In 2008, Miguel Ramos saw in the newspaper that a piece of amber 65438+ 1 100 million years ago was found a few hours' drive from Madrid where he lived, with primitive Mesozoic insects on it. As a physicist who specializes in glass, Ramos has been wanting ancient amber for many years. He contacted paleontologists working in this place and they invited him to visit.
"They provided me with clear samples that they didn't need," he said. There are no interesting insects or things in amber. However, they are perfect for me. " .
In the next few years, Ramos intermittently engaged in the measurement of ancient glass. He hopes that after such a long time of aging, this gum fossil can approach the imaginary form of a substance called ideal glass.
Physicists have always dreamed of this perfect amorphous solid. They long for the ideal glass, not because of it (although it has unique and very useful characteristics), but because its existence will solve a profound mystery. Every window, every mirror, every piece of plastic, every piece of hard candy, and even the cytoplasm of every cell constitute a mystery. Technically, all these materials are glass, because glass is solid and rigid, but it is composed of disordered molecules, just like molecules in liquid. Glass is a suspended liquid, a liquid whose molecules are incredibly stable. The ideal glass, if it exists, will tell us why.
Inconveniently, the ideal glass takes a long time to form, and may not have been formed in the whole history of the universe. Physicists can only look for circumstantial evidence to prove that this would be the case if given infinite time. Ramos is an experimental physicist at the Autonomous University of Madrid. He hopes that after 1. 1 100 million years of aging, Spanish amber may have begun to show perfect light. If so, he can know what the molecules in ordinary glass are doing when they seem to be doing nothing.
Ramos's measurement of amber is part of the surge in interest in ideal glass. In the past few years, the new methods of making glass and simulating glass on computer have made unexpected progress. In the past few years, new glass manufacturing methods and computer simulation methods have brought unexpected progress. There are some important clues about the properties of ideal glass and its relationship with ordinary glass. These studies provide new support for the hypothesis that the ideal glass state exists.
When you cool a liquid, it either crystallizes or hardens into glass. Which of these two situations depends on the essence and exquisiteness of the process, which glass blowers learned after thousands of years of trial and error. For them, avoiding crystallization is a dark art.
These two situations are quite different.
Crystallization is a dramatic transformation, from a liquid phase with disordered and free-flowing molecules to a crystalline phase with molecules locked in regular and repetitive patterns. For example, water freezes at 0 degrees Celsius, because at this temperature, water molecules stop shaking and are locked up just by feeling each other's strength.
Other liquids become glass more easily after cooling. For example, silicon dioxide (window glass) is a molten liquid, and its temperature is much higher than the initial 1000 degrees Celsius. When it cools, its disordered molecules shrink slightly and squeeze more tightly, which makes the liquid more and more viscous. Eventually, the molecules stop moving completely. In this gradual glass transition, molecules will not recombine. They just stopped slowly.
The exact reason why the coolant hardens is not clear. If the molecules in the glass are too cold to flow, it should still be possible to squeeze them into a new arrangement. However, the glass will not be squashed. Although it looks like a molecule in a liquid, its messy molecules are indeed rigid. Camille Scalliet, a glass theorist at Cambridge University, explained: "Liquid and glass have the same structure, but their behaviors are different. The key is to understand this. "
1948, a young chemist named Walter Kauzman discovered the so-called entropy crisis, which is a glass-like paradox. Later, researchers realized that the ideal glass seemed to solve this specious problem.
Kautzman knows that the slower the coolant is, the more it can be cooled before it becomes glass. Slowly formed glass will have a higher and more stable final density. Because its molecules take longer to move (when the liquid is still viscous) and find a tighter and lower energy arrangement. The measurement results show that the entropy or disorder degree of glass is correspondingly lower than that of slowly formed glass-molecules are reduced under the same low-energy arrangement.
According to this trend, Kautzmann realized that if the speed of coolant is slow enough, it can be cooled to the temperature now called "Kautzmann temperature" before it completely hardens. At this temperature, the entropy of the obtained glass will be as low as that of the crystal. But the crystal is a neat and orderly structure. By definition, glass is disordered. How can there be the same order?
Ordinary glass can't do this, which means something special will happen at Kautzmann temperature. If a liquid reaches the ideal glassy state after reaching that temperature, that is, the random packing state with the largest molecular density, then the crisis can be avoided. This state will show "long-range amorphous order", that is, each molecule feels and affects the position of other molecules, so in order to move, they must move as a whole. The long-range order hidden in this hypothetical state can be comparable to the more obvious order of crystals. Mark Ediger, a chemical physicist at the University of Wisconsin-Madison, said, "This discovery is the core reason why people think that there should be ideal glass."
According to this theory first put forward by Julian Gibbs and Edmund DiMarzio in 1958, ideal glass is a real material phase, similar to liquid phase and crystal phase. The transition to this stage takes too long and requires too slow cooling process, so scientists have never seen it. Daniel stan, a condensed matter physicist at new york University, said that the ideal glass transition was "masked" because the liquid became "very viscous and everything was blocked".
Stan said, "It's a bit like looking through the glass in the dark. We can't find (ideal glass) or see it. But in theory, we can try to build an accurate model for the situation there. "
This experiment brought unexpected help. Humans have used coolant for thousands of years, and there is no hope of forming ideal glass. In order to prevent the liquid from hardening before reaching the Kautzmann temperature, you must cool the liquid very slowly, even infinitely slowly. But in 2007, Wisconsin physicist Ediger developed a new glass manufacturing method. He said: "We have come up with another method to make high-density glass that is close to the ideal state. This is a completely different route. "
Ediger and his team found that they could create an "ultra-stable glass" between ordinary and ideal. They used a method called vapor deposition, which dripped molecules one by one onto the surface, just like playing Tetris, so that each molecule could stick tightly to the formed glass before the next molecule came down. The final glass is denser, more stable and has lower entropy than all the glasses in human history. Ediger said, "If you extract a liquid and cool it for a million years, these materials will have the characteristics you expect."
Another characteristic of ultra-stable glass will finally reveal the most promising road map of ideal glass.
In 20 14, two teams led by Miguel Ramos of Madrid discovered this feature. At that time, they found that ultra-stable glass deviated from the general characteristics of all ordinary glass.
For decades, physicists have known that ultracold glass has a high heat capacity, that is, the heat needed to raise its temperature. Glass can absorb more heat than crystals close to absolute zero, and the heat capacity is proportional to the temperature.
Theorists, including the respected Nobel Prize winner and condensed matter physicist Phil Anderson, put forward an explanation in the early 1970s. They believe that glass contains many "two-level systems", that is, small clusters of atoms or molecules that can slide back and forth between two optional and equally stable configurations. Frances Hellman of the University of California, Berkeley said, "You can imagine a whole string of atoms changing from one configuration to a very different configuration, which does not exist in crystal materials."
Although atoms or molecules are too tightly bound by their neighbors to make too many transformations by themselves, at room temperature, heat activates the two-stage system and provides energy for atoms to move. With the decrease of glass transition temperature, this activity gradually weakens. However, near absolute zero, quantum effect becomes very important: atomic groups in glass can "tunnel" between two different configurations through quantum mechanics, directly pass through any obstacles, and even occupy two energy levels in two energy-level systems at the same time. The tunnel absorbs a lot of heat, resulting in the unique high heat capacity of glass.
A few years after Ediger found a way to make ultra-stable glass, Hailmann group in Berkeley and Ramos group in Madrid set out to study whether glass would deviate from the universal heat capacity close to absolute zero. In their respective experiments, they studied the low-temperature characteristics of ultra-stable silicon and ultra-stable indomethacin, a chemical that is also used as an anti-inflammatory drug. Sure enough, they found that the heat capacities of these two kinds of glasses are much lower than the normal absolute zero, which is equivalent to that of crystals. This shows that there are fewer tunnels between the two energy level systems of ultra-stable glass. These molecules have a tight structure and almost no competitors.
If the abnormal low heat capacity of ultra-stable glass really comes from fewer two-level systems, then the ideal glass naturally corresponds to the state of no two-level system at all. David Reichman, a theorist at Columbia University, said, "Somehow, it is where all the atoms are out of order. It has no crystal structure, but nothing is moving. "
In addition, the reason driving this ideal long-range amorphous ordered state is that each molecule will affect the position of all other molecules, which may be the reason why the liquid hardens into common glass around us.
When the liquid turns into glass, it is actually trying to become an ideal glass phase, which is attracted by the long-range and orderly basic tension. The ideal glass is the end point, but when molecules try to get together, they will stick together; The increase of viscosity prevents the system from reaching the ideal state.
Recently, breakthrough computer simulations have been used to test these ideas. In the past, it was not feasible to simulate ultra-stable glass on computer, because it took a lot of calculation time for the simulated molecules to get together. However, a skill two years ago accelerated the calculation process 1 trillion times. The algorithm randomly selects two particles and exchanges their positions. These shakes help to keep the simulated liquid loose, so that the molecules can be stably formed into more suitable shapes.
In a paper published in Physical Review Letters, the co-authors reported that the more stable the simulated glass is, the less two-level systems it has. Like Herman and Ramos' heat capacity measurement, computer simulation shows that the competitive molecular group configuration of two energy level systems is the source of glass entropy. The fewer these substitution states, the stronger the stability and long-range order of amorphous state, and the closer it is to the ideal state.
On 20 14, Ramos and his collaborators published their comparison between the old samples and the "recycled" samples of yellow glass in Physical Review Letters. They found that the density of this amber with a history of 1. 1 100 million years increased by about 2%, which was consistent with the ultra-stable glass. This should show that amber has indeed stabilized over time, because a small group of molecules slipped into a lower energy arrangement one after another.
However, when the Madrid team cooled the ancient glass to near absolute zero and measured its heat capacity, the result told a different story. Old amber, new amber and all other ordinary glasses have high heat capacity. Its molecules seem to travel between many two-level systems as usual.
Why didn't the number of secondary systems decrease with the stability and density of amber? The results of the survey are inconsistent with this.
"I really like the experiment on amber, but the process of making amber glass is a bit confusing," said Eddie, the inventor of vapor deposition. "It's basically a kind of chewing gum. Chemical changes will occur over time and will solidify over time. " He thinks that impurities in Spanish amber may have polluted the measurement of heat capacity.
The researchers plan to do further experiments on amber and glass made and simulated in the laboratory, hoping to find more details of the two-stage system and get closer to the assumed ideal state. Lekman pointed out that its existence may never be proved with complete certainty. Maybe one day, we will know, at least on the computer, how to package particles accurately and make them the ideal glass we are looking for. But we have to wait a long time to see if it remains stable.