Can You Really Slip on a Banana Peel?

On September 18, four Nobel laureates made an announcement in front of a crowd of more than 1,000 people at Harvard University's Sanders Theater. The announcement had been eagerly awaited all year: the recipients of the 2014 Ig Nobel Prizes.

The Ig Nobel Prizes -- not to be confused with the better-known Nobel Prizes -- have been awarded every year since 1991 for research that makes people laugh, and then makes them think. The annual awards ceremony may be the only one that both opens and closes with a barrage of paper airplanes from the audience. The ceremony is organized by the journal Annals of Improbable Research, and co-sponsored by the Harvard-Radcliffe Society of Physics Students and the Harvard-Radcliffe Science Fiction Association. The awards, which recognize offbeat and eccentric research, are given out by actual Nobel Prize winners.

This year's Ig Nobel Prize in Physics was accepted by Dr. Kiyoshi Mabuchi, a professor of biomedical engineering at Kitasato University in Japan. He received the award on behalf of his co-authors Kensei Tanaka, Daichi Uchijima, and Rina Sakai for their 2012 paper in Tribology Online, the online journal of the Japanese Society of Tribologists.

The paper's topic should be familiar to fans of Looney Tunes and other cartoons: the slipperiness of banana peels.

Banana peels' reputation for being exceptionally slippery and potentially dangerous dates back to the 19th century. On April 26, 1879, an article in Harper's Weekly magazine warned, "whosoever throws banana skins on the sidewalk does a great unkindness to the public, and is quite likely to be responsible for a broken limb." Since then, slipping on banana peels became a comedy gag in vaudeville shows, and later in movies and cartoons.

That's not to say that banana peels are always a laughing matter. In December 2013, a New York man narrowly escaped death after allegedly slipping on a banana peel and falling onto subway tracks. On the other hand, a Washington, DC-area man, who claimed to have suffered a similar incident, was charged with fraud after unsuccessfully suing the local public transit agency.

Just how slippery are banana peels? Dr. Mabuchi and his co-workers decided to find out. Using a 6-axis force transducer, they measured the friction coefficient between a shoe sole, a banana peel, and a linoleum floor tile. The result was a friction coefficient of approximately 0.07, compared to a friction coefficient of about 0.41 between the shoe and the floor with no banana peel. This means that the banana peel reduced friction by about a factor of six.

They also answered the age-old question: is a banana peel slipperier with the yellow side up or down? The results strongly favor the "yellow side up" hypothesis. The friction coefficient was nearly twice as high at 0.12 with the yellow side down. In addition, they measured the friction coefficient of other fruit peels, including apple, tangerine, and citron, a thick-skinned relative of the lemon. None of the other fruit peels were as slippery as banana peels.

A banana peel's moisture content is critical to its slipperiness. Drying a banana peel for one month increased the friction coefficient to just under that of the linoleum floor. The authors believe that this is because water is necessary to the lubrication mechanism. They suggest that the low friction of banana peels is due to a layer of follicular gel, several microns thick, located between the yellow outside layer of the peel and its white inside layer. Applying pressure, for instance by stepping on the peel, causes water to exude from these follicles, and transforms the gel into a homogenous sol.

Banana peels are not Dr. Mabuchi's primary research interest, nor are most of his papers as humorous as this one. He's a noted expert in the field of biotribology, and has done a great deal of work on orthopedic implants, including hip replacement. The effect that is responsible for the slipperiness of banana peels is similar to the lubrication effects that take place in human joints.

Tribology is the study of friction, wear, and lubrication. It has a wide range of applications in engineering, manufacturing, and medicine, among other fields. Hopefully, Dr. Mabuchi's work will not only make people laugh, it will also make them think about the importance of tribology in the world around them.

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