Shulou(Shulou.com)11/24 Report--

Ultrasonic cleaning, that is, the use of ultrasonic to clean the surface of objects, I believe we are no stranger, whether in life or industry, there are a wide range of applications.

Shan (chang) mo (yu) editor is dealing with ultrasonic cleaning machine every day, which has made great contributions to cleaning all kinds of experimental equipment for us.

But the ultrasonic cleaning machine has a disadvantage, that is, it is really noisy when working.

We learned in the second year of junior high school that ultrasound is a sound wave or vibration whose frequency exceeds 20kHz and belongs to a range of frequencies that cannot be heard by human ears.

Since it should not be heard, why does it still make a shrill "Zizi" sound at work, and where does this sound come from?

01. If you want to explore this problem, you must first have a clear understanding of the protagonist, Sound Wave. Next, let's review the acoustics that we learned in the second year of junior high school.

Sound waves are mechanical waves in which particles at the sound source vibrate and propagate around through the medium (gas, solid, liquid).

If the direction of particle vibration is parallel to the direction of wave propagation, it is called longitudinal wave, and mechanical longitudinal wave can propagate in gas, liquid and solid.

Diagram of longitudinal wave propagation (particle vibrates left and right, wave propagates to the right) if the direction of particle vibration is perpendicular to the direction of wave propagation, it is called shear wave. Because the propagation of mechanical shear wave requires the shear elasticity of the medium, the mechanical shear wave can only propagate in the solid.

A schematic diagram of shear wave propagation (particles vibrate up and down, waves propagate to the right). In general, we use physical quantities such as frequency and amplitude to describe a wave. Corresponding to sound waves, their frequency and amplitude determine the tone and loudness of the sound.

The tone is determined by the frequency of the sound wave (in hertz Hz). The higher the frequency, the higher the pitch.

Loudness is determined by the amplitude of sound waves. Loudness is measured in decibels (dB). The greater the amplitude, the greater the loudness.

The range of frequencies that can be heard by the human ear is 20Hz~20kHz. According to this range, we define infrasound (sound waves with frequencies lower than 20Hz) and ultrasonic waves (sound waves with frequencies higher than 20kHz).

Typical sound in different frequency range | Source: the propagation speed of wikipedia sound wave in different media is different. At room temperature and pressure, the propagation speed of sound wave in solid is the fastest, followed by liquid, and gas is the slowest.

Because liquid has better sound transmission effect than gas, ultrasonic cleaning machine needs to add liquid as transmission medium in order to achieve better cleaning effect.

02, small bubbles, a lot of energy review the relevant knowledge of sound waves, let's take a look at what strange phenomena happen when such high-frequency sound waves act on this pool of liquid.

By observing the ultrasonic cleaning machine in the laboratory, the editor found that when the machine was working, there were ripples on the surface of the liquid, and there were many quivering bubbles in the liquid that had no bubbles.

You can't seem to see anything with the naked eye, so you have to put on some props at this time.

Tu Yuan: the manufacturing principle really startles the ears! The bubble, which seems to be trembling all the time, has gone through a process of sinking in from the middle, then perforating, and finally breaking into countless small bubbles.

What is the mechanism behind such a magical phenomenon? It's not complicated.

As we said in the previous section, sound waves can only travel in the form of longitudinal waves in the liquid, and the propagation of longitudinal waves will cause the imbalance of local pressure in the liquid.

Schematic diagram of pressure in different regions during longitudinal wave propagation | Source: reference [2] due to the rapid decrease of pressure, bubbles with a vacuum can usually be formed in these low-pressure areas.

And because longitudinal waves are propagating all the time, these areas of low pressure will soon feel a wave of high pressure, so there is a process of depression from the middle, and then perforation causes the whole bubble to burst.

After the high pressure passes, another low pressure arrives, and a new round of process begins and the cycle goes back and forth.

The whole process is called ultrasonic cavitation, the reason why the human eye can not observe the details, because the ultrasonic frequency is too high, high pressure and low pressure alternate so fast that the human eye can not distinguish.

Although this process is difficult for the human eye to capture, do not underestimate the power of these bubbles when they burst.

Because of the great difference between internal and external pressure during bubble formation and the high concentration of energy during final rupture, cavitation can produce instantaneous extremely high pressure (thousands of atmospheric pressure) and extremely high temperature (thousands of degrees Celsius) in a small range.

It is precisely because such extreme conditions can be produced in the liquid that the dirt on the surface of the object can be removed, thus playing the role of cleaning.

03. Can you answer the questions at the beginning of the article? I feel that I have not faced the question of the source of the "Zi" sound for a long time, in fact, the editor did not deliberately avoid answering, but the "Zi" sound is closely related to the cavitation effect.

This harsh "Zizi" sound actually stems from the impact of small bubbles produced by cavitation on the inner wall of the machine.

Bubbles produced by cavitation collide with the inner wall of the machine | Source: as we said earlier in the pulpit, these small bubbles produced by cavitation can produce a small range of instantaneous extreme high pressure and high temperature.

The inner wall of the machine is generally made of stainless steel, which is strong, so it will produce a sharper sound when it collides.

And the frequency of this sound is just within the range of frequencies that can be heard by human ears, so we will hear this harsh "Zizi" sound.

Unexpectedly, a common ultrasonic cleaning contains so much interesting knowledge, but in spite of this, it is still very noisy.

references

[1] Sound-Wikipedia

[2] Fuchs F J. Ultrasonic cleaning, fundamental theory and application[C] / / NASA CONFERENCE PUBLICATION. NASA, 1995: 369369.

Xu Zhonghua, Zhang Hongbo. Cavitation mechanism of ultrasonic cleaning [J]. Harbin Railway Science and Technology, 2009 (04): 3-5.

[4] Brennen C E. Cavitation and bubble dynamics [M]. Cambridge University Press, 2014.

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Eric

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