Inspired by art, researchers find that the snap of a finger has the strongest acceleration that the human body produces

The snap of a finger was first depicted in ancient Greek art around 300 BC Today, that same snap initiates evil forces for villainous Thanos in Marvel’s latest film. Avengers movie. The two media inspired a group of researchers at the Georgia Institute of Technology to study the physics of a finger snap and determine how friction plays a critical role.

Using an intermediate amount of friction, neither too high nor too low, a finger snap produces the highest rotational accelerations seen in humans, even faster than the arm of a professional baseball pitcher. The results were published on November 17 in the Journal of the Royal Society Interface.

The research was led by undergraduate student at Georgia Tech, Raghav Acharya, as well as doctoral student Elio Challita, assistant professor Saad Bhamla of the School of Chemical and Biomolecular Engineering and assistant professor Mark Ilton of Harvey Mudd College in Claremont, California.

Their findings may one day inform the design of prostheses intended to mimic the extensive capabilities of the human hand. Bhamla said the project is also a great example of what he calls curiosity-driven science, where everyday events and biological behaviors can serve as data sources for new discoveries.

“Over the past few years, I have been fascinated by the way we can snap our fingers,” Bhamla said. “It really is an extraordinary physical puzzle at your fingertips that hasn’t been studied closely.”

In previous work, Bhamla, Ilton and other colleagues had developed a general framework for explaining the surprisingly powerful and ultra-rapid movements observed in living organisms. The framework seemed to apply naturally to the snap. He postulates that organisms depend on the use of a spring and a locking mechanism to store energy, which they can then quickly release.

Acharya and Bhamla felt a special push to apply this frame to a snap after watching the movie Avengers: Infinity War, released in April 2018 and produced by Marvel Studios. In it, Thanos, a villainous character, seeks to obtain six Special Stones and place them in his Metal Gauntlet. After collecting them all, he snaps his fingers and unleashes universe-wide consequences.

But would it be possible to break while wearing an armor gauntlet, the researchers asked? In the case of a finger snap, they suspected that the friction of the skin played a more important role compared to other spring and latch systems. With the frictional properties of a metal gauntlet, they imagined that might be impossible.

Using high-speed imaging, automated image processing, and dynamic force sensors, the researchers analyzed a variety of finger snaps. They explored the role of friction by coating the fingers with different materials, including metal thimbles to simulate the effects of trying to snap while wearing a metal gauntlet, much like Thanos.

For an ordinary bare-fingered snap, the researchers measured maximum rotational speeds of 7,800 degrees per second and rotational accelerations of 1.6 million degrees per second squared. The spin speed is lower than that measured for the fastest spinning motions seen in humans, which originate from the arms of professional baseball players during the act of throwing. However, instantaneous acceleration is the fastest human angular acceleration ever measured, almost three times faster than the rotational acceleration of the arm of a professional baseball pitcher.

“When I saw the data for the first time, I jumped out of my chair,” said Bhamla, who studies ultrafast movements in a variety of living systems, from single cells to insects. “The snap occurs in just seven milliseconds, over twenty times faster than a blink, which takes over 150 milliseconds.”

When the subjects ‘fingertips were covered with metal thimbles, their maximum rotational speeds decreased dramatically, confirming the researchers’ hunches.

“Our results suggest that Thanos could not have cracked because of his armored metal fingers,” said Acharya, the study’s first author. “So it’s probably Hollywood special effects, rather than actual physics, at play! Sorry for the spoiler.”

They explained this decrease by considering the reduced contact area that exists between the fingers covered with thimble.

“The compression of the skin makes the system a bit more fault tolerant,” said Challita, co-author of the book. “The reduction in both the compressibility and the friction of the skin makes it much more difficult to build up enough force in your fingers to actually snap.”

Surprisingly, the increased fingertip friction with rubber covers also reduced speed and acceleration. The researchers concluded that a Goldilocks friction zone was necessary: ​​too little friction and not enough energy was stored to fuel the snap, and too much friction resulted in energy dissipation as the fingers put in more time to glide over each other, wasting the energy stored in Heating.

The researchers experimented with a variety of mathematical models of the capture process to explain their observations. They found that a model comprising a spring and a soft friction contact latch could reproduce the qualitative characteristics of their results.

“We included a soft frictional contact in our mathematical model, and the results reinforced the central role that friction plays in achieving ultra-fast movements,” said Ilton. “This model can now help us understand how other animals such as termites and ants break their mandibles, as well as rationally bioinspired actuators for engineering applications.”

John Long, program director in the Division of Integrative Organism Systems at the National Science Foundation, oversees research in the Physiological Mechanisms and Biomechanics program, which is currently funding Bhamla’s investigations of ultrafast behaviors in animals.

“This research is a great example of what we can learn with smart experiments and insightful computer modeling,” he said. “By showing that varying degrees of friction between the fingers impairs the elastic performance of a snap, these scientists have opened the door to the discovery of the principles operating in other organisms and to the implementation of this mechanism in systems of ‘engineering such as bioinspired robots. “

John Long, program director in the Biological Sciences Branch of the National Science Foundation, oversees research in the Physiological and Biomechanical Mechanisms program, which is currently funding Bhamla’s research into ultrafast behaviors in animals.

“The research of Dr Bhamla and his colleagues is a great example of what we can learn with smart experiments and insightful computer modeling,” he said. “By showing that varying degrees of friction between the fingers impair the elastic performance of the snap fastener, they opened the door to the discovery of these principles operating in other organisms and to the implementation of this flexible, sophisticated and flexible mechanism. adjustable in systems such as as bio-inspired robots. ”

The researchers believe the findings open up a variety of opportunities for future studies, including understanding why humans crack at all and whether humans are the only primates to have developed this physical ability.

“Based on ancient Greek art from 300 BC. “This is the only science project in my lab where we could snap our fingers and get data.”