Physics no longer exists? Chinese scientists discover "frictionless" ice for the first time

Produced by: Science Popularization China

Author: Denovo Team

Producer: China Science Expo

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When we watch ice skating competitions, have you ever wondered why skaters can run so fast on the ice? In fact, this is closely related to the physical properties of ice and the changes in friction.

When the skater's weight is concentrated on the narrow blade of the skate, the huge pressure generated will partially melt the ice, so that the solid ice is partially converted into liquid water when it touches the skate, forming a thin layer of water film. This water film acts as a lubricant, greatly reducing the friction between the skate and the ice surface, allowing the skater to slide smoothly.

Partial melting of ice reduces friction when skating

(Image source: AI generated)

During skating, although friction is small, it still exists, and if the action is suspended, the skater will eventually stop. So, is there a kind of "frictionless" ice?

Friction of water in the microscopic world

Let’s start with water. In the macroscopic world, you may not find “frictionless” water, but unlike the transport of water in the macroscopic world, in the microscopic world, when the size of the water channel is as small as a few nanometers or even sub-nanometers, many interesting phenomena will occur, such as the “superlubrication effect”.

The difference between friction in the macro world and the micro world

(Image source: AI generated)

In nanoscale channels, water flows with significantly reduced friction, exhibiting a phenomenon similar to "superlubricity." This means that water can pass through these narrow channels at very high speeds with very little force required. This phenomenon is particularly evident in carbon nanotubes and other nanostructures.

Scientists have found that when different materials are used to make nano-scale channels, the transport properties of water are very different. For example, when graphene and hexagonal boron nitride, which have similar structures, are used to make nano-channels, the water permeability in graphene channels is 10-100 times higher than that in boron nitride. So will the friction in graphene channels be one hundredth of that in boron nitride? According to theoretical predictions, the friction between the two systems only differs by 3-5 times. Is the fact really as predicted by theory?

How does water achieve super lubricity?

On June 14, 2024, Science magazine published the research results of a research team composed of Professor Jiang Ying and Academician Wang Enge from the Center for Quantum Materials Science, School of Physics, Peking University. They used the domestically developed qPlus scanning probe microscope to discover the super-lubricating behavior of two-dimensional ice on the graphene surface, clarifying the origin of the ultrafast water transport properties under low-dimensional confined conditions.

Two-dimensional ice structures on graphene and boron nitride surfaces

(Image source: Reference 1)

What is two-dimensional ice? What is superlubricity? What is low-dimensional confinement? Let's explain them one by one.

"Low-dimensional confinement" refers to the fact that water molecules are restricted by geometric structures in nanoscale or smaller dimensions (such as two-dimensional or one-dimensional). This restriction usually occurs on the surface of extremely thin graphene materials (two-dimensional) or in very narrow nanochannels (one-dimensional). Due to the restrictions of the geometric structure, the movement of water molecules is constrained to a specific dimension and direction, and they cannot move freely like in three-dimensional space. Water often presents an ice-like structure.

Two-dimensional ice refers to the ordered structure of water molecules on a two-dimensional plane at the nanoscale. Due to the limitations of nanochannels or planar materials (such as graphene), water molecules cannot move freely as they do in three-dimensional space, but are arranged in a honeycomb hexagonal lattice, similar to the structure of ice. In simple terms, you can imagine two-dimensional ice as a thin and soft "ice cloth".

Superlubricity refers to the phenomenon that the friction between two surfaces is extremely low, almost zero. This usually occurs under specific conditions, such as nanoscale, extremely smooth surfaces or special interface structures, so that the interaction forces between atoms or molecules cannot form enough resistance to achieve frictionless sliding.

Illustration of ice sliding on a smooth surface

(Image source: AI generated)

That is to say, at the nanoscale, a piece of "ice cloth" can slide on the surface of graphene without friction!

How to measure the friction of microscopic two-dimensional ice?

Jiang Ying's team used the domestically produced qPlus scanning probe microscope to directly see the two-dimensional ice atomic structure on the surface of graphene and boron nitride. They both showed a double-layer interlocking hexagonal ice phase, which formed a very weak van der Waals interaction with the surface.

Top and side views of the most stable two-dimensional water structures on graphene surfaces (G\I) and boron nitride (H/J).

(Image source: Reference 1)

At the same time, in order to measure friction, you need to find something to push the two-dimensional ice on the graphene surface, but when faced with large areas of fragile two-dimensional ice, it is not easy to achieve stable and precise control and friction measurement. They prepared a special scanning probe microscope tip to move the two-dimensional ice on the microscopic surface.

In the macroscopic world, friction is independent of surface area, while in the microscopic world, friction is related to surface area due to the actual contact area and the interaction between molecules. The researchers found that on the graphene surface, as the area of ​​2D ice increases, the friction per unit area decreases rapidly and eventually falls below 1 piconewton, which is consistent with the theoretical prediction of superlubricity.

The static friction coefficient of larger two-dimensional ice on the graphene surface can even be lower than 0.01, confirming its superlubricity. On the boron nitride surface, the friction force per unit area of ​​two-dimensional ice always remains at a high constant value, showing traditional friction behavior. These experimental results are highly consistent with theoretical simulations.

Schematic diagram of the 2D ice on a microscopic surface as the tip of a special scanning probe microscope moves.

(Image source: Reference 1)

Why does 2D ice on graphene exhibit superlubricity but not on boron nitride? By imaging the 2D ice and the substrate on which they rest, the researchers found that the superlubricity of 2D ice on graphene stems from the weak van der Waals interactions between water molecules and graphene and the incommensurability between the 2D ice and graphene lattices.

Weak van der Waals interactions mean that the attraction between them is weak, and incommensurability means that the two different structures cannot be perfectly aligned, just like two mismatched puzzle pieces cannot be put together perfectly. Although the lattice of boron nitride is very similar to that of graphene, the polarity of the boron-nitrogen bond leads to good commensurability of the two-dimensional ice/boron nitride system. The two-dimensional ice matches the lattice of boron nitride very well and is "stuck" by certain structures, making it impossible to achieve superlubricity on the surface of boron nitride.

Huge application potential of "super lubrication"

The "superlubricity" phenomenon of microscopic water shows great application potential in many fields.

First, in microfluidics and nanotechnology, superlubricity can significantly reduce flow resistance and improve liquid transfer efficiency. This is of great significance for the development of efficient microfluidic devices, such as chip lab systems for precision medicine and advanced drug delivery systems, which can significantly improve the speed and accuracy of diagnosis and treatment.

Secondly, by utilizing the super-lubricating properties between water and graphene, future seawater desalination equipment will achieve more efficient and environmentally friendly water resource utilization.

When seawater passes through graphene nanochannels, water molecules can pass through without hindrance, while salt and other impurities are effectively isolated outside the channels. This not only significantly reduces energy consumption, but also reduces the generation of wastewater and waste, truly achieving green and sustainable seawater desalination. The application of this technology will provide an innovative solution to the global water shortage problem, while contributing to environmental protection and resource conservation.

The superlubricity of water has important application prospects in materials science and nanotechnology. It is believed that in the future, these applications will be of great benefit to the environment and humans.

References:

1.Wu, Da, et al. "Probing structural superlubricity of two-dimensional water transport with atomic resolution." Science 384.6701 (2024): 1254-1259.

2.Hummer, Gerhard, Jayendran C. Rasaiah, and Jerzy P. Noworyta. "Water conduction through the hydrophobic channel of a carbon nanotube." nature 414.6860 (2001): 188-190.

3. Majumder, Mainak, et al. "Enhanced flow in carbon nanotubes." Nature 438.7064 (2005): 44-44.

4.Xie, Quan, et al. "Fast water transport in graphene nanofluidic channels." Nature nanotechnology 13.3 (2018): 238-245.

5. Ramezani, Maziar, et al. "Superlubricity of materials: progress, potential, and challenges." Materials 16.14 (2023): 5145.