I think when you listen to music, not everyone has a line or a certain pattern in their mind. It may also be that they tend to some emotion – for example, they hear a sense of mystery in the music. In fact, for those of us who study sound, sound does not need to be patterned with this intention or imagination. Sound itself can realize self patterning. < / P > < p > for example, we sprinkle salt particles on aluminum plates. Salt particles on the surface of aluminum plate will form a very interesting pattern – the center is a circle, the outside is also a circle. < / P > < p > if we continue to increase the frequency, we can see that the pattern will change. This pattern is becoming more and more complicated. There are about one circle, two circles, three circles and four circles. < / P > < p > this pattern will continue to change, because with the increase of frequency, the wavelength will become shorter, and within a limited range, it will appear more patterns and become more and more complex. This is the real sound pattern. For example, the sound of a butterfly flapping its wings is about 5 to 6 times per second, which is too low for us to hear; the sound wave with vibration frequency less than 20 times per second is called infrasonic wave. Because infrasound waves are beyond our hearing range, we can’t hear them. < p > < p > for example, elephants are not as reticent as we seem. In fact, elephants are quite talkative. It’s just that it communicates with infrasound waves, which we can’t hear. < / P > < p > in nature, in addition to bats and dolphins, there are other organisms that can emit ultrasound, such as rats, earthworms, crickets, and so on. < / P > < p > then why does ultrasound remove the stains? How is it removed? Can’t we hear ultrasound? So why did it make such a noise just now? When ultrasonic wave propagates in liquid, it will make the driving fluid flow. This macro flow is called “acoustic flow effect”. The higher the frequency is, the faster the flow rate is. We can even use this way to make ultrasonic pump. < / P > < p > in addition, ultrasonic can also make the liquid vibrate, tens of thousands of times per second or even higher. This high-speed vibration and shear effect is also part of cleaning, but not the most important; < / P > < p > in addition, there is also a “thermal effect”, which belongs to a kind of mechanical energy, which will eventually dissipate into heat energy, that is, the irregular movement of molecules; < / P > in addition, this kind of high-speed vibration and shear effect is also part of cleaning, but it is not the most important; < / P > < p > in addition, there is a “< p> Cavitation effect is very special, it is associated with some small bubbles in the liquid, so I study these small bubbles rather than acoustics. < / P > < p > the noise from the ultrasonic cleaning machine mentioned just now is actually made by these small bubbles. Before we introduce these bubbles, let’s take a look at what they do? < / P > < p > the following figure shows the blade of the propeller. The “Scar” on its edge is called erosion pit. These pits are not caused by chemical corrosion, but by this small bubble. < / P > < p > the propeller is located at the tail of the ship. The propeller needs to rotate at a high speed during the ship’s running. There will be a large number of such bubbles on the back of the blade and the tip of the blade. It is these bubbles that destroy the propeller. < / P > < p > it can be seen from the schematic diagram that at the end of the anti arc section of the spillway tunnel, the water velocity suddenly changes, resulting in a low-pressure area, where small bubbles are generated, and erosion pits are also found here. < / P > < p > when the low pressure comes, if the pressure is low to a certain degree, it will produce these small bubbles; when the high pressure comes, it will become smaller. < / P > < p > in the process of growing up and becoming smaller, it will produce deformation. Let’s look at the situation in a period. After deformation, micro jet is generated and shock wave is released at the same time. < / P > < p > If a bubble is near the wall, due to the imbalance of momentum, deformation and depression will occur at the top of the bubble, and a high-speed jet penetrating the liquid will be formed to hit the wall. < / P > < p > are these bubbles like a sweeping robot? These vacuoles adhere to the wall surface, constantly swim, impact the wall tens of thousands of times per second, and tirelessly clean the wall. < / P > < p > for example, my fist is a bubble that sticks to the wall. Then, when you stand up, it sticks to the wall, and vice versa, it sticks to the wall, because of the force. < / P > < p > because the volume of the bubble changes greatly, the large bubble will rapidly become smaller. In the process of decreasing, the high concentration of energy will form a high temperature of several thousand degrees centigrade and a pressure of several thousand atmospheres in the interior of the cavity, and at the same time, it will release strong shock wave and high-speed micro jet. The extreme physical environment of < / P > < p > makes these bubbles like chemical reactors, which makes chemical reactions that cannot or are difficult to occur at normal temperature and pressure. Based on hydrodynamics, cavitation dynamics, heat and mass transfer and nucleation theory, it has been widely used in various industries, such as ultrasonic cleaning, sonochemistry, process intensification, mixing, demulsification, emulsification and so on. < / P > < p > if the sphere represents a stone in the human body, there is no need to operate to remove the stone, but with the help of these bubbles, the stone can be broken. These bubbles seem weak, but in fact, they are still very powerful. When these bubbles grow up, they will move macroscopically while expanding and shrinking. A large number of bubbles interact to form spatial distribution, which is called “cavitation structure”. < / P > < p > if properly controlled, we can make these small bubbles do a lot of things, and they can transport particles along the preset route repeatedly. < / P > < p > we can even make it into the letters of the cavitation cloud: these little bubbles are arranged in the letters IOA, which is the abbreviation of the Institute of acoustics of the Chinese Academy of Sciences. < / P > < p > I ask myself, are these bubbles alive? Do they have life? How do they behave? Why are their actions so coordinated and unified? < / P > < p > along this line of thought, I take these bubbles as a whole to study their macro stability, memory effect, surface tension and colloidal properties. Huawei has finally made a choice! Xiaomi and ov have also followed up. Have you ever thought about today?