In 1927, when a strong ultrasonic transducer immersed in liquid emits towards the liquid surface, there will be a layer of mist on the liquid surface. The intension of mist is related to the intensity of ultrasonic wave, while the size of mist droplets is related to the frequency of ultrasonic wave and the surface tension of liquid. At this time, surface wave propagates on the surface of liquid, and the wavelength of surface wave is also related to the frequency and surface tension of ultrasonic wave. Of It has been proved that the diameter of droplets is slightly smaller than half wavelength of surface wave, which makes people tend to think that droplets are ejections of surface wave at the peak. Ultrasound atomization is the process of forming micro-droplets of liquid by using ultrasonic energy. Ultrasound can atomize liquid in two ways:
1. The thin liquid layer on the vibrating surface excites a capillary gravity wave under ultrasonic vibration.
2. The atomization mode is the fogging of the ultrasonic fountain.
There are two theoretical explanations for the principle of ultrasonic atomization. They are micro shock wave theory and surface tension wave theory. On the one hand, the theory of micro-shock wave explains that the cavitation effect of ultrasound in liquid medium leads to the generation of micro-shock wave and the atomization phenomenon. The theory holds that the cavitation effect is the direct cause of liquid atomization. In addition to thermal and optical radiation, the other parts of the cavitation collapse radiate in the form of micro-shock wave. When the micro-shock wave reaches a certain intensity, it causes liquid atomization. On the other hand, the theory of surface tension holds that the droplet is generated by the instability of liquid surface wave, which causes the liquid to atomize. Specifically, when the ultrasonic wave of a certain sound intensity directs at the gas-liquid interface through the liquid, it forms a surface tension wave at the interface perpendicular to the surface tension wave, once the amplitude of the vibration surface reaches a certain value, the droplet will fly out of the peak. The formation of atomization. This theory holds that the surface tension wave produces droplets at its peak, and the size of droplets is proportional to the wavelength. Surface tension wave model and surface tension wave atomization model diagram.
D is droplet diameter; T is surface tension coefficient; P is liquid density; f is acoustic wave rate.
Fountain atomization, which is a common form of ultrasonic atomization, uses piezoelectric wafers as transducers to generate megahertz-level ultrasound. Usually, the formation mechanism of fountain atomization is as follows: when the frequency of ultrasonic wave emitted by ultrasonic transducer is MHz, the directivity of ultrasonic wave and its cavitation field is very good, so the solution contacted with it will be sprayed up to form "ultrasonic fountain". A kind of
A large number of aerosols are produced simultaneously with the generation of ultrasonic fountains. Among them, "ultrasonic fountain" can be regarded as an upward jet of ultrasonic cavitation field, which has a unidirectional radiation force and symmetrical cyclonic sound flow. In this kind of cavitation field, the distribution of cavitation bubbles is very different. When water and other liquids are cavitated, due to the effect of acoustic radiation pressure and the physical effect of ultrasonic radiation force and spotlight injection, the concentrated thermal effect and mechanical effect of a large number of cavitation bubbles are more prominent at the front of the fountain, and the acoustic energy density is greatly increased along the direction of injection due to ultrasonic free injection and spotlight injection.
In the ultrasonic fountain, the main mechanism of the ultrasonic fountain is the collapse of a large number of cavitation bubbles, the high temperature acoustic impulse flow and high pressure shock wave during the burst. Other mechanical stirring effects, thermal effects and so on also exist at the same time. The ultrasonic humidifier designed with this principle is often used as indoor humidifier. It can humidify the computer room and wool textile workshop to remove the static electricity of the equipment, disinfect indoors with human drugs, make facial beauty and shape bonsai, etc.