At Chongqing University's State Key Laboratory of Coal Mine Disaster Dynamics and Control, water has become an extremely sharp "knife" which can cut rock, steel plate, and aeronautical materials.

"High-pressure water jet cutting is also known as 'water knife,'" said Long Haiyang, an engineer from the laboratory. "It is a high-velocity flow from a nozzle of a certain shape that uses water as a medium, and high-pressure generator as equipment to create high pressure and huge power."

According to Long, there are low- or high-pressure "water knives," and some high-pressure water jet cutting machines can reach a pressure of more than 200MPa.

According to Long, there are low- or high-pressure "water knives," and some high-pressure water jet cutting machines can reach a pressure of more than 200MPa. (Photo/ Visual Chongqing)

High-pressure water jet cutting

What is such pressure like? "A pressure of 1MPa is equivalent to a force of 10 kilograms acting on an object with the size of your fingernail," he explained.

Long introduced that so far, high-pressure water jet technology has been increasingly widely used in coal, petroleum, new materials, and other fields.

For example, "water knives" can create seams, which significantly improves the efficiency of coalbed methane exploitation and reduces the risk of gas disasters in a coal mine.

Furthermore, a "head" is crucial to making a "water knife" sharp enough to penetrate a steel plate quickly.

Long showed that there is a tiny hole in the center of the "head by dissembling in the center of the " head. That is the so-called "nozzle," according to him.

"With a diameter of only 0.35 millimeters, the gem nozzle is very powerful," said Long.

A good "knife head" will form converging jets of high-pressure water with a stronger force. On the contrary, a bad "head" will disperse the high-pressure water, reducing the sharpness of a "knife."

World's largest test system for deep geotechnical engineering

In another room, the most valuable scientific research equipment of this national key laboratory came into sight.

"This is the world's largest multifunctional physical simulation test system for deep geotechnical engineering," said Zhou Junping, professor of Chongqing University. "We independently developed this equipment worth about 20 million yuan (about 3.1 million U.S. dollars)."

This is the world's largest multifunctional physical simulation test system for deep geotechnical engineering.

China's exploitation of resources is going deeper into the earth. However, the deeper it goes, the more complex the stratigraphic environment becomes.

Zhou explained that with this test system, they could better restore the deep stratum resource mining environment in the laboratory to carry out research effectively.

The core part of the system is a huge cylindrical device, which is about two or three people high. In particular, the structure actually has a square space of 1.2m×1.2m×2.06m inside.

"The design can more truly reflect the stratigraphic environment," said Zhou. "In this way, the stratigraphic environment of the real test site can be moved to and reproduced in the laboratory. Parameters that are difficult to observe on the test site are much easier to acquire in the laboratory."

However, this design makes the development of the equipment more difficult. The greatest challenge lies in sealing air, where the equipment must fit together seamlessly that there is no leakage anywhere.

The project was initiated in 2011 and accepted after inspection upon completion in 2016. It took five years to develop the system, including conceptual design, construction and assembly, and debugging repeatedly.