How can beer bubble?

When you pour beer from a bottle into a cup, impatient people hold the bottle very high, a bit like pouring a big bowl of tea, and make the beer column rush to the bottom of the cup. As a result, a cup of foam is always full and the foam flows all over the table. After the foam disappeared, there was little beer left in the glass.

Skilled waiters tilt the cup as far as possible, put the bottle mouth close to the rim of the cup, let the beer slowly flow to the bottom of the cup along the cup wall, and then slowly adjust the inclination of the cup to the vertical position with the increase of beer in the cup, so that a cup of beer without too much foam can be filled. People once humorously summed up this trick of pouring beer into three homophonic idioms: "crooked door (evil), dirty glass wall (despicable), crooked (evil) right."

Beer, champagne, cola and other cool drinks are all supersaturated solutions of carbon dioxide. If it is not sealed, carbon dioxide will slowly separate and escape into the air. The more carbon dioxide this refreshing drink contains, the higher its quality. That's why pouring beer into a glass can cause trouble.

We call the two methods of pouring beer mentioned above straight and oblique. Why does the oblique sliding method produce less foam while the straight sliding method produces more foam? To answer this question, we have to start with the solubility of gas.

The dissolved amount of carbon dioxide in water is usually measured by the volume of carbon dioxide dissolved in unit volume of water, which is called solubility and is related to temperature and pressure. High solubility at low temperature and low solubility at high temperature. It has high solubility at high pressure and low solubility at low pressure. If the pressure in draught beer suddenly drops under high pressure, carbon dioxide will precipitate and bubble. In a closed container, after the pressure in the container is increased by bubbles, the solubility is also increased, and bubbles no longer emerge. When we opened the champagne bottle, we heard a bang. The newspaper also published the news that when the beer bottle was opened, the bottle cap flew out and hurt people, all because of the high pressure in the container.

There is an interesting thing in history. In the middle of last century, a tunnel was dug under the Thames in London. When the tunnel becomes boring, local politicians will hold celebrations in it. It was disappointing to find that all the champagne brought to the tunnel was tasteless. However, when people walked out of the tunnel and returned to the ground after the celebration, something unfortunate happened. Wine expands in their stomachs, and gas keeps coming out of their noses and mouths. Some people wear inflatable vests, while others have to go back to the tunnel to relieve the sudden pain.

So this phenomenon occurs because the tunnel is hundreds of meters lower than the ground, where the air pressure is high and the solubility of carbon dioxide is high, so champagne is as tasteless as running away. When we returned to the ground, the air pressure was low, and the carbon dioxide separated, which propped up the gentlemen's stomachs. Normally, the air pressure drops by 2 190Pa every time the sea level rises100m. For supersaturated carbon dioxide solution, gas separation is obvious.

Now let's discuss the problem of pouring beer into a cup. The pressure of beer standing in the cup is basically uniform everywhere, and the upper pressure is slightly less than the bottom of the cup, so there are a little more bubbles on the surface. But if the beer in the cup flows unevenly, the pressure at each point will be different. According to Bernoulli's law of fluid mechanics, along a streamline, where the speed is high, the pressure is low, so these high speeds will easily produce a large number of carbon dioxide bubbles. To illustrate this fact, take a glass of beer that is still fresh, and we can see that it basically does not foam. If you stir it with chopsticks, you will find that a lot of bubbles will come out at the end of chopsticks movement, which is precisely because of the low pressure there. If the chopsticks are stirred back and forth in the cup to make the beer in the cup spin, and then the chopsticks are taken out, the beer will form a vortex in the cup. According to theoretical analysis, the pressure in the center of the vortex is very small, so there will be a series of bubbles, just like tornadoes seen on land, which is very interesting. People who swim in the river will have a cordial experience of the small pressure in the center of the whirlpool. Swimming to the edge of the vortex will be sucked in by the center of the vortex, which is very dangerous.

That is to say, if you want to fill the glass with beer without bubbling, you should try to reduce the relative velocity of the liquid in the beer glass and make the process of filling the glass quasi-static as much as possible. The reason why the straight-through type mentioned above is not applicable is that it makes the beer column have greater momentum, thus increasing the speed difference of beer in the cup, that is, it is easy to form a large number of small vortices. On the one hand, the oblique sliding type reduces the drop from the bottle mouth to the cup and reduces the kinetic energy of beer entering the cup; On the other hand, the tilt of the cup can change the frontal impact of the beer column on the cup into an oblique impact, thus reducing the instantaneous momentum change of beer contact; Furthermore, in the process of oblique sliding, the distance of beer sliding to the bottom of the cup increases, and the resistance of the boundary viscous layer near the cup wall to beer during this sliding process can also reduce the speed of beer reaching the bottom of the cup. So basically meet the requirements of quasi-static as much as possible, so that the whole process has less bubbles.

Beer contains a lot of carbon dioxide, why does it feel comfortable to drink? One of the important reasons is the dependence of carbon dioxide solubility on temperature. When you fill a glass of iced wine, try to insert a chopstick into the glass, and you will find small bubbles crawling around the chopsticks. This is because the initial temperature of chopsticks is higher than that of beer, and the solubility of carbon dioxide in beer around chopsticks is small at high temperature, so it is separated and climbed on chopsticks. Similarly, when beer is drunk into the body, the temperature in the body is higher than that of beer, and a large number of bubbles will quickly attach to the mucosa from the oral cavity, esophagus and stomach wall. We also know that the heat transfer efficiency of bubbles is relatively low, which is why you won't feel too cold when you drink a cold drink that is much lower than your body temperature. We also know that the sudden drop of mucosal temperature will cause the blood vessels nearby to contract, reduce nerve activity, and at the same time, the digestive ability and appetite will become dull accordingly. The function of bubbles in beer is to make people feel cool without turning off their appetite, thus maintaining strong digestion. Because of this, you may notice that in hot summer, when you finish a cup of ice cream, you will feel poor appetite, but after eating cold beer, you will still enjoy it, because the latter will produce foam.

Therefore, in order to make beer delicious, we must pay attention to a series of links, such as brewing, storage and transportation, pouring from bottles to cups, so that carbon dioxide can not escape and produce more small bubbles after it enters the mouth. In the process of storage and transportation, it is necessary to avoid sun exposure and properly cool down; Don't shake it too violently, so as not to escape too much carbon dioxide. Even in a closed container, the separated carbon dioxide gas will cause an explosion accident due to too high pressure. It should also be noted that when pouring beer, too much carbon dioxide is run away in an oblique way instead of in the last process before the "entrance". It should also be mentioned that the formation of bubbles in beer is not only related to pressure and temperature, but also to a certain gasification core. Bubbles always form near tiny solids or burrs on the inner surface of bottles first. Try to put a pinch of sand in the beer glass. As the sand sinks, beer will bubble like a jar. Moreover, once tiny bubbles are formed, the bubbles themselves can act as gasification nuclei and accelerate the formation of bubbles. So beer bubbling is actually a nonlinear process similar to avalanche. That is, the more bubbles, the easier it is to increase bubbles. So once a large number of bubbles come out, they will overflow the cup in lightning speed. Even if you stop pouring beer, you will take off for a while until the carbon dioxide runs almost.

In life, this more or less nonlinear phenomenon is called Matthew effect. It comes from a sentence in the gospel of Matthew in the Bible: "Whoever owns it, give it to him to make him more than enough;" If you don't, even what he has will be taken away. "This effect can be seen everywhere in mechanics and physics. The river course is curved, and it is even more curved due to the erosion of water flow; If the land is uneven, it will be even more uneven under the erosion of runoff; When the atmosphere is ionized, it is easier to ionize locally until it is discharged. To a certain extent, the gap between the rich and the poor has increased, the stock market has soared and plummeted, and the economic crisis is Matthew effect. So is the beer bubbling we are talking about here. It is not easy to accurately describe the nonlinear process of beer foaming, because foam is a fractal structure, and the foam behavior at different scales is also different.

After discussing beer, let's look at water, which is the same liquid as beer. Carbon dioxide dissolves in beer. What about water? A small amount of air is usually dissolved in water, and further, the molecular groups of water can be converted into gas-water vapor. At this point, it is no different from beer. The difference is that water molecules will turn into gas at low pressure, producing bubbles, which are called vacuoles, also known as holes. The diameter of air cavity is sometimes as small as 10-5 cm. Don't underestimate this humble hole. It was and still is a terrible obstacle to navigation.

1894, when Brave, a small destroyer with a British tonnage of 240 tons, made its first trial, the propeller speed could only reach 384 rpm, which was 1.54% lower than the rated design speed. After several debugging, it was not until 1897 that chief engineer Barnaby published a paper at the meeting of shipbuilding engineers, explaining that the initial poor performance was due to propeller cavitation. Twenty years later, 19 15, the new British torpedo boat "Linde" sailed into the Atlantic Ocean for trial. Its design speed is twice that of the previous model, but when the ship's machinery works at the maximum speed, the stern shakes and the seawater foam at the tail churns, just like pouring beer, at the same speed as the previous model. When the torpedo boat returned to the base, the propeller was in tatters. This is another bubble that creates trouble. It was not until 197 1 that thousands of ships were investigated, and it was found that after one year of use, 30% of propellers were damaged to varying degrees due to cavitation.

In order to study the mechanism and function of cavitation, people have started theoretical and experimental research since the last century. 1895, Britain built a small water tunnel dedicated to cavitation research, and then in the 1920s and 1930s, Britain, Germany, France, the Soviet Union, the United States and other countries successively built larger cavitation water tunnels. At the same time, theoretical research has also made corresponding progress.

Why does high-speed water bubble? It turns out that water boils at standard atmospheric pressure (1 atmospheric pressure is equivalent to1kloc-0/325pa) and the temperature reaches 100℃. "Boiling" is the phenomenon that bubbles can appear in water. The boiling pressure of water is different at different temperatures, which is called saturated vapor pressure, also called vapor pressure. The saturated vapor pressure of water at different temperatures is shown in the following table.

As can be seen from the above table, when the pressure is 2338. 1Pa, water will boil at 20℃, and this phenomenon of boiling at room temperature can be called "cold boiling". On the plateau above 4000 meters above sea level, it is not easy to cook there because of the low air pressure and the boiling point of only 86℃. When the pressure reaches 198490Pa, that is, less than two atmospheres, water does not boil until 120℃, which is almost the pressure of an ordinary pressure cooker.

As mentioned above, the high-speed movement of fluid will reduce the local pressure, especially when high-speed ships, propellers and torpedoes move in the water, which will cause the local water pressure to be very high and reach the steam pressure at room temperature. This is the reason why bubbles are produced in high-speed sailing water.

Once cavitation occurs, the resistance will increase, bubbles will consume a lot of energy, and the speed of the ship will never go up again. If special measures are not taken to solve the cavitation problem, the speed of most large ships will not exceed 26 knots (about 14 m/s).

However, the harm of cavitation to navigation does not stop there. The problem is that after cavitation is formed in the low pressure area, it flows to the high pressure area with the water flow, where the pressure increases and bubbles cannot exist and close. The closure of bubbles will cause high pressure similar to explosion, even reaching 1000000 atmospheric pressure. Under this atmospheric pressure, any metal material will be destroyed, so the propeller will soon be bitten by cavitation. Similar problems occur in large hydropower stations and dams. For example, the speed of water flow in spillway tunnel is high, and bubbles can erode the wall of the tunnel, and the turbine blades of hydropower station can be eaten by bubbles for dozens of millimeters in a few days.

Drops of water wear through the stone, and uninterrupted drops of water can wear through the stone. At first, people thought it was caused by the long-term erosion of water, but it was actually due to cavitation. With the development of high-speed cameras, some people aim at the place where water drops "land" with 1500 cameras per second. The water drops flatten from a circle and then disperse. At this time, some local velocities near the center of the droplet are quite large enough to reach the low pressure of cavitation. So the cavitation gradually eroded the hard stone. In the fast-flowing river, the running water beats against the rock bank, "the rock surprises and the waves crack the bank." I am afraid that this effect of water is also caused by cavitation.

Attentive readers may notice that when a cup of freshly boiled water falls on the ground, it is plopped, while when cold water falls on the ground, it is heard as a crisp plop. The difference in sound is also due to bubbles. If you pour a fresh glass of beer on the ground, it will sound like boiling water. The newly boiled water is nearly 100℃. When it is poured into the ground, the local velocity of the fluid is relatively high, so the pressure decreases. This small pressure will make the liquid boil again. A layer of bubbles between the ground and water certainly sounds different from no bubbles, but when cold water hits the ground, the local pressure drop is not enough to make the fluid boil.

Take a aluminum pot and boil a pot of water. When the water boils, you lift the pot away from the stove with one hand and gently put the other hand on the bottom of the pot. At this time, you will find that this hand can hold up the whole aluminum pot without feeling hot. This is the bubble at work again. There is a layer of tiny bubbles crawling on the bottom wall of the newly cooked pot, which has good heat preservation performance. When you hold the bottom of the pot with your hands, the heat capacity of aluminum at the bottom of the pot is very small, and it will soon reach a balance with the hand temperature, but the heat of water in the pot cannot be felt because of the heat insulation of a layer of bubbles. If you don't believe me, please try.

At the beginning of this century, people gradually realized the ultrasonic phenomenon. 19 17 French scientist Ron Wan Zhi invented the piezoelectric crystal ultrasonic generator, and then the ultrasonic wave entered the application research stage. It is worth noting that the propagation of ultrasonic waves in water causes local high-frequency oscillation of water, and the negative pressure generated by this oscillation is enough to produce cavitation, which makes ultrasonic waves widely used in parts cleaning, emulsification, accelerating chemical reaction, crushing and so on.

It was also in 19 17 that the British scholar Rayleigh first calculated the infinite pressure at the center when the spherical cavity in incompressible fluid was closed. When a liquid is compressible, the pressure is not infinite, but it is still very large. The understanding of cavitation is far from over. As early as 60 years ago, people found that when ultrasonic waves were put into water, the water could glow instantly. This phenomenon has not been reasonably explained. It was not until 1959 that people demonstrated for the first time that light was emitted by the huge energy concentration generated when bubbles burst.

According to a recent report by the British magazine New Scientist, in recent years, people gradually use more accurate models to calculate cavitation. Three Americans got different results. 1986 An American calculated that the bursting of bubbles could cause a high temperature of 5000K K. 1993, someone improved the calculation and said that it could reach a high temperature of 7000K, which is already the temperature on the surface of the sun. 1994165438+10 announced at the National Acoustics Conference in the United States in October that the temperature at which bubbles burst can reach 2 million K, which is half the temperature required for fusion thermonuclear reaction.

If the above calculation theory is correct and we can rely on modern technology to realize it, perhaps cavitation is still a feasible method to control thermonuclear reaction! What an attractive prospect this is! To say the least, even if we can't reach the calculated high temperature, people can use this extraordinary high temperature to open up many new application fields!

Bubbles in beer can bring delicious food and trouble. Similarly, cavitation can be dangerous and beneficial to human beings. World affairs have advantages and disadvantages. How to seek advantages and avoid disadvantages depends on a full understanding of its mechanism. The same is true for pouring beer, especially for dealing with bubbles.