How slippery is a banana peel? To tell you the answer, he "stepped" on a banana peel 60 times

Have you ever slipped on a banana peel? Not only banana peels, but also watermelon peels, apple peels, "onion snot", "cabbage butts" and other kitchen waste can easily make people slip. In movies, scenes of people slipping on banana peels often appear.

Image source: Veer Gallery

Why do these kitchen wastes make people slip? Which one is more likely to make people slip: banana peels, apple peels, or orange peels? What inspiration can these phenomena bring to humans?

Dangerous "green onion snot", source: Japanese Twitter

The "bored" old man who won the "Nobel Prize" with a banana peel

How slippery are banana peels?

There are scientists who have actually conducted serious experimental research. Japanese scientist, Professor Ma Liuqingzi of the Faculty of Health Sciences at Kitasato University, won the 2014 Ig Nobel Prize in Physics for measuring the friction coefficient of banana peels and revealing their lubrication principle.

Professor Ma Liu is actually engaged in the study of the lubrication mechanism of articular cartilage and artificial joints. When talking about his research results with others, he often said: "This thing (joint) is as slippery as stepping on a banana peel when it turns." One day, he suddenly realized something: how slippery is a banana peel? Is my example correct?

Curiosity and thirst for knowledge prompted him to try to find literature data on the friction force related to stepping on banana peels. But no one had seriously studied how slippery banana peels are, so he decided to study it himself.

At first, Professor Ma Liu wanted to use the inclined plane to measure the friction coefficient of banana peels directly. He put the banana on an inclined plane and kept increasing the inclination angle of the inclined plane. Finally, he found that the banana could slide on the inclined plane only when the inclination angle exceeded 45 degrees. After conversion, the static friction coefficient of the banana was about 1.2. This value far exceeds the friction coefficient between the sole and the flat ground, which is 0.4 (the smaller the friction coefficient value, the more slippery it is). Can banana peels be anti-slip? This conclusion obviously goes against common sense. The static banana peel cannot reflect its true smoothness at all.

Two methods for determining the coefficient of friction of banana peel with and without load, source: Reference 1

Therefore, Professor Ma Liu decided to use special equipment in the laboratory to measure the lubrication level of artificial joints to simulate the situation of a person stepping on a banana peel to measure the friction coefficient of the banana peel.

After measuring 60 sets of data, Professor Ma Liu found that the friction coefficient of banana peel under simulated conditions was only 0.066, which is much lower than the friction between the sole of the foot and the flat ground under normal circumstances, and even smaller than the friction under the feet when skiing or skating. Banana peel is indeed very slippery.

In addition to banana peels, Professor Ma Liu also tested the friction coefficients of various biomasses, including apple peels and orange peels, whose friction coefficients are around 0.1 and 0.2 respectively.

From the perspective of safety engineering, there is a 90% chance of slipping on surfaces with a friction coefficient of less than 0.1. In other words, the probability of slipping after stepping on a banana peel or even an apple peel is more than 90%.

Friction coefficients of various biomass surfaces, source: Reference 1

Why are banana peels so slippery?

This has to start with the structure of banana peels. Through an electron microscope, Professor Ma Liu observed that there are many vesicular cells on the inner surface of banana peels, which are filled with a large amount of polysaccharide biological mucus. When squeezed, the small bubbles containing mucus will rupture and release mucus. The lubricating effect of these mucus is the culprit that makes you and me fall on our backs.

Comparison of microbubbles in banana peel before and after bursting, source: Reference 1

It is very common for biomass lubrication to rely on mucus. The composition of biomass mucus is relatively complex, but in general, the mucus of fruits is mainly formed by large molecules such as polysaccharides dissolved in cell fluid. The long carbon chains of polysaccharides allow them to form a complex three-dimensional polymer network at the microscopic level, thereby obtaining a higher viscosity.

You may wonder, fluids with high viscosity are not easy to flow, doesn't this just mean that there is a lot of friction inside them? Why can mucus also have a lubricating effect?

This is because in general solid contact friction, the entire surface is not actually in contact, but only the protruding parts of the surface are in friction. If a liquid is introduced between the solid surfaces to form a liquid film, direct solid contact can be minimized.

The reason why mucus has a better lubrication effect than ordinary clear liquid is that the higher viscosity and internal friction of mucus make it more difficult to be expelled from the contact surface when under pressure, thus having a larger lubricating film thickness and lubricating surface ratio.

However, once these biomasses are completely dehydrated, their lubricating effectiveness disappears.

You may not know it, but your joints are slipperier than banana peels

Biomass lubricants such as banana peels and apple peels only cause us some minor troubles in life, but in some scenarios, biomass lubricants can cause significant losses.

Millipedes are found in some places in the central mountainous areas of Japan every eight years. This is because the life cycle of these millipedes is eight years, and the eighth year is when they emerge from the soil to mate. Millipedes are extremely spectacular, with hundreds of them in one square meter. The biggest trouble that millipedes bring to the local area is when they occur in large numbers on the railway.

Millipedes. Considering the crowd-dense nature, I won’t post pictures of millipedes clustering. Source: Veer Gallery

The local area is a mountainous area and the railway tracks have a certain slope. Once a large number of millipedes accumulate, the train will slip due to the mucus produced after crushing the millipedes. In serious cases, the train wheels will completely idle when going uphill.

Local trains have to suspend operations during the two-month breeding season of the millipedes, which has earned the millipedes their common name "train millipedes".

On the other hand, our ability to walk flexibly is inseparable from biological lubrication. The most amazing lubricant in the human body is synovial fluid. Synovial fluid fills the entire joint cavity and provides lubrication for joint movement. Human joints need to withstand hundreds of Newtons of pressure for decades, and the reason why they can maintain good operation for a long time is all thanks to synovial fluid. Under the lubrication of synovial fluid (of course, including the unique structure of the entire joint cavity), the friction coefficient of the joint is only about one-tenth of that of a banana peel, which is several times lower than the friction coefficient when skating and skiing.

Joint fluid contains a large amount of hyaluronic acid, which is also a biomacromolecule with lubricating properties. In addition to polysaccharides and hyaluronic acid, there are many other aqueous solutions of biomacromolecules in organisms, such as proteins and nucleic acids, which can also form mucus with lubricating effects. In this sense, all biological organisms will be "slippery" under certain conditions.

Studying lubrication phenomena related to biomass is closely related to human material production and physical health. No matter how unreliable the research object may seem, the motivation behind it is serious.

References:

The skin of the person is slippery. The joints are slippery?ナナの slippery, さらにlife science の広か?りへScience and technologyとculture-ハ?ナナのpiか? The world of extension - Biological science? What is the surface microstructure?ロシ? What's the relationship?ルのFrictionとWater and lubrication~Biological sliding らかなkinetic explanation へのアフ?ローチ