Ice-X

14

Snowflakes under a magnifying glass
seemed much larger
what they really were
and waited for luxurious flowers
or tenth stars.
See how well it has been done! —
Kai said. — Snowmen are proud
more interesting than real flowers!
And what accuracy!
Oh, if only
they didn’t hide!

G.-H. Andersen.
“The Snow Queen”

15

Ice has always seemed to people as a symbol of beauty, precision, and size. But soup-like beauty, beauty that carries within itself coldness, evil, and sometimes death. No wonder Dante imagined that ice should be in the very center of the Earth, at the last step leading to Hell, and Satan’s treasures were hidden in an ice chest.

Undoubtedly, the artist’s exploration of natural phenomena and the laws of the world goes through his emotional perception of the surrounding world. A scientist must be dispassionate, both when choosing an object of study and when analyzing numerous (and sometimes contradictory) data.

Why might ice be interesting from a physics point of view?

The chemical formula of ice is H₂O. When cooled, water freezes, or, more precisely, crystallizes. Currently, thirteen (!) different structural forms of ice have been discovered. Even if this number is considered unlucky by some, it puts ice in a special position - not a single substance, simply in its chemical composition, can be similar to such common phase transitions. This is the first reason to take a closer look at this object.

figure 1 shows almost all the phase diagram of H₂O. It has been studied quite well, although it is still rich in white spots. Suffice it to say that ice-X and ice-XI were discovered only a few years ago. The ice that we are all accustomed to is also indicated in the diagram - this is ice-I, or, more precisely, ice-Iₙ. At normal pressure and 0 °C, its specific density is 0.917×10³ kg m⁻³, it is lighter than water, i.e., during crystallization, the density of water decreases.

This completely unlearned property of ice (after all, crystallization generally leads to an increase in density) is of enormous importance for life on Earth. The ice shell that forms on the surface of the water creates a column of effective thermal insulation, so that reservoirs rarely freeze to their entire depth. I don’t even want to think what would happen to the underwater world if ice were even a little heavier than water!

Fig. 1. Fig. 2.

18

nom on figure 3, b, leads to a violation of paragraph 3.

But it is known that ice is not an ideal dielectric, but has a completely different electrical conductivity, and proton conductivity. Read, protons can move! So, are the BFP rules wrong? The correct answer is this: they are almost always correct, but it is the exceptions to these rules that are responsible for the amazing electrical properties of ice.

At the end of the fifties, the Tsarist physicist Jacquard proposed a very elegant way to describe these electrical properties by the movement of free defects that arise precisely when the BFP rules are violated. This is a very unteachable mechanism of electrical conduction, so let’s look at it in more detail.

Let’s “spoil” the ice lattice by violating 2nd of the BFP rules: we remove a proton from one oxygen atom and add it to another - as in figure 3, b. We will get so-called ion defects (they are designated OH⁻ and H₃O⁺). Now let’s spoil the lattice by violating rule 3, as shown in figure 3, f. In this case, orientation defects will arise (the so-called L- and D-defects).

Defects are always present in the structure of ice, but there are very few of them - in chemically pure ice at a temperature of -10 °C, for every 3 × 10¹¹ water molecules there are about 10⁵ orientational defects and only a couple of ion defects.

At figure 4, a it is shown how the movement of protons can occur. Let’s consider the ion defect H₃O⁺. Proton 1 moves to the place indicated by the cross. Previously, the H₃O⁺-ion was at the oxygen atom I, but now it will be at the oxygen atom II. Next, proton 2 moves to the place also marked with a cross - now it is H₃O⁺ at atom III, etc. We see that each time different protons move, as if passing the baton to each other. And as a result, the H₃O⁺ ion “goes” all the way “by itself”. figure 4, b shows the ice lattice after the H₃O⁺ ion moves from position I to position X.

For clarity, we took a hypothetical flat square lattice of ice. In this lattice, each oxygen atom has clear equivalent oxygen neighbors, as in a three-dimensional lattice, and therefore all the conclusions obtained here are valid for the true lattice of ice. Try to figure out for yourself how OH⁻-ions, L- and D-defects move.

It turned out that in some other substances the conduction mechanism is the same. Nowadays, even a lot of blame follows the study of the mechanism of electrical conductivity of ice, since in a number of biological objects the transference is very similar to the movement of protons in ice.

Jaccard’s theory required a lot, but every year the number of experimental facts that require further deepening of existing ideas is growing. New, and sometimes very uneducated, hypotheses appear. According to one of these bold hypotheses, the true charge carriers in ice are electrons that “mount” defects moving protons. Whether this is so, time will tell.

19

The bulk properties of ice are quite necessary and even extincious, but the properties of the ice surface are still surprising.

Wake up any person in the middle of the night and ask at what temperature does ice melt? “Of course, at 0°C.” But the question “at what temperature does the ice surface begin to melt?” cannot be answered unambiguously even by experts in ice physics.

From the point of view of fundamental science, studies of the ice surface are extremely interesting. But these studies are of equal, if not greater, practical importance. The movement of icebreakers, anti-icing of ships and aircraft, construction in the polar and polar regions, the movement of glaciers - in all these cases it is important to know what happens on the surface of the ice, i.e. at the interface between ice - metal, plastic, various groups, etc. It is necessary to know the mechanisms of adhesion of ice to various structural materials, the nature and magnitude of friction and similar characteristics. And how necessary are materials with a minimum friction coefficient for various sports equipment!

By the way, why do many materials glide so well on ice?

This question may seem simple to many. After all, good gliding is when there is a layer of water between the body and the ice. And when a body presses hard on ice, it melts - under pressure, melting begins at a lower temperature.

Let’s figure it out. As follows from the physical diagram, the melting temperature of ice increases by 1 degree with additional pressure Δp=10⁷ Pa. Let the mass of the ridge be M=60 kg, and the section of the ridge S=3·200 mm²=6·10⁻⁴ m²; then the additional pressure will be Δp=Mg/S=10⁶ Pa. This means that under the ridge, the melting point is only… 0.1 °C. But what about skiing at -10°C? Yes, the explanation for good gliding is apparently something else.

It is believed that in the extreme temperature range there is a thin quasi-id layer on the surface of crystalline ice (“quasi” means “almost” or “as if”). It is unlikely that this thin membrane is a common liquid. However, many of its properties are very close to those of water. In some experiments, the existence of a quasi-id layer can be traced even at -30 °C, i.e. 30 degrees before melting!

Perhaps the presence of a quasi-id layer causes a small friction coefficient? Such a proposal can explain a lot, but, unfortunately, the true nature is much more complex and remains to be understood.

By the way, the debate between the concepts of “quasi-id layer” and “pressure melting” has been going on for more than a century. At the origins of this dispute there are replacement physicists - Faraday and the Thomson brothers (one of them is known to everyone as Lord Kelvin). Yes, yes, these world famous scientists also studied ice.

I have no doubt that you all played snowballs. But have you ever wondered why it’s so easy to split a snowball? Snow is not dough, not plates, but small crystals (!) of ice. After all, you can’t make a “snowball” out of metal filings, but you can make a snowball out of ice. Don’t think so?

But Michael Faraday, more than a hundred years ago, apparently began to think. In 1850, he drew attention to the following fact: pieces of ice freeze if they are brought into contact at a temperature of about 0 ° C. Why? It was to explain this fact that Faraday suggested

20

THAT THERE IS A SPECIAL - QUASI-LIQUID - LAYER ON THE SURFACE OF THE LBDA.

Let’s now look at figure 5, and I think it will immediately become clear to you why the presence of a quasi-liquid layer leads to the freezing of two pieces of ice.

It is by the presence of a quasi-liquid layer that I try to explain the very high plasticity of ice and the movement of glaciers.

Faraday’s authority was very great, but he had an opponent - James Thomson (brother of Lord Kelvin). He drew attention to the then known part of the phase diagram of ice and theoretically proved that ice should melt as it slides down. This means that if you squeeze two pieces of ice, they should fuse (look at figure 6).

Faraday conducted a series of experiments refuting the point of view

When Georg changed his lightning rods, a lightning rod had already appeared on the spire of the Peter and Paul Fortress in St. Petersburg (probably the first in our country).

It should be noted that in reality the shape of the lightning rod usually does not matter. But if the top of the lightning rod is at such a great distance from the ground that it is comparable to the height of a thundercloud (for example, in the case of lightning rods on the tallest skyscrapers), then it is possible to observe a quiet discharge from the tip of the lightning rod that does not turn into lightning, as Franklin assumed when he advocated a “quiet” lightning rod with a pointed top. Franklin wrote with amazing accuracy in 1767: “A rod thus sharpened either prevents the lightning from striking a cloud, or, when struck, conducts it into the ground without damage to the building.”

(For the end, see p. 33)