Surface tension area. Start in science. Water surface tension coefficient

Surface tension of water is one of the most interesting properties of water.

Here are several definitions of this term from competent sources.

Surface tension is...

Great Medical Encyclopedia
Surface tension (S.T.) is the force of attraction with which each section of the surface film (the free surface of a liquid or any interface between two phases) acts on adjacent parts of the surface. Internal pressure and P. n. The surface layer of liquid behaves like an elastic stretched membrane. According to the idea developed by Chap. arr. Laplace, this property of liquid surfaces depends on “molecular forces of attraction, rapidly decreasing with distance. Inside a homogeneous liquid, the forces acting on each molecule from the molecules surrounding it are mutually balanced. But near the surface, the resultant forces of molecular attraction are directed inward; it tends to draw surface molecules into the thickness of the liquid. As a result, the entire surface layer, like an elastic stretched film, exerts a very significant pressure on the internal mass of the liquid in the direction normal to the surface. According to calculations, this “internal pressure”, under which the entire mass of liquid is located, reaches several thousand atmospheres. It increases on a convex surface and decreases on a concave surface. Due to the tendency of free energy to a minimum, any liquid tends to take a form in which its surface - the place of action of surface forces - has the smallest possible size. The larger the surface of a liquid, the larger the area its surface film occupies, the greater the supply of free surface energy released during its contraction. The tension with which each section of the contracting surface film acts on adjacent parts (in a direction parallel to the free surface) is called tension. In contrast to the elastic tension of an elastic stretched body, P. n. does not weaken as the surface film contracts. ... Surface tension equals the work that must be done to increase the free surface of a liquid by one. P.n. observed at the interface of a liquid with a gas (also with its own vapor), with another immiscible liquid, or with a solid. In the same way, a solid body has P. n. at the border with gases and liquids. In contrast to P. n., in which a liquid (or solid) has on its free surface bordering a gaseous medium, the tension at the internal boundary of two liquid (or liquid and solid) phases is conveniently designated by a special term adopted in German literature , the term “border tension” (Grenzflachenspannung). If a substance is dissolved in a liquid that reduces its P. n., then the free energy decreases not only by reducing the size of the boundary surface, but also through adsorption: a surfactant (or capillary active) substance collects in increased concentration in the surface layer...
…
Big medical encyclopedia. 1970

All of the above can be summarized in this way - molecules that are on the surface of any liquid, including water, are attracted by other molecules inside the liquid, as a result of which surface tension arises. We emphasize that this is a simplified understanding of this property.

Surface tension of water

To better understand this property, here are several manifestations of surface tension of water in real life:

When we see water dripping from the tip of a faucet rather than flowing, this is the surface tension of water;
When a raindrop in flight takes on a round, slightly elongated shape, this is the surface tension of water;
When water on a waterproof surface takes on a spherical shape, this is the surface tension of the water;
The ripples that appear when the wind blows on the surface of reservoirs are also a manifestation of the surface tension of water;
Water in space takes on a spherical shape due to surface tension;
The water strider insect floats on the surface of the water thanks to precisely this property of water;
If you carefully place a needle on the surface of the water, it will float;
If we alternately pour liquids of different densities and colors into a glass, we will see that they do not mix;
Rainbow soap bubbles are also a wonderful manifestation of surface tension.

Surface tension coefficient

Polytechnic terminological explanatory dictionary
Surface tension coefficient is the linear density of the surface tension force at the surface of a liquid or at the interface between two immiscible liquids.
Polytechnic terminological explanatory dictionary. Compilation: V. Butakov, I. Fagradyants. 2014

Below we present the values of the coefficient of surface tension (K.s.n.) for various liquids at a temperature of 20°C:

Ph.D. acetone - 0.0233 Newton / Meter;
Ph.D. benzene - 0.0289 Newton / Meter;
Ph.D. distilled water - 0.0727 Newton / Meter;
Ph.D. glycerol - 0.0657 Newton / Meter;
Ph.D. kerosene - 0.0289 Newton / Meter;
Ph.D. mercury - 0.4650 Newton / Meter;
Ph.D. ethyl alcohol - 0.0223 Newton / Meter;
Ph.D. ether - 0.0171 Newton / Meter.

Water surface tension coefficient

The surface tension coefficient depends on the temperature of the liquid. Let us present its values at different water temperatures.

At a temperature of 0°C - 75.64 σ, 10 –3 Newton / Meter;
At a temperature of 10°C - 74.22 σ, 10 –3 Newton / Meter;
At a temperature of 20°C - 72.25 σ, 10 –3 Newton / Meter;
At a temperature of 30°C - 71.18 σ, 10 –3 Newton / Meter;
At a temperature of 40°C - 69.56 σ, 10 –3 Newton / Meter;
At a temperature of 50°C - 67.91 σ, 10 –3 Newton / Meter;
At a temperature of 60°C - 66.18 σ, 10 –3 Newton / Meter;
At a temperature of 70°C - 64.42 σ, 10 –3 Newton / Meter;
At a temperature of 80°C - 62.61 σ, 10 –3 Newton / Meter;
At a temperature of 90°C - 60.75 σ, 10 –3 Newton / Meter;
At a temperature of 100°C - 58.85 σ, 10 -3 Newton / Meter.

This lesson will discuss liquids and their properties. From the point of view of modern physics, liquids are the most difficult subject of research, because in comparison with gases it is no longer possible to talk about negligible energy of interaction between molecules, and in comparison with solids it is impossible to talk about the ordered arrangement of liquid molecules (there is no long-range order in a liquid) . This leads to the fact that liquids have a number of interesting properties and their manifestations. One such property will be discussed in this lesson.

To begin with, let's discuss the special properties that molecules in the surface layer of a liquid have compared to molecules located in the volume.

Rice. 1. Difference between molecules of the surface layer and molecules located in the bulk of the liquid

Let's consider two molecules A and B. Molecule A is inside the liquid, molecule B is on its surface (Fig. 1). Molecule A is uniformly surrounded by other molecules of the liquid, therefore the forces acting on molecule A from molecules falling into the sphere of intermolecular interaction are compensated, or their resultant is zero.

What happens to molecule B, which is located at the surface of the liquid? Let us recall that the concentration of gas molecules located above the liquid is much less than the concentration of liquid molecules. Molecule B is surrounded on one side by liquid molecules, and on the other side by highly rarefied gas molecules. Since many more molecules act on it from the side of the liquid, the resultant of all intermolecular forces will be directed into the liquid.

Thus, in order for a molecule from the depths of the liquid to enter the surface layer, work must be done against uncompensated intermolecular forces.

Recall that work is the change in potential energy taken with a minus sign.

This means that the molecules of the surface layer, compared to the molecules inside the liquid, have excess potential energy.

This excess energy is a component of the internal energy of the liquid and is called surface energy. It is designated as , and is measured, like any other energy, in joules.

Obviously, the larger the surface area of the liquid, the more molecules that have excess potential energy, and therefore the greater the surface energy. This fact can be written in the form of the following relation:

where is the surface area, and is the proportionality coefficient, which we will call surface tension coefficient, this coefficient characterizes this or that liquid. Let us write down a strict definition of this quantity.

Surface tension of a liquid (coefficient of surface tension of a liquid) is a physical quantity that characterizes a given liquid and is equal to the ratio of surface energy to the surface area of the liquid

The coefficient of surface tension is measured in newtons divided by meter.

Let's discuss what the coefficient of surface tension of a liquid depends on. To begin with, let us remember that the surface tension coefficient characterizes the specific interaction energy of molecules, which means that factors that change this energy will also change the surface tension coefficient of the liquid.

So, the surface tension coefficient depends on:

1. The nature of the liquid ("volatile" liquids, such as ether, alcohol and gasoline, have less surface tension than "non-volatile" liquids - water, mercury and liquid metals).

2. Temperatures (the higher the temperature, the lower the surface tension).

3. The presence of surfactants that reduce surface tension (surfactants), such as soap or washing powder.

4. Properties of gas bordering liquid.

Note that the surface tension coefficient does not depend on the surface area, since for one individual near-surface molecule it is absolutely unimportant how many similar molecules there are around. Pay attention to the table, which shows the surface tension coefficients of various substances at temperature:

Table 1. Surface tension coefficients of liquids at the interface with air, at

So, the molecules of the surface layer have excess potential energy compared to the molecules in the bulk of the liquid. In the mechanics course it was shown that any system tends to a minimum of potential energy. For example, a body thrown from a certain height will tend to fall down. In addition, you feel much more comfortable lying down, since in this case the center of mass of your body is as low as possible. What does the desire to reduce one's potential energy lead to in the case of a liquid? Since surface energy depends on surface area, it is energetically disadvantageous for any liquid to have a large surface area. In other words, in a free state, the liquid will tend to make its surface minimal.

You can easily verify this by experimenting with soap film. If you dip a certain wire frame into a soap solution, a soap film will form on it, and the film will take on a shape such that its surface area is minimal (Fig. 2).

Rice. 2. Figures from soap solution

You can verify the existence of surface tension forces using a simple experiment. If a thread is tied to a wire ring in two places, so that the length of the thread is slightly greater than the length of the chord connecting the points of attachment of the thread, and dip the wire ring in a soap solution (Fig. 3a), the soap film will cover the entire surface of the ring and the thread will lie on soap film. If you now tear the film on one side of the thread, the soap film remaining on the other side of the thread will contract and tighten the thread (Fig. 3b).

Rice. 3. Experiment to detect surface tension forces

Why did this happen? The fact is that the soap solution remaining on top, that is, the liquid, tends to reduce its surface area. Thus, the thread is pulled upward.

So, we are convinced of the existence of surface tension. Now let's learn how to calculate it. To do this, let's conduct a thought experiment. Let's lower a wire frame into the soap solution, one of the sides of which is movable (Fig. 4). We will stretch the soap film by applying a force to the moving side of the frame. Thus, three forces act on the crossbar - an external force and two surface tension forces acting along each surface of the film. Using Newton's second law, we can write that

Rice. 4. Calculation of surface tension force

If, under the influence of an external force, the crossbar moves a distance, then this external force will do work

Naturally, due to this work, the surface area of the film will increase, which means the surface energy will also increase, which we can determine through the surface tension coefficient:

The change in area, in turn, can be determined as follows:

where is the length of the movable part of the wire frame. Taking this into account, we can write that the work done by the external force is equal to

Equating the right-hand sides in (*) and (**), we obtain an expression for the surface tension force:

Thus, the surface tension coefficient is numerically equal to the surface tension force, which acts per unit length of the line delimiting the surface

So, we are once again convinced that the liquid tends to take such a shape that its surface area is minimal. It can be shown that for a given volume the surface area of a sphere will be minimal. Thus, if no other forces act on the liquid or their effect is small, the liquid will tend to take on a spherical shape. This is how, for example, water will behave in zero gravity (Fig. 5) or soap bubbles (Fig. 6).

Rice. 5. Water in zero gravity

Rice. 6. Soap bubbles

The presence of surface tension forces can also explain why a metal needle “lies” on the surface of the water (Fig. 7). A needle, which is carefully placed on a surface, deforms it, thereby increasing the area of this surface. Thus, a surface tension force arises, which tends to reduce such a change in area. The resultant forces of surface tension will be directed upward, and it will compensate for the force of gravity.

Rice. 7. Needle on the surface of the water

The principle of operation of a pipette can be explained in the same way. The droplet, which is affected by gravity, is pulled down, thereby increasing its surface area. Naturally, surface tension forces arise, the resultant of which is opposite to the direction of gravity, and which prevent the droplet from stretching (Fig. 8). When you press down on the rubber cap of the pipette, you create additional pressure, which helps gravity, and as a result, the drop falls down.

Rice. 8. How the pipette works

Let's give another example from everyday life. If you dip a paint brush into a glass of water, the hairs will fluff up. If you now take this brush out of the water, you will notice that all the hairs are stuck to each other. This is due to the fact that the surface area of water adhering to the brush will then be minimal.

And one more example. If you want to build a castle out of dry sand, you are unlikely to succeed, since the sand will crumble under the influence of gravity. However, if you wet sand, it will maintain its shape due to the forces of surface tension of the water between the grains of sand.

Finally, we note that the theory of surface tension helps to find beautiful and simple analogies for solving more complex physical problems. For example, when you need to build a lightweight and at the same time strong structure, the physics of what happens in soap bubbles comes to the rescue. And it was possible to construct the first adequate model of the atomic nucleus by likening this atomic nucleus to a drop of charged liquid.

Bibliography

G. Ya. Myakishev, B. B. Bukhovtsev, N. N. Sotsky. "Physics 10". - M.: Education, 2008.
Ya. E. Geguzin “Bubbles”, Quantum Library. - M.: Nauka, 1985.
B. M. Yavorsky, A. A. Pinsky “Fundamentals of Physics” vol. 1.
G. S. Landsberg “Elementary textbook of physics” vol. 1.

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Homework

Having solved the problems for this lesson, you can prepare for questions 7,8,9 of the State Examination and questions A8, A9, A10 of the Unified State Exam.
Gelfgat I.M., Nenashev I.Yu. "Physics. Collection of problems for grade 10" 5.34, 5.43, 5.44, 5.47 ()
Based on problem 5.47, determine the coefficient of surface tension of water and soap solution.

List of questions and answers

Question: Why does surface tension change with temperature?

Answer: As the temperature increases, the molecules of the liquid begin to move faster, and therefore the molecules more easily overcome the potential forces of attraction. Which leads to a decrease in surface tension forces, which are potential forces that bind molecules of the surface layer of a liquid.

Question: Does the coefficient of surface tension depend on the density of the liquid?

Answer: Yes, it does, since the energy of the molecules in the surface layer of the liquid depends on the density of the liquid.

Question: What methods exist for determining the surface tension coefficient of a liquid?

Answer: In the school course, they study two ways to determine the surface tension coefficient of a liquid. The first is the wire tearing method, its principle is described in problem 5.44 from homework, the second is the drop counting method, described in problem 5.47.

Question: Why do soap bubbles collapse after a while?

Answer: The fact is that after some time, under the influence of gravity, the bubble becomes thicker at the bottom than at the top, and then, under the influence of evaporation, it collapses at some point. This leads to the fact that the entire bubble, like a balloon, collapses under the influence of uncompensated surface tension forces.

Surface tension describes the ability of a liquid to resist gravity. For example, water on a table surface forms droplets because the water molecules are attracted to each other, which counteracts the force of gravity. It is thanks to surface tension that heavier objects, such as insects, can be held on the surface of the water. Surface tension is measured in force (N) divided by unit length (m), or the amount of energy per unit area. The force with which water molecules interact (cohesive force) causes tension, resulting in the formation of droplets of water (or other liquids). Surface tension can be measured using a few simple items found in almost every home and a calculator.

Steps

Using a rocker

Write down the equation for surface tension. In this experiment, the equation for determining surface tension is as follows: F = 2Sd, Where F- force in newtons (N), S- surface tension in newtons per meter (N/m), d- length of the needle used in the experiment. Let us express surface tension from this equation: S = F/2d.

The force will be calculated at the end of the experiment.
Before starting the experiment, use a ruler to measure the length of the needle in meters.

Construct a small rocker arm. In this experiment, a rocker and a small needle that floats on the surface of the water are used to determine surface tension. It is necessary to carefully consider the construction of the rocker, since the accuracy of the result depends on this. You can use various materials, the main thing is to make a horizontal crossbar from something hard: wood, plastic or thick cardboard.
- Locate the center of the rod (such as a straw or plastic ruler) that you intend to use as the crossbar and drill or poke a hole at that location; this will be the fulcrum of the crossbar on which it will rotate freely. If you are using a plastic straw, simply poke it with a pin or nail.
- Drill or poke holes at the ends of the crossbar so that they are the same distance from the center. Thread threads through the holes to hang the weight cup and needle.
- If necessary, support the rocker arm with books or other sufficiently hard objects to keep the crossbar horizontal. It is necessary that the crossbar rotates freely around a nail or rod inserted into its middle.
Take a piece of aluminum foil and roll it into a box or saucer shape. It is not at all necessary that this saucer has the correct square or round shape. You'll fill it with water or other weight, so make sure it can support the weight.
- Hang a foil box or saucer from one end of the bar. Make small holes along the edges of the saucer and thread a thread through them so that the saucer hangs on the crossbar.
Hang a needle or paperclip from the other end of the bar so that it is horizontal. Tie a needle or paper clip horizontally to the thread that hangs from the other end of the crossbar. For the experiment to be successful, it is necessary to position the needle or paper clip exactly horizontally.
Place something, such as playdough, on the bar to balance the aluminum foil container. Before starting the experiment, it is necessary to ensure that the crossbar is horizontal. The foil saucer is heavier than the needle, so on its side the crossbar will go down. Attach enough plasticine to the opposite side of the crossbar so that it is horizontal.
- This is called balancing.
Place a needle or paper clip hanging from a thread in a container of water. This step will require extra effort to position the needle on the surface of the water. Make sure that the needle does not submerge in water. Fill a container with water (or another liquid with an unknown surface tension) and place it under the hanging needle so that the needle is directly on the surface of the liquid.
- Make sure that the rope holding the needle remains in place and is sufficiently taut.
Weigh a few pins or a small amount of measured drops of water on a small scale. You will add one pin or drop of water to the aluminum saucer on the rocker arm. In this case, it is necessary to know the exact weight at which the needle will come off the surface of the water.
- Count the number of pins or drops of water and weigh them.
- Determine the weight of one pin or drop of water. To do this, divide the total weight by the number of pins or drops.
- Let's say 30 pins weigh 15 grams, then 15/30 = 0.5, that is, one pin weighs 0.5 grams.
Add pins or drops of water, one at a time, to the aluminum foil saucer until the pin lifts off the surface of the water. Gradually add one pin or drop of water at a time. Watch the needle carefully so as not to miss the moment when, after the next increase in the load, it comes off the water. Once the needle leaves the surface of the liquid, stop adding pins or drops of water.
- Count the number of pins or drops of water before the needle at the opposite end of the bar breaks away from the surface of the water.
- Write down the result.
- Repeat the experiment several (5 or 6) times to get more accurate results.
- Calculate the average of the results obtained. To do this, add up the number of pins or drops in all experiments and divide the sum by the number of experiments.
Convert the number of pins to strength. To do this, multiply the number of grams by 0.00981 N/g. To calculate surface tension, you need to know the force that was required to lift the needle from the surface of the water. Since you calculated the weight of the pins in the previous step, to determine the force, simply multiply that weight by 0.00981 N/g.
- Multiply the number of pins placed in the saucer by the weight of one pin. For example, if you put 5 pins weighing 0.5 grams, their total weight will be 0.5 g/pin = 5 x 0.5 = 2.5 grams.
- Multiply the number of grams by the factor of 0.00981 N/g: 2.5 x 0.00981 = 0.025 N.
Substitute the resulting values into the equation and find the desired value. Using the results obtained during the experiment, surface tension can be determined. Simply plug in the values found and calculate the result.
- Let's say that in the example above, the length of the needle is 0.025 meters. We substitute the values into the equation and get: S = F/2d = 0.025 N/(2 x 0.025) = 0.05 N/m. Thus, the surface tension of the liquid is 0.05 N/m.

Liquid–a substance in a liquid state of aggregation, occupying an intermediate position between solid and gaseous states. The main property of a liquid, which distinguishes it from substances in other states of aggregation, is the ability to unlimitedly change its shape under the influence of tangential mechanical stresses, even arbitrarily small, while practically maintaining its volume.

General information about the liquid state

The liquid state is usually considered intermediate between a solid and a gas: a gas retains neither volume nor shape, but a solid retains both.

The shape of liquid bodies can be determined entirely or partly by the fact that their surface behaves like an elastic membrane. So, water can collect in drops. But a liquid is capable of flowing even under its stationary surface, and this also means that the form (the internal parts of the liquid body) is not preserved.

Liquid molecules do not have a definite position, but at the same time they do not have complete freedom of movement. There is an attraction between them, strong enough to keep them close.

A substance in the liquid state exists in a certain temperature range, below which it turns into a solid state (crystallization occurs or transformation into a solid amorphous state - glass), above which it turns into a gaseous state (evaporation occurs). The boundaries of this interval depend on pressure.

As a rule, a substance in the liquid state has only one modification. (The most important exceptions are quantum liquids and liquid crystals.) Therefore, in most cases, a liquid is not only a state of aggregation, but also a thermodynamic phase (liquid phase).

All liquids are usually divided into pure liquids and mixtures. Some mixtures of fluids are of great importance for life: blood, sea water etc. Liquids can act as solvents.

Physical properties liquids

1 ).Fluidity

The main property of liquids is fluidity. If an external force is applied to a section of a liquid that is in equilibrium, then a flow of liquid particles arises in the direction in which this force is applied: the liquid flows. Thus, under the influence of unbalanced external forces the liquid does not retain its shape and relative arrangement of parts, and therefore takes the shape of the vessel in which it is located.

Unlike plastic solids, a liquid does not have a yield limit: it is enough to apply an arbitrarily small external force for the liquid to flow.

2).Volume conservation

One of the characteristic properties of a liquid is that it has a certain volume (under constant external conditions). A liquid is extremely difficult to compress mechanically because, unlike a gas, there is very little between the molecules free space. The pressure exerted on a liquid enclosed in a vessel is transmitted without change to each point in the volume of this liquid (Pascal’s law is also valid for gases). This feature, along with very low compressibility, is used in hydraulic machines.

Liquids generally increase in volume (expand) when heated and decrease in volume (contract) when cooled. However, there are exceptions, for example, water contracts when heated, at normal pressure and temperature from to approximately .

3).Viscosity

In addition, liquids (like gases) are characterized by viscosity. It is defined as the ability to resist the movement of one part relative to another - that is, as internal friction.

When adjacent layers of liquid move relative to each other, collisions of molecules inevitably occur in addition to that caused by thermal motion. Forces arise that inhibit orderly movement. In this case, the kinetic energy of ordered movement transforms into thermal energy of chaotic movement of molecules.

The liquid in the vessel, set in motion and left to its own devices, will gradually stop, but its temperature will increase.

4).Miscibility

Miscibility is the ability of liquids to dissolve in each other. An example of miscible liquids: water and ethyl alcohol, an example of immiscible liquids: water and liquid oil.

5).Free surface formation and surface tension

Due to the conservation of volume, the liquid is able to form a free surface. Such a surface is the interface between the phases of a given substance: on one side there is a liquid phase, on the other there is a gaseous phase (steam), and, possibly, other gases, for example, air.

If the liquid and gaseous phases of the same substance come into contact, forces arise that tend to reduce the interface area - surface tension forces. The interface behaves like an elastic membrane that tends to contract.

6).Density waves

Although a liquid is extremely difficult to compress, its volume and density still change when the pressure changes. This doesn't happen instantly; So, if one area is compressed, then such compression is transmitted to other areas with a delay. This means that elastic waves, more specifically density waves, can propagate inside the liquid. Along with density, other physical quantities, such as temperature, also change.

If, as the wave propagates, the density changes quite slightly, such a wave is called a sound wave, or sound.

If the density changes strongly enough, then such a wave is called a shock wave. The shock wave is described by other equations.

Density waves in a liquid are longitudinal, that is, the density changes along the direction of propagation of the wave. There are no transverse elastic waves in the liquid due to non-conservation of shape.

Elastic waves in a liquid fade over time, their energy gradually turns into thermal energy. The reasons for attenuation are viscosity, “classical absorption”, molecular relaxation and others. In this case, the so-called second, or volumetric viscosity works - internal friction when the density changes. The shock wave, as a result of attenuation, after some time turns into a sound wave.

Elastic waves in a liquid are also subject to scattering by inhomogeneities resulting from the chaotic thermal motion of molecules.

Structure of liquids

Experimental studies of the liquid state of matter, based on the observation of x-ray diffraction and neutron fluxes as they pass through liquid media, have discovered the presence of short-range order, i.e. the presence of some order in the arrangement of particles only at a small distance from any selected position (Fig. 140).

The mutual arrangement of neighboring particles in liquids is similar to the ordered arrangement of neighboring particles in crystals. However, this ordering in liquids is observed only within small volumes. At distances: from some selected “central” molecule, ordering is disrupted (is the effective diameter of the molecule). Such ordering in the arrangement of particles in liquids is called short-range order. .

Due to the lack of long-range order, liquids, with few exceptions, do not exhibit the anisotropy characteristic of crystals. For this reason, the structure of the liquid is sometimes called quasicrystalline or crystal-like .

For the first time, the idea of the similarity of some properties of liquids (especially metal melts) and crystalline solids was expressed and then developed in the works of the Soviet physicist Ya.I. Frenkel back in the 1930s–1940s. According to Frenkel's views, which have now received universal recognition, the thermal motion of atoms and molecules in a liquid consists of irregular vibrations with an average frequency close to the frequency of vibrations of atoms in crystalline bodies. The center of oscillations is determined by the force field of neighboring particles and shifts along with the displacements of these particles.

In a simplified way, one can imagine such thermal motion as the superposition of relatively rare jumps of particles from one temporary equilibrium position to another and thermal oscillations in the intervals between jumps. The average time of “settled” stay of a liquid molecule near a certain equilibrium position is called time of relaxation. After time, the molecule changes its place of equilibrium, moving abruptly to a new position, separated from the previous one by a distance of the order of the size of the molecules themselves. Thus, the molecule moves slowly inside the liquid. With increasing temperature, time decreases, the mobility of molecules increases, which entails a decrease in the viscosity of liquids (fluidity increases). According to the figurative expression of Ya.I. Frenkel, molecules wander throughout the entire volume of liquid, leading a nomadic lifestyle, in which short-term movements are replaced by relatively long periods of sedentary life.

Amorphous solids (glass, resins, bitumen, etc.) can be considered as supercooled liquids, the particles of which have limited mobility due to their greatly increased viscosity.

Due to the low order of the liquid state, the theory of liquids turns out to be less developed than the theory of gases and crystalline solids. There is no complete theory of liquid yet.

A special type of liquids are certain organic compounds consisting of elongated or disk-shaped molecules, or so-called liquid crystals. The interaction between molecules in such liquids tends to align the long axes of the molecules in a certain order. At high temperatures, thermal movement prevents this, and the substance is an ordinary liquid. At temperatures below critical, a preferred direction appears in the liquid and long-range orientational order arises. While retaining the basic features of a liquid, for example, fluidity, liquid crystals have the characteristic properties of solid crystals - anisotropy of magnetic, electrical and optical properties. These properties (along with fluidity) are found in numerous technical applications, for example, in electronic watches, calculators, mobile phones, as well as in personal computer monitors, televisions, as indicators, scoreboards and screens for displaying digital, alphabetic and analog information.

Surface tension

The most interesting feature of liquids is the presence free surface. Connected to the surface of the liquid free energy, proportional to the free surface area of the liquid: . Since the free energy of an isolated system tends to a minimum, the liquid (in the absence of external fields) tends to take a form that has a minimum surface area. Thus, the problem of the shape of a liquid is reduced to an isoperimetric problem under given additional conditions (initial distribution, volume, etc.). A free drop takes the shape of a sphere, but under more complex conditions the problem of the shape of the liquid surface becomes extremely difficult.

Liquid, unlike gases, does not fill the entire volume of the container into which it is poured. An interface is formed between liquid and gas (or vapor), which is in special conditions compared to the rest of the liquid. Molecules in the boundary layer of a liquid, unlike molecules in its depth, are not surrounded by other molecules of the same liquid on all sides. The forces of intermolecular interaction acting on one of the molecules inside a liquid from neighboring molecules are, on average, mutually compensated (Fig. 141).

But all molecules, including molecules of the boundary layer, must be in a state of equilibrium. This equilibrium is achieved by slightly reducing the distance between the molecules of the surface layer and their nearest neighbors inside the liquid. As the distance between molecules decreases, repulsive forces arise. The molecules of the surface layer are packed somewhat more densely, and therefore they have an additional supply of potential energy compared to the internal molecules. Hence, molecules of the surface layer of a liquid have excess potential energy compared to the molecules inside the liquid, equal to free energy. .Thus, the potential energy of the surface of a liquid is proportional to its area: .

It is known from mechanics that the equilibrium states of a system correspond to the minimum value of its potential energy, i.e. the free surface of the liquid tends to reduce its area. The liquid behaves as if forces acting tangentially to its surface are contracting (pulling) this surface. These forces are called surface tension forces .