Friday, July 28, 2017
by Tia Ghose
The mystery of tiny glassteardrop-shaped glass confections that can survive a hammer blow, yet shatter to smithereens with the slightest touch to the stem, has finally been solved.
The strange shapes, called Prince Rupert's drops, have posed a riddle that has stymied scientists for 400 years.
"On one hand, the head can withstand hammering, and on the other hand, the tail can be broken with just the slightest finger pressure, and within a few microseconds the entire thing shatters into fine powder with an accompanying sharp popping noise," study co-author Srinivasan Chandrasekar, a professor of industrial engineering and director of the Center for Materials Processing and Tribology at Purdue University in Indiana, said in a statement.
Now, a new study reveals that the head of these little glass tadpoles has such indomitable strength because of the compressive forces acting on the outside of the drops. These forces rival the compressive forces withstood by some forms of steel, the study found.
Prince Rupert's drops first gained widespread fame in 1660, when Prince Rupert of the Rhine (of Germany) brought a few of the curiosities to King Charles II of England. (The teardrops, which are made by pouring molten glass into cold water, had likely been known to glassblowers centuries earlier.) Charles then handed them over to the Royal Society, which published its first scholarly investigation of their properties in 1661.
Prince Rupert of The Rhine
Over the centuries, scientists puzzled over the riddle of Prince Rupert's drops. In 1994, Chandrasekar and a colleague used a high-speed camera to capture 1 million frames per second of the drops as they shattered. The footage revealed that tiny cracks that form in the tail rapidly spread into the head.
Once those cracks reach high enough speeds (about 1.5 kilometers per second), they split in two, Chandrasekhar said. Then those two cracks reach a high enough speed and split in two, and so forth. Eventually, the entire structure is completely overtaken by myriad tiny cracks, he said.
"The tail will snap off but the head will explode into powder, and that part is actually quite spectacular," Chandrasekhar told Live Science.
Tht finding explained why the tail's snapping destroys the structure so easily. However, since that investigation, scientists have tried to explain these glass baubles' paradoxical combination of strength and fragility, but have never come up with a satisfactory explanation of the head's nearly shatterproof properties.
In the new study, Chandrasekar relied on a slightly different technique called integrated photoelasticity, to reveal the mysteries of the glass tadpoles' heads. The technique calls for placing the object in a pool of water and then passing polarized light waves, or light that is oriented in a single plane, through the material. Internal stresses inside the material change the polarization of the light. Looking at the polarization of the outgoing light waves through special filters reveals the internal stresses inside the object - in this case, the head of the drop and the tail.
It turned out that the heads of the Prince Rupert's drops sustained extraordinary levels of compressive stress - about 50 tons per square inch. (Compressive stress is the force that squishes things together).
These stresses formed because the type of glass used in these teardrops - which expands dramatically with heat - also shrinks dramatically when exposed to cold water. During the process to make these drops, the molten glass is dipped in cold water. When the glass hits the water, the outside cools faster than the inside. The outside layer of the glass then forms a kind of "jacket" that squishes the inside. Because the inside is still cooling, and because the total forces acting on the object have to equal zero, the head forms tensile stresses on its interior, the researchers reported in their paper, which was published online in Applied Physics Letters. (In general terms, tensile stress is the internal force per unit area that pulls things apart – think of the act of tearing a piece of paper in half. Tensile and compressive stresses act in opposite directions and so cancel each other out.)
The reason the compressive stress on the outside of the drops prevents fracturing is somewhat intuitive; the compression is squishing the atoms of the glass closer together – so they have no place to go. Fractures also don't move as easily through materials under compression. By contrast, most materials tend to break more easily when they are being pulled apart in tension.
However, even these shatter-resistent confections will eventually crack under pressure; for instance, if the heads of the drops are put inside a vise with enough pressure, they too will eventually turn to powder, though not quite as spectacularly as in the tail-snapping process, Chandrasekar said.
"Nothing is unbreakable," Chandrasekar said.
Tia Ghose has interned at Science News, Wired.com, and the Milwaukee Journal Sentinel and has written for the Center for Investigative Reporting, Scientific American, and ScienceNow. She has a master's degree in bioengineering from the University of Washington and a graduate certificate in science writing from the University of California Santa Cruz.
Thursday, July 27, 2017
Tuesday, July 25, 2017
The link between football and traumatic brain injury continues to strengthen. Now, one of the largest studies on the subject to date finds that 110 out of 111 deceased NFL players had chronic traumatic encephalopathy (CTE), a degenerative brain disorder associated with repetitive head trauma.
Several studies have linked CTE to suicidal behavior, dementia and declines in memory, executive function and mood. Professional athletes may be at higher risk for CTE because of their high likelihood for concussions and other traumatic brain injuries; u p to 3.8 million sports-related concussions occur in the United States each year. In 2016, a health official with the NFL acknowledged the link between football and CTE for the first time.
In the new study, published in the Journal of the American Medical Association, researchers looked at the brains of 202 deceased people who had played football at various levels, from high school to the NFL. (The brains had been donated to a brain bank at Boston University for further study.) The researchers analyzed the brains for signs of CTE and also spoke to family members about the players’ histories. They diagnosed CTE in 87% of the players. Among the 111 NFL players, 99% had CTE.
"This study more than doubles the number of cases reported in the literature of CTE," says study author Dr. Jesse Mez, an assistant professor of neurology at Boston University School of Medicine. "It suggests, with a lot of caveats, that this is probably not a rare disease, at least among those who are exposed to a lot of football."
The severity of CTE symptoms appeared to progress the more a person played the sport. High school players included in the study tended to have mild disease, and most college, semi-professional and professional players had severe symptoms. The study authors also found that mood, behavior and cognition problems were common among the players with mild to severe CTE.
Among players with severe CTE, 85% had signs of dementia, and 89% had behavioral or mood symptoms, or both. They were also likely to have issues in brain regions associated with depressive symptoms, impulsivity and anxiety. 95% had cognitive symptoms, like issues with memory, executive function and attention.
The study has key limitations. Researchers studied a limited and possibly skewed sample of brains; news about repetitive head trauma and CTE has become increasingly prevalent, and families of players with symptoms of brain injury may have felt more motivated to participate in the brain bank study. It’s also still difficult to say how common CTE is among all football players.
"The numbers are not meant to represent the prevalence of CTE in football players," says Mez. "But it does begin to suggest a relationship between football and this disease, and that's an important step for research that will look at this in the future."
Mez says the brain bank, which is ongoing, receives between 50 to 100 donations every year. Having access to brain tissue allows the researchers to study possible mechanisms for CTE, and why some players develop it while others do not. "We are really early in understanding this disease," says Mez.
For years, the NFL has stood by the contention that there is no direct evidence proving that playing football is linked to traumatic brain injury or the devastating brain disorder chronic traumatic encephalopathy, which is increasingly being diagnosed in former players. And they were right, in a sense. The evidence that existed was circumstantial, and most involved finding signs of TBI in deceased players, making it impossible to know for sure whether their time in the league was responsible or whether other factors played a role. But now, scientists have revealed a strong link between playing football and trauma to the brain. The reaction by the NFL is the study is inaccurate and so are its conclusions.
As to the data on concussions, TBI and football continues to grow because it bring in a lot of money but parents, coaches and professional athletes have to started talking about ways to reduce trauma to the brain. That means starting to talk about limiting contact to the head during practice, perhaps even eliminating it, while the research continues. However, that is not likely to happen any time soon.