in patent literature. These documents address key challenges such as ultrasonic stack design, melt–sonotrode interaction, thermal management, resonance stability under variable load, and the integration of external heat sources. A selection of representative patents and technical disclosures relevant to the development of ultrasonic atomization is summarized below. [28][29][30][31][32][33][34][35][36][37][38]
↑EP4192985A1,ZRODOWSKI, Lukasz,"Ultrasound system for metal and their alloys processing and method of liquid metals and their alloys processing",issued 2023-06-14
↑EP3638442A1,ZRODOWSKI, Lukasz; RALOWICZ, Robert& ROZPENDOWSKI, Jakubet al.,"Device for the manufacturing of spherical metal powders by an ultrasonic atomization method",issued 2020-04-22
↑EP3766610A1,KACZYNSKI, Konrad; Rukat, Michal& RALOWICZ, Robertet al.,"Sonotrode for ultrasonic atomization of metals and their alloys",issued 2021-01-20
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