The bottle cap can be tightened automatically without any manual effort. What is the principle behind this?

While poking around on the internet, I came across some interesting short videos showing a “magic jam” that had an automatically twisting lid.

The lid of the jam jar can be screwed on automatically? | ktzwcrrk

This jam comes from the Japanese brand "Aohata". It is said that its bottle design is very special. As long as you gently place the lid on the bottle mouth, the bottle cap will rotate and tighten by itself, and there is no need to manually tighten the bottle cap.

This magical phenomenon has received a lot of attention on various social platforms.

Is the automatic twist-off lid really so smooth? What is the principle behind it? I decided to find this jam jar and verify it myself.

Automatic capping, proven to be effective

Actual tests show that the jam bottle mentioned on the Internet does often have the phenomenon of automatically twisting the lid.

As long as you gently place the bottle cap on the bottle mouth, there is a high probability that the cap will automatically rotate downward. However, this phenomenon does not necessarily occur every time it is placed, and the automatic rotation cannot really tighten the cap completely.

As long as you put the bottle cap on the bottle mouth, there is a high probability that the cap will rotate automatically | Photo by the author

At the same time, I also found four other similar glass canning bottles as a control group, and the caps of these bottles could not rotate downward automatically. This at least shows that the automatic rotation of glass canning bottle caps is not a common phenomenon.

Glass canning jars used as a control group | Photo by the author

Not a clever design

A bottle with a self-screw cap may seem like a clever convenience—but it’s not.

In 2022, the Japanese website "まいどなュース" reported this phenomenon and interviewed the jam manufacturer. The other party clearly stated that the automatic rotation of the lid "is not an intentional design function" and they are not sure why this phenomenon occurs. The manufacturer recommends that consumers still tighten the bottle by hand and store the opened jam in the refrigerator.

Manufacturer: I don’t know anything | まいどなニュース

Through actual experience, I can also feel that the automatic rotation of the jam lid is not very convenient. The automatic rotation alone cannot completely tighten the bottle cap and cannot form a seal, which is not conducive to preserving jam. If the bottle cap is not completely tightened by hand, the bottle body may slip off, which is more likely to break the glass bottle than not tightening the bottle cap at all.

So the automatic rotation of the bottle cap is just an accidental phenomenon. Although it is interesting, it has no practical value.

What is the principle?

So, what is the principle behind this phenomenon? The answer lies in the threads of the bottle mouth.

Through observation, it can be found that the biggest difference between the jam jar with automatic capping and the control group is the angle of inclination of the thread: the former has a larger angle of inclination, while the latter is more gentle. The automatic rotation of the bottle cap should be caused by the larger angle of inclination of the bottle mouth thread.

Comparison of an automatic twist-off jam jar (left) and a control jar of the same diameter (right). The former has six threads at the mouth, which looks steeper; the latter has four threads at the mouth, which looks more gentle. | Photo by the author

More precisely, the thread of the automatic twist-off jam jar seems to consist of three sections with different slopes. After a rough measurement, it was found that the inclination angles of the first and last sections of the thread were both above 10°, while the slope of the middle section was relatively gentle, with an inclination angle of about 6.5°.

The thread on the mouth of the automatic twist-off jam jar appears to consist of three sections with different slopes | Photo by the author

The threads of the control group bottles all looked relatively flat, with no significant difference in the inclination angles of each part. Their thread inclination angles were generally around 5°~6°.

Slider ramp problem

We can use the ramp slider commonly seen in physics problems to understand this process. The "automatic rotation" of the bottle cap is actually the process in which the bottle cap overcomes friction under the action of gravity and slides down the "slope" formed by the thread.

The downward rotation of the bottle cap can be roughly understood as sliding down a slope | Wikipedia

Objects on a slope sometimes slide down, and sometimes stay where they are. The key factors that cause this difference are the coefficient of friction and the angle of the slope. Increasing the slope angle will increase the component of gravity in the direction of the slope, and at the same time will cause the pressure exerted by the object on the slope to decrease, which will in turn reduce the maximum value of static friction. When the angle is large enough, the friction is no longer enough to offset the power of sliding down, and the bottle cap will naturally slide down and rotate.

The "automatic capping" phenomenon of the Aohata jam jar does not occur every time the bottle cap is put in, which should be related to the three-section slope of the bottle mouth thread. At the head and tail parts where the slope is steeper, the inclination angle exceeds the critical value, allowing the bottle cap to slide down; while the slope of the middle section of the thread is relatively gentle, which may not be enough for the bottle cap to slide on its own.

The place where the jam jar cap contacts the slope of the thread is the small bumps on the underside of the cap (as shown in the picture below). I marked the position of these bumps and then placed it at different positions on the threads to verify.

The small bump under the bottle cap is the contact point between the bottle cap and the thread slope | Photo by the author

As expected, placing the contact point on the steepest slope of the thread made it easy for the cap to slide down automatically.

Photographed by the author

If the contact point is placed on the gentle slope in the middle, the bottle cap can stay in place without sliding.

Photographed by the author

So far, the mystery of the automatic jam jar cap has been solved. It's not very useful, but we can still thank it for helping us review some physics knowledge.

References

[1] https://maidonanews.jp/article/14570579

[2] https://www.youtube.com/shorts/s_1C6pIW4zc

[3] https://thesmartlocal.jp/self-closing-jam-jars/

Planning and production

Source: Guokr (ID: Guokr42)

Author: Window Knocking Rain, Master of Pharmacy, Chief Isopod Enthusiast of Guokr

Editor: Yinuo

Proofread by Xu Lai and Lin Lin

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