Is glass a solid or a liquid

In the morning, the sun shines on your face through the glass window, music rings, your fingers flick on the glass screen of your mobile phone, turn off the alarm clock, the long milk fragrance overflows from the glass on the dining table, wake up the new day In life, people have been used to glassy matter for a long time, but you may not know that these almost everywhere "glass" has always puzzled physicists. Even science magazine, which has always been very cold, has listed "what is the nature of glassy matter" as one of the most challenging scientific frontier issues on the 125th anniversary of its founding.

Is it a solid or a liquid

From the macroscopic point of view, a solid has a fixed shape, while a liquid has no definite shape; from the microscopic point of view, some solids have a very tight atomic structure, with atoms arranged regularly and periodically, and this regular and orderly structure is relatively stable. In contrast, the arrangement of the atoms in the liquid is more chaotic. They gather irregularly and the positions of the atoms change constantly.

And glass has shape and hard, with the properties of solid. But the strange thing is that the arrangement of the atoms is the same as the disordered arrangement of the liquid. That is to say, from the surface, glass is more like a solid, but the arrangement of internal atoms is much like a liquid.

Some experts think that the solid matter in nature can be roughly divided into two categories according to the characteristics of its microstructure: one is the orderly arrangement of atoms or particles, just like the square array of military parade, i.e. crystalline solid; the other is the disordered arrangement of atoms or particles, just like the bustling crowd on the street, i.e. amorphous solid. Glass is a typical amorphous solid.

"In fact, glass is a very slow liquid for atoms," Wang Weihua, academician of the Institute of physics of the Chinese Academy of Sciences, told Science and Technology Daily on August 15. The atoms in the liquid are constantly moving, but the atoms in the glass move extremely slowly, which is 20-30 orders of magnitude slower than that in the ordinary liquid. " In order to explain the huge gap between the velocity of atoms in general liquid and that in glass, Wang Weihua made an image analogy: "atoms move faster than rockets in general liquid, but slower than snails in glass."

Philip Gibbs, an American scientist, published "is glass a solid or a liquid?" on the homepage of the Mathematics Department of the University of California, riverside? 》It is also pointed out that from some aspects of molecular dynamics and thermodynamics, glass can be regarded as a high viscosity liquid, an amorphous solid, or another state that is neither a liquid nor a solid.

The process of morphological transformation is confusing

In addition to the fact that glass is a solid or a liquid, the transformation of glass from a liquid to a solid like state of glass is also confusing. "People living in Mesopotamia started using glass 4000 years ago, but we still don't fully understand how liquids change into glass," said Kenneth Kelton, a physicist at the University of Washington, St. Louis. This is one of the most interesting dynamical processes. "

When some materials change from liquid to solid, their atoms are arranged in a highly regular pattern, which is called "lattice". That is to say, when these substances are in liquid state, the atoms can move freely, and then at some time, the atoms will suddenly find themselves trapped, so a regular lattice arrangement is formed, which is called "crystallization". This kind of change occurs in the process of cooling molten steel into solid steel.

However, in the process of changing from a hot liquid to a glass, the moving glass atoms are not trapped suddenly, but slow down with the decrease of temperature. Finally, these atoms are still arranged irregularly like a liquid. In other words, we have a strange phenomenon in glass - the irregular arrangement of liquid atoms is magically fixed.

Although the atoms in the glass are arranged randomly to some extent, they are actually more stable than the surface, Wang said. The vast majority of atoms may be confined to certain places by their "neighbors". What about the slow motion of atoms in glass? How do they move?

In other words, how does glass flow? This is also the unsolved mystery of condensed matter physics and material science. Current research shows that if an atom wants to move, the atoms around it also have to move. Just as you're going to get out of an extremely crowded bus, other passengers have to move to get out of the way. The slow movement of glass atoms involves the common movement of a large number of atoms inside, but the way of their movement is still unclear.

Some conjectures on how glass flows

In the process of glass formation, how is the irregular arrangement like liquid fixed? Scientists have put forward many theories to explain this outstanding problem.

In 1959, Cohen, a scientist at the University of Chicago, put forward the theory of free volume. According to the theory, there are many extra volumes in the liquid that are not necessary for the arrangement of atoms. These volumes can be redistributed without additional energy, so they are called free volumes. With the decrease of liquid temperature, the free volume of atoms decreases gradually. When the free volume decreases below a certain critical value, glass is formed. But how to strictly define and measure the free volume can not be done.

The modal coupling theory, which appeared in 1980s, is considered to be the most useful theory to describe the glass transition. The physical image of modal coupling theory can be summed up as "cage effect": every particle in the liquid is located in the cage formed by its neighboring particles, and the life of the cage increases with the decrease of temperature. When the temperature is close to a certain critical temperature, the cage life will tend to be infinite. In the liquid with high flow performance, in addition to the regular vibration and random "swimming" of particles in the cage, the position of the cage where particles are also changed with the rearrangement of the surrounding particles, that is, in addition to the vibration in the cage, particles will also change the cage where they are for the random diffusion motion. When the temperature is lower than the critical temperature, the life of the cage is infinite, which makes the liquid frozen into glass. Because of the confinement and limitation of the cage, the disordered arrangement of particles is preserved.

In addition to these theories, scientists have made some discoveries about this problem in their experiments. Early experiments carried out by American physicist Kelton and his team showed that atoms can form islands by gathering together in an orderly structure. These islands seem to stop the liquid from turning into a solid, leaving the liquid in a more chaotic state. But researchers did not agree on how islands work or whether they exist in all glass.

Patrick royal of the University of Bristol and others in the experiment, in order to observe the real movement of micro glass atoms, large colloidal particles were used to simulate the glass atoms, and observed with a high-power microscope. It was found that the gel formed by these particles formed a twenty surface structure and could not form crystallization.

The discovery and application of glassy matter and its related research have gone through a long history and made fruitful achievements. However, there are still many problems in the nature and basic laws of glassy matter that deserve further consideration. Perhaps in the near future, scientists will give us a satisfactory answer about how glass forms and flows. With the answers to these questions, we will bring more new glass materials and change our lives.