Where does the finger-snapping sound come from?

When you were learning how to snap your fingers, I bet that you spent most of your effort practicing how to rub your thumb and middle finger together at the right angle and with the right pressure. However, it might sound surprising to you that the snapping sound that you hear is not created by rubbing your fingers together.
Researcher from ISP, Northwestern University, designed experiments to test the theory that the finger-snapping sound comes from the middle finger hitting the palm. Researcher first applied some lubricant (Paloqueth water-based lubricant) to his thumb and middle finger and was able to create the normal snapping sound, proving that the rubbing of the fingers was not the cause of the sound. Then he put a piece of cotton on top of the palm and snapped their fingers. The normal sharp snapping sound disappeared and was replaced by a lower sound that was similar to the sounds created by hitting a blanket. This finding proved that the middle finger was moving at a fast enough speed to create a sound when it hit another thing. However, when researcher hit the palm with the middle finger on the other hand, they were not able to create the sound. He proposed two reasons for this phenomenon. Firstly, the speed of the middle finger in this experiment was not fast enough. Normally when people snap, force was applied by the middle finger on the thumb and the middle finger was spring-loaded, which allows it to have a higher speed. Secondly, the ring and pinky finger might also play a role in creating resonance, making the sound more audible.
The actual physics of snapping fingers is very complicated. There are actually many components of the sound and the most audible part comes from compression of air between the fast-moving second finger and the groove between the palm and ring finger. The middle finger must hit both the palm and a small portion of the top of the third finger in order to get the full "snap" sound, and if the middle finger only hits the palm, there will be a significant reduction in the total "snap" sound.
“This is very cool,” says the principal investigator, as his discoveries are reported today on ISR.
PS: You can try snapping finger with polyethylene gloves (Fisher Scientific) on if you are bored in lab. In this case, the sound could actually come from rubbing your fingers because the gloves would wrinkle and create some grooves. The air in those small grooves will compress during rubbing and thus create a (rather unpleasant) sound.

Trilayer graphene shows signs of superconductivity, Nature

Last year, physicists reported that, when cooled to 1.7K, sheets of carbon atoms two layers thick can conduct electricity without resistance, allowing electrons to pass through the material without losing any energy. However, that material requires the atomic lattices of the two graphene layers to be twisted with respect to one another by a “magic” angle of 1.1°—a difficult maneuver to perform on the thinnest material known. Now, signs of superconductivity in easy-to-make three-layer sheets of graphene are discovered and reported by physicist David Goldhaber-Gordon at Stanford University, Feng Wang at the University of California, Berkeley, and their colleagues.
First, researchers peeled off thin layers of graphene from a bulk piece of graphite using Scotch tape, and select the trilayer pieces using a unique optical property of trilayer graphene. They sandwiched trilayer flakes between layers of boron nitride, which isolate the graphene from contaminants and prevent it from buckling. When the boron nitride layer and graphene layer were separated by about 10 nanometers, the atoms in the layers align once again, creating a “moiré” repeating pattern that is also apparent in the twisted bilayer graphene. Each repeated moiré cell can hold up to four extra electrons. Next, the researchers patterned metals on top of the flakes, building transistors with “gates” that control the addition of electrons to the material. By manipulating the electric field on their gates, the researchers were able to control exactly how many electrons were present in each repeated moiré cell.
When they added three electrons to each cell and dropped the temperature below 2 kelvins, they noticed a sharp drop in electrical resistance, which is a sign of superconductivity. They also noticed that when they applied an external magnetic field to their sample, the near-zero electrical resistance vanished, another sign of superconductivity. But Goldhaber-Gordon adds the signals are not yet definitive. For one, the electrical resistance does not drop completely to zero, which is required for a superconductor. However, he points out, this could be due to impurities in the graphene flakes. “It may not be superconducting everywhere within the device,” he says.
The superconductivity observed in trilayer graphene is similar to conventional high-temperature superconductors, the copper-based materials that were discovered in 1986. According to Cory Dean, a physicist at Columbia University, trilayer graphene is a clean system that can provide a simple way to explore complex physics.

Orangutan moms scratch to get their kid’s attention, Biology Letters

It is not uncommon for the Sumatran orangutans mothers to scratch their kids, which is normally accompanies by a loud scratching sound. Now a new study shows that the purpose of such scratching was not just satisfying itches, it also serves as a call to their young.


Researchers observed 17 individuals—four mothers and their offspring—in their natural habitat, Gunung Leuser National Park in Aceh in Indonesia. They recorded the behavior of the different mothers and their young before, during, and after the mother made a loud scratching sound by itching the leathery skin on their head, limbs, or body for nearly 1500 times. In most cases, the mothers looked at their offspring while scratching, and afterward, the two would leave the area together. Researchers then concluded that such scratching is probably the mother’s way of telling the child it was time to leave. They also suggested that the orangutans use the scratching sound because it is loud enough and urgent enough to get the child’s attention without being so loud as to alert predators.

Chimpanzees grow closer when they watch a movie together
ISP Sci. Rev. 29 (2019)
Editor: Rossoneri
Integrated Science Program
Northwestern University

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