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Ultrasonic atomization is a very common atomization method in liquid atomization. It is widely used in various applications of humidification, disinfection, aromatherapy, beauty, spray coating, spray drying. In order to facilitate you to understand ultrasonic atomization technology faster, in this paper, we will introduce the basic principle, types and characteristics of ultrasonic atomization in detail.
All technology or methods for atomizing by ultrasonic can be called "ultrasonic atomization". There are many types of specific methods and technologies. What we mainly discuss here and what we usually say "ultrasonic atomization" refers to the ultrasonic atomization based on piezoelectric ceramic transducer. There are also many types of ultrasonic atomization based on piezoelectric ceramic transducer. At present, the main methods of ultrasonic atomization in the industry can be roughly divided into three types: single piezo type, mesh type and Langevin transducer type. Next, we will introduce the principles and characteristics of these three types of ultrasonic atomization.
Figure 1. Single Piezo Ultrsaonic Transducer
Single Piezo Type
Firstly, the ultrasonic atomizer composed of single piezoelectric ceramic transducer can be considered as the most common and earliest ultrasonic atomization method, which is also commonly known as ultrasonic atomizer. In this technology, the piezoelectric ceramic transducer (atomizer) vibrates and emits ultrasonic energy in the liquid. When the ultrasonic energy is transmitted to the interface between liquid and air, due to the great difference of acoustic impedance of different media, the ultrasonic energy will quickly gather at the interface and tear the liquid into tiny droplets to form atomization. The earliest industrial application of this single piezo ultrasonic atomization technology can be traced back to 1960s - 1970s. It was used in medical atomization inhalation, that is, atomized drug inhalation industry. Subsequently, Japan and other countries began to use this technology to humidify the environment, which began to be widely used in ultrasonic atomization. Siansonic invented the ultrasonic atomization transducer based on nickel electrode piezoelectric ceramics in the 1990s, which is far more anticorrosive and durable than the traditional silver electrode. Therefore, the ultrasonic atomization technique based on singe piezoelectric ceramic has a broader place to play. Later, various ultrasonic atomizers and humidifiers based on this technology have been continuously developed and created. The advantage of this ultrasonic atomization method is that the atomizer has simple structure and low cost. Only piezoelectric ceramic disc is used as the atomization transducer, and the ultrasonic frequency is high, generally 1-3MHz. The atomized particles are small, and the droplet size is generally between 3-5 microns.
Figure 2. Partice Size Distribution of Siansonic Ultrasonic Transducer
By increasing the resonant frequency of piezoelectric ceramics and special structural design, the atomized droplets size can be further reduced. Siansonic has successfully create the aerosol by ultrasonic atomization in sub-micron scale (mean droplet size is only 0.5 microns). Figure 2 shows the atomized droplets size distribution of Siansonic nano aerosol generator based on single piezo type of ultrasonic atomization. With the advantage of very small atomized particles, single piezo type ultrasonic atomization is used in the field of advanced material manufacturing for the preparation of ultra-fine powders by spray pyrolysis. Spray pyrolysis is to atomize the precursor liquid, which is usually a salt solution, into tiny droplets, and then send them to a high-temperature furnace for thermal decomposition reaction. After the reaction, the droplets of metal salt solution will be dried and pyrolysis into metal oxide particles, so as to achieve the preparation of ultra-fine powder. Figure 3 shows Siansonic ultrasonic spray pyrolysis system used for the pilot-scale production of metal oxide ultra-fine powder.
Figure 3. The Submicron Aerosol Generator Applied in Spray Pyrolysis System Based on Ultrasonic Atomization Technology
However, the disadvantage of single piezo type ultrasonic atomization technology is that additional structure is required to form a complete atomization device, which is usually complex, because the single piezo type atomizing transducer (ultrasonic atomizer) must be immersed into the liquid, and there must be a certain liquid level and atomization height (ultrasonic energy will excite the liquid into a column fountain, and the height of liquid column is the atomization height) to realize atomization, Therefore, the atomization direction is usually limited, and the downward spray is not allowed. At the same time, the atomized liquid needs to accumulate to a certain amount before it can be atomized. Another major disadvantage is the low conversion efficiency of ultrasonic energy, resulting in low atomization efficiency and atomization capacity. Generally, the atomization flow rate of 300ml/h needs to consume more than 20W electric power. The oscillation capacity of ultrasonic is limited, and the maximum viscosity of liquid that can be atomized is only 1.2 cps. Therefore, only liquids similar to water can be atomized, and the scope of application is greatly limited. Therefore, the main applications of this technology are still limited to humidification, atomization inhalation, atomization landscaping and other fields.
Figure 4. Ultrasonic Mesh Type Piezo Under Microscope
The second ultrasonic atomization method is an ultrasonic atomization device formed by bonding piezoelectric ceramic ring with a micropores mesh plate. This technology was improved from piezoelectric inkjet printing and introduced into the field of ultrasonic atomization at the beginning of this century. It uses the radial vibration of piezoelectric ceramic to drive the axial vibration of mesh plate (generally made of stainless steel, titanium alloy and other metal), and then the mesh absorbs the liquid on one side and sprays it through the micropores. Because there are many micropores and the pore size is very small (generally 5-10 microns), the tiny droplets screened out by the microporous mesh also form a liquid mist. Therefore, this atomization method is actually a kid of spray valve rather than the mist generated by vibration tearing in the traditional atomization. Therefore, this atomization method is different from other ultrasonic atomization methods. Its atomization droplets size has nothing to do with the ultrasonic frequency, but only related to the diameter of micropores, and the atomization droplets size is basically close to the pore diameter. This atomization method is mainly invented to solve the disadvantage of low energy conversion efficiency of the singe piezo type atomization above. Compared with singe piezo type atomization, the main advantage of mesh type atomization is high atomization efficiency. It only needs 3-5V voltage excitation and 1-2W electric power to produce good atomization effect. Moreover, the atomization device made by this technology can be freer in the spray direction, and it does not need to accumulate a certain amount of liquid to atomize.
However, this atomization method also has many disadvantages. For example, although the atomization efficiency is high, its maximum vibration force is much less than that of piezoelectric ceramics because it actually depends on the vibration of metal sheets. Therefore, the atomization amount and atomization ability it can provide are very low. The maximum atomization flow rate is usually less than 10ml/h, and the maximum viscosity of the liquid that can be atomized is only 1-2 cps. Therefore, only a small amount of liquid similar as water can be atomized. In addition, because the micropores are too small, the solutes or impurities in the atomized liquid are easy to block the micropores and make the atomization device unable to atomize. When spray downward, if there is too much liquid, it will accumulate on the microporous mesh, which will also cause the failure of vibration atomization. Therefore, the application of this ultrasonic atomization method is relatively limited, and it is only suitable for some consumer applications of portable micro atomization with low quality requirements, such as small aromatherapy device, household portable inhaler, beauty replenishment instrument and so on.
Figure 5. The Ultrasonic Atomizer Nozzle Made by Langevin Transducer
Langevin Transducer Type
The third ultrasonic atomization method is an atomization method using Langevin ultrasonic transducer. This technology was firstly proposed in the United States around the 1990s. A liquid channel is applied in the traditional Langevin ultrasonic transducer. When the liquid is delivered to the front tip of the transducer horn where has the maximum oscillation amplitude, it is torn by ultrasonic vibration and atomized. The first advantage of this technology is that it solves the problem of low atomization ability of the above two types of ultrasonic atomization. Using Langevin transducer and horn, the oscillation amplitude of the ultrasonic transducer is dramatically improved, so as to increase the maximum atomizable viscosity of liquid, which can reach 30-50 cps, that is, 30-50 times of the above two ultrasonic atomization abilities, and 1W electric power can generate an amazing atomization flow rate 1200ml/h. At the same time, the atomization device of this technology can realize spray in any direction, and good atomization can be achieved without accumulating a certain amount of liquid. The liquid will be completely atomized out as soon as it reaches the atomizer. Therefore, the accurate real-time atomization flow rate can be achieved by using additional metering pump to deliver liquid, and the accuracy of atomization flow rate can be in nanoliter per second. Combined with the design of various carrier gas channels, the atomized droplets can be evenly and directionally distributed. The ultrasonic atomization device based on this technology is usually called ultrasonic spray nozzle. Sono-Tek, USI, and Siansonic have this ultrasonic atomization technology. With the unique advantages of this ultrasonic atomization technology, various solutions, sols, suspensions can be atomized and deposited onto the surface of the substrate to form a uniform thin film coating, so as to bring the ultrasonic atomization technology from the traditional applications of humidification and medical inhalation to the novel and broad application of advanced materials manufacturing. The preparation process of thin film coating based on this ultrasonic atomization technology is called "ultrasonic coating", which has been widely used in various industries such as biomedical, new energy, microelectronics and semiconductors, glass manufacturing, nano materials, etc. At the same time, the technology can also be applied to the advanced material manufacturing processes of ultra-fine powder such as spray pyrolysis and spray drying.
The ultrasonic atomization technology of Langevin transducer also has its own drawbacks. Its main disadvantage is that the atomization droplet size is relatively large. This is because the resonance frequency of Langevin transducer cannot be very high, usually only between 20-200khz, so the minimum droplets that can be achieved have to be more than 10 microns. Therefore, this atomization method is not suitable for the applications requiring very small atomization droplets.
In conclusion, the three types of ultrasonic atomization technologies have their own advantages and disadvantages, and their technical characteristics are shown in Table 1. Siansonic has accumulated nearly 40 years of technical experience in ultrasonic atomization. It not only has all the technologies and corresponding products of the above three types of ultrasonic atomization, but also continues to try and explore new ultrasonic atomization technology, such as ultrasonic atomization based on surface acoustic wave, high-intensity focused ultrasonic atomization, non-contact acoustic field atomization. Siansonic aims to provide more advanced and suitable atomization solutions for various industries, and help to create a great future using ultrasonic technology.