What is under the quicksand, why does it sink when people step on it? Pull-down force of 100,000 Newtons

Everyone should have heard or seen stories about quicksand, because quicksand is very popular in traditional cultures all over the world, and it is believed that in the 1960s, one in every 35 movies was about quicksand.

Quicksand gives us the impression that if you step on it, it will slowly engulf us and sink even harder if you move around.

Since the land of Africa is relatively dry, and the entire northern part is basically occupied by the Sahara Desert, we often think that Africa is a place full of quicksand, which is more dangerous.

In fact, many of our subjective perceptions of quicksand are misled by some literature and film and television works. Quicksand is indeed dangerous, but it is different from what many of us imagine.

First of all, the first is that there are actually very few quicksands in the desert, at least not so common in other areas.

Quicksand doesn't mean sand that's actually flowing, it actually refers to an area where the friction between the sands is too low, causing people or other things to settle on them to sink.

Under natural conditions, sand is an area composed of small, coarse sand particles mixed with air or moisture.

In most cases water or air cannot wrap all the sand, which allows the sand to interact and create a lot of friction.

We don't sink when we step on normal sand, it's the friction that holds us up?

However, when the sand is soaked in water, it will be different. The sand particles will no longer be in contact with each other, and the strong friction will be ineffective. At this time, it is actually more like a quagmire. It may be more appropriate to call it a "sand pool". At this time, if people stand on it, it will sink, and it will become the quicksand in our rumors.

It is not difficult to find that in most cases, quicksand needs water to develop, and the desert is very short of water, so there is not much quicksand in the desert.

In fact, most of the quicksands appear around lakes, forests, river valleys or beaches, and the places where quicksand appears in deserts are basically those near cenotes or shallow groundwater. These places with water sources are covered with sand and appear invisible on the surface. There is nothing unusual, but when people step on it, it will sink.

Can quicksand really escape on its own?

As early as more than a decade ago, Daniel Bonn of the University of Amsterdam has simulated quicksand in the laboratory. It turns out that although quicksand is very difficult to save itself, it is equally difficult to completely engulf a person.

After taking quicksand samples in a quicksand area, Daniel analyzed the ratio of clay, salt water and sand and recreated a quicksand in the lab.

He tested the properties of artificial quicksand, and the results showed that when an aluminum ball with a density similar to that of humans was placed on the artificial quicksand, the quicksand could not completely engulf the aluminum ball.

The team also simulated what would happen if a human struggled by shaking the artificial quicksand, and the results showed that the aluminum ball did start to be completely engulfed, but as the sand gradually began to mix with the water, the buoyancy increased and the aluminum ball floated back again. to the top.

The contact area between the sphere and the quicksand is larger, which means that its buoyancy will be greater. If it is replaced by a human, we need to increase our contact area as much as possible - for example, try to lean back as much as possible, and basically it will not sink.

Although it is almost impossible for quicksand to completely engulf people, it is very difficult to escape from quicksand by oneself, and it can even be said to be almost impossible, because quicksand provides strong suction.

In Daniel's lab, when he tried to pull out something deep in it, he needed to provide a force of around 100,000 Newtons -- the equivalent of lifting a mid-sized car.

This is because the person is tightly surrounded by the quicksand mixture and provides much greater pressure than water in all directions, and when the person tries to pull out of it, these pressures pull the person downwards.

The only way to escape from such quicksand is to move slowly with the help of others, giving the water and sand a chance to flow into the space squeezed out by the movement.

Worse than wet quicksand is dry quicksand

All of what we have discussed before is due to the friction caused by the water to reduce the resulting quicksand. This is called "wet quicksand". There are many factors that make wet quicksand stable. Most of the time, the most important thing is its salt content. ingredients, but in a small number of cases, the action of some bacteria will also create a stable quicksand.

However, as we said before, the sand particles in the sand are still separated by air. When a certain volume of sand contains a high proportion of air, this will also cause the friction between the sand particles to decrease. In this case, quicksand will also form, and this is called "dry quicksand".

In the lab, the researchers injected air into sand samples so that the sand made up only 41 percent of the total volume and the rest was air.

In this case, put an aluminum ball equivalent to a person into it, and the aluminum ball will be swallowed and sink to the bottom in an instant.

"Dry quicksand" is often found in desert areas, and it's mostly caused by dust storms, which blow sand up and then fall to the ground to form a fluffy area.

"Dry quicksand" is not as stable as "wet quicksand", and over time, the air in it will gradually fill up, increasing friction, and the quicksand will naturally disappear.

However, encountering "dry quicksand" can be more dangerous than encountering "wet quicksand."

Now the rescue method of "dry quicksand" is basically to wrap a cylinder for the person caught in it, and then pump the sand out of the cylinder. This can prevent new sand from filling it, but it is basically impossible to complete this kind of work in the wild. Work.

at last

In the process of human production, there is a typical and dangerous "dry quicksand", that is, a granary full of grain. The grain particles themselves do not have the friction force of sand, and their shape also allows more air to be stored in them.

Therefore, it is very dangerous if a person falls into the grain pile, and the grain will engulf the person in a short time.