“Don’t stick your smartwatch on the high-speed train window”, why?

Review expert: Zhou Hongzhi, senior laboratory technician, master's supervisor, School of Physics and Optoelectronic Engineering, Beijing University of Technology

"Don't stick your smartwatch on the high-speed train window" has become the most popular "Don't swallow the light bulb incident" at the moment. Although many people have warned others not to try it based on their personal experience, people are still curious and have tried to verify it. As a result, the watch is stuck on the window and it takes a lot of effort to remove it intact. So, why does the watch get sucked?

Image source: Xiaohongshu

Is it because of the "glossy glue phenomenon"?

Some netizens pointed out that the two were sucked together because of the "optical glue phenomenon".

The glossy phenomenon is a physical phenomenon. When the smoothness of an object's surface exceeds a certain limit, the smoother it is, the greater the friction will be.

Because the molecules on two highly smooth surfaces will be closely adjacent to each other, when the distance between them is small enough, there will not only be atmospheric pressure between the two surfaces, but the molecules on the two surfaces will also attract each other due to the electromagnetic force (Lorentz force).

At this point, it will be difficult to separate the two objects or move them relative to each other, because the combined force of attraction between the many molecules on the contact surface of the two objects is greater than the adhesive force of the super glue.

Optical lenses used in microscopes or telescopes are generally required to be very smooth, and scientists often need to combine two lenses of different magnifications. When the lens surface is very smooth and the contact surface is large enough, the optical lenses will stick together tightly as if the middle contact surface is coated with strong glue, and it is difficult to separate them, just like they have become one lens, hence the name "optical glue phenomenon".

Later, this technology became a production process for optical instruments, called the "optical glue" process. In this process, no actual glue is used.

However, because smart watches are often worn and used, their surfaces cannot reach the mirror surface that can be adsorbed, and the same is true for the glass of high-speed trains. The smoothness of the surfaces of both is difficult to reach the standard of "glossy glue phenomenon". Obviously, the adsorption of smart watches is not a glossy glue phenomenon.

Therefore, if adsorption occurs, you need to consider whether a protective film is applied.

Is it because of the film on the watch?

After observing the posts posted by everyone, some netizens found that only watches with hydrogel film can be firmly attached to the glass. Watches without film or other films do not have this ability.

Nowadays, most people who wear watches are used to applying film to the dial, and hydrogel film is the first choice for many people. The reason why it is sucked and cannot be removed is that the hydrogel film is sticking?

The hydrogel film is made of TPU (thermoplastic polyurethane elastomer) as the base material and combined with hydrogel. It has super ductility and retraction, self-repairing function, impact resistance, durability, better toughness, and a certain buffering effect on collisions with sharp objects.

Relying on this characteristic, when the hydrogel film is attached to the glass, the air in between can be expelled to achieve the effect of vacuum adsorption.

The meaning of vacuum is the gas state with a pressure lower than one atmosphere in a given space. People usually call this rarefied gas state vacuum.

The gas pressure in a vacuum state is lower than one atmosphere. Therefore, various vacuum containers on the surface of the earth will be affected by atmospheric pressure, and the magnitude of the pressure difference depends on the pressure difference between the inside and outside of the container.

The vacuum hooks commonly used in homes utilize the vacuum principle, leave no marks, do not damage the wall, and are suitable for all kinds of smooth surfaces.

If your watch is also firmly attached to the glass and cannot be removed, pulling hard may easily pull the dial off and cause damage. You can try rotating it left and right, or find a thin wire and "cut" it back and forth to "scrape" the watch off.

In general, although it is an experiment with the spirit of scientific inquiry and exploration, it is still not recommended for everyone to try it.