As the surface tension depends on the temperature of the fluid. Surface tension
Definition 1.
Surface tension - liquid glowing reduce its own free surface, that is, to reduce the excess of potential energy at the separation boundary with the gaseous phase.
Not only solid physical bodies are equipped with elastic characteristics, but also the surface of the fluid itself. Each in his life saw how the soap film is stretched with a slight blowing bubbles. Surface tension forces that occur in a soap film are held for a certain period of time air, similar to how the rubber stretched camera saves the air in a soccer ball.
Surface tension appears on the boundary of the main phases partition, for example, gaseous and liquid, or liquid and solid. This is directly due to the fact that the elementary particles of the surface layer of the liquid always experience different strengths of attraction from the inside and outside.
This physical process is possible to consider the example of a drop of water, where the fluid moves itself as if it is in an elastic shell. Here, the atoms of the surface layer of the liquid substance are attracted to its own inner neighbors stronger than to the outer particles of air.
In general, the surface tension can be explained as an infinitely small or elementary work of $ \\ Sigma A $, which must be made to increase the total surface area of \u200b\u200bthe liquid on an infinitely small value of $ DS $ at a consistent temperature of $ dt $.
The mechanism for the emergence of surface tension in liquids
Figure 2. Scalar positive value. Author24 - Student Internet Exchange
Liquid, unlike solid tel and gases, it is not able to fill the entire volume of the vessel in which it was placed. A certain border of the section is formed between the ferry and the liquid substance, which acts under special conditions compared with another mass of fluid. Consider for a more visual example, two $ a $ and $ b $ molecules. The $ a $ particle is inside the fluid itself, the $ b $ molecule is directly on its surface. The first element is surrounded by other liquid atoms uniformly, therefore the force acting on the molecule on the part of the particles falling into the sphere of intermolecular interaction are always compensated, or, in other words, their equal power is zero.
The $ b $ molecule on the one hand is framed by fluid molecules, and on the other side of the gas-atoms, the total concentration of which is largely lower than the combination of elementary particles of the fluid. Since the fluid on the fluid on the $ b $ b will act much more molecules than from the ideal gas, which is unforgettable all intermolecular forces can already be equal to zero, since this parameter is directed inside the volume of the substance. Thus, in order for the molecule from the depth of the liquid in the surface layer, it is necessary to work against the uncompensated forces. And this means that the atoms of the near-surface level, compared with the particles inside the liquid, are equipped with an excessive potential energy that is called surface energy.
Surface tension coefficient
Figure 3. Surface voltage. Author24 - Student Internet Exchange
Definition 2.
The surface tension coefficient is a physical indicator characterizing a certain liquid and numerically equal to the ratio of surface energy to the total area of \u200b\u200bthe fluid.
In physics, the main unit of measurement of the surface tension coefficient in the C concept is (n) / (m).
The specified value directly depends on:
- the nature of the liquid (in the "volatile elements of such as alcohol, ether, gasoline, the surface tension coefficient is significantly less than that of non-volatile - mercury, water);
- the temperature of the liquid substance (the higher the temperature, the less the final surface tension);
- properties of the ideal gas, bordering this fluid;
- the presence of stable surfactant elements such as washing powder or soap that can reduce surface tension.
Note 1.
It should also be noted that the surface tension parameter does not depend on the initial area of \u200b\u200bthe fluid fluid.
It is also known from the mechanics that the minimum value of its internal energy always corresponds to the unchanged states of the system. Due to such a physical process, the liquid body often takes shape with a minimum surface. If the extraneous forces do not affect the liquid or their effect is extremely small, its elements to the form of a sphere in the form of a drop of water or soap bubble. Similarly, water begins to behave in weightlessness. The fluid moves as if, on tangent of its main surface, factors that reduce this Wednesday act. These forces are called superficial tension.
Consequently, the surface tension coefficient is also possible to determine as the main module of the surface tension force, which actually acts per unit of the length of the initial circuit that limits the free medium of the fluid. The presence of these parameters makes the surface of the liquid substance similar to a stretched elastic film, with the only difference that the unchanged forces in the film directly depend on its area of \u200b\u200bits system, and the surface tension forces themselves are capable of working independently. If you put a small sewing needle on the surface of the water, the smooth will come and will not let her drown.
Action external factor You can describe the slide of light insects such as water meters, along the entire surface of the reservoirs. The foot of these arthropods deforms the water surface, thereby increasing its area. As a result, the strength of the surface tension occurs, seeking to reduce such a change in the area. The relaxing force will always be directed exclusively by compensating for the effect of gravity.
Surface Tension Result
Under the influence of surface tension, small amounts of liquid media seek to take a spherical shape that will be ideal to fit the smallest value. ambient. The approach to the ball configuration is achieved by the greater, the more weaker the initial strength of gravity, since the small drops of the surface tension force significantly exceeds the effect of gravity.
Surface tension is considered one of the most important characteristics of the surfaces of the phase partition. It directly affects the formation of fine particles of physical bodies and liquids during their separation, as well as to the fusion of elements or bubbles in the fogs, emulsions, foams, on the processes of adhesion.
Note 2.
Surface tension sets the form of future biological cells and their main parts.
The change in the forces of this physical process affects phagocytosis and on the processes of alveolar respiration. Due to this phenomenon, porous substances may hold a huge amount of liquid for a long time, even from air vapor, capillary phenomena, imposing changes in the height of the fluid level in capillaries compared to the level of fluid in a wider vessel, are very common. Through these processes, water raising in the soil is due to the root system of plants, the movement of biological fluids on the system of small tubules and vessels.
Cap, Cap ... Here is another drop gathered on the spout of the crane, the swollen and fell down. A similar picture is familiar to anyone. Or a warm summer rain waters down the earthly land - and again drops. Why exactly drops? What is the reason? Everything is very simple: the reason for this is the surface tension of water.
This is one of the properties of water or, in general, all liquids. As you know, the gas fills the entire volume in which it does not fall, but this can not do this. Molecules inside the volume of water are surrounded by the same molecules from all sides. But those on the surface, on the boundary of the liquid and gas, are influenced not from all sides, but only by the part of those molecules that are located inside the volume, there are no effects on them.
At the same time, the force on the surface of the liquid will act along it perpendicular to the surface of the surface to which it acts. As a result of the action of this force and the surface tension occurs. The external manifestation will be the formation of the likeness of an invisible, elastic film on the border of the section. Due to the effects of surface tension, the water drop will take the form of the sphere as a body having the smallest area at a given volume.
Now you can determine that the surface tension is work to change the surface of the liquid. On the other hand, it can be defined as the energy necessary to break the units of the surface. Surface tension is possible on the boundary of the liquid and gas. It is determined by the force acting between molecules, and it means responsible for volatility (evaporation). The smaller the size of the surface tension, the more bats there will be fluid.
It is possible to determine what is equal to the formula for its calculation involves the surface area and as already mentioned earlier, the coefficient does not depend on the shape and the size of the surface, but is determined by the force of intermolecular interaction, i.e. type of liquid. For different liquids, its value will be different.
Surface tension of water can be changed. This is achieved by heating, by adding biologically active substances, such as soap, powder, paste. Its value depends on the degree of purity of water. The cleaner water, the amount of surface tension is greater, and it is inferior only to mercury in its meaning.
Curious effect is observed when the liquid comes into contact with solid substance, and gas. If we strike a drop of water to the surface of the paraffin, then it will take the shape of the ball. This is caused by the fact that the forces acting between paraffin and a drop, less than the interaction between themselves as a result of which the ball appears. When the forces acting between the surface and the drops are greater than the forces of the intermolecular interaction, the water is evenly spread over the surface. This phenomenon is called wetting.
The effect of wettability to some extent can characterize the degree of purity of the surface. On the pure surface of the drop spread uniformly, and if the surface is contaminated or covered with substance, not wetting water, then the latter is going to the balls.
As an example of using surface tension in the industry, it is possible to cast a casting of spherical parts, for example, fractions for guns. The molten metal drops simply frozen on the fly, taking a spherical shape.
Surface tension of water, like any other liquid, is one of its important parameters. It determines some characteristics of the liquid - such as volatility (evaporation) and wettability. Its value depends only on the parameters of intermolecular interaction.
The most characteristic property of a liquid that distinguishes it from gas is that on the border with gas, the liquid forms a free surface, the presence of which leads to the occurrence of the phenomena of a special kind, called superficial. They are required by their appearance physical conditionsin which there are molecules near the free surface.
For each fluid molecule, the strengths of attraction from the surrounding molecules arranged from it at a distance of about 10 -9 m (molecular radius). On a molecule M. 1, located inside the liquid (Fig. 1), there are forces on the side of the same molecules, and the relaxing of these forces is close to zero.
For molecules M. 2 The resultant forces are different from zero and are directed inside the liquid, perpendicular to its surface. Thus, all fluid molecules in the surface layer are drawn into the liquid. But the space inside the liquid is occupied by other molecules, so surface layer creates pressure on the liquid (molecular pressure).
To move the molecule M. 3, located directly under the surface layer, to the surface, it is necessary to work against molecular pressure forces. Consequently, the molecules of the surface layer of fluid have additional potential energy compared to molecules inside the liquid. This energy is called surface energy.
Obviously, the size of the surface energy is the greater, the larger the area of \u200b\u200bthe free surface. Let the free surface area changed to Δ S.At the same time, the surface energy has changed to \\ (~ \\ deelta w_p \u003d \\ sigma \\ cdot \\ delta s \\), where σ is the surface tension coefficient. As it is necessary to make a change for this
\\ (~ A \u003d \\ Delta W_P, \\) That \\ (~ a \u003d \\ sigma \\ cdot \\ delta s. \\)
Hence the \\ (~ \\ sigma \u003d \\ dfrac (A) (\\ Delta S) \\).
The unit of the surface tension coefficient in C is a joule per square meter (J / M 2).
- The value is numerically equal to the work performed by molecular forces when changing the area of \u200b\u200bthe free surface of the fluid at 1 m 2 at a constant temperature.
Since any system provided herself seeks to take such a position in which its potential energy is the smallest, the liquid reveals the desire to reduce the free surface. The surface layer of fluid behaves like a stretched rubber film, i.e. All the time seeks to reduce the area of \u200b\u200bits surface to the minimum sizes possible at a given volume.
For example, a drop of fluid in a state of weightless has a spherical shape.
Surface tension
The property of the liquid surface can be declined to interpret as the existence of forces seeking to reduce this surface. Molecule M. 1 (Fig. 2), located on the surface of the liquid, interacts not only with molecules that are inside the liquid, but also with molecules on the surface of the liquid located within the sphere of molecular action. For molecule M. 1 is the resultant \\ (~ \\ VEC R \\) of molecular forces directed along the free surface of the fluid, is zero, and for a molecule M. 2, located at the border of the surface of the liquid, \\ (~ \\ VEC R \\ Ne 0 \\) and \\ (~ \\ VEC R \\) directed by normal to the boundaries of the free surface and on the tangent of the surface of the liquid.
The resultant forces acting on all molecules on the border of the free surface and there is power surface tension. In general, it acts so that it seeks to reduce the surface of the fluid.
It can be assumed that the strength of the surface tension \\ (~ \\ VEC F \\) is directly proportional to the length l. The boundaries of the surface layer of the fluid, because on all parts of the surface layer of the liquid, the molecule is in the same conditions:
\\ (~ F \\ SIM L. \\)
Indeed, consider the vertical rectangular frame (Fig. 3, a, b), which is balanced movable side. After removing the frame from a solution of a soap film, the movable part moves from the position 1 in the Regulation 2 . Considering that the film is a thin layer of fluid and has two free surfaces, we will find the work performed when moving across the distance h. = a. 1 ⋅ a. 2: A. = 2F⋅h.where F. - force acting on the frame from each surface layer. On the other hand, \\ (~ a \u003d \\ sigma \\ cdot \\ delta s \u003d \\ sigma \\ cdot 2l \\ cdot h \\).
Therefore, \\ (~ 2f \\ Cdot H \u003d \\ Sigma \\ Cdot 2L \\ CDOT H \\ RIGHTARROW F \u003d \\ SIGMA \\ CDOT L \\), from where \\ (~ \\ sigma \u003d \\ dfrac fl \\).
According to this formula, the unit of surface tension coefficient in C is Newton to a meter (N / M).
Surface tension coefficient σ is numerically equal to the strength of the surface tension acting per unit of the limits of the free surface of the fluid. The surface tension coefficient depends on the nature of the fluid, on temperature and on the presence of impurities. With increasing temperature, it decreases.
- At critical temperature when the difference between the liquid and steam disappears, σ \u003d 0.
The impurities are mainly reduced (some increase) the coefficient of surface tension.
Thus, the surface layer of fluid is a elastic stretched film covering the whole liquid and seeking to assemble it into one "drop". Such a model (elastic stretched film) allows you to determine the direction of the forces of the surface tension. For example, if the film is stretched under the action of external forces, the strength of the surface tension will be directed along the surface of the liquid against stretching. However, this condition differs significantly from the tension of the elastic rubber film. The elastic film is stretched due to an increase in the distance between the particles, and the strength of the tension increases, with the stretching of the liquid film, the distance between the particles does not change, and the surface increase is achieved as a result of the transition of molecules from the thickness of the liquid into the surface layer. Therefore, with an increase in the surface of the liquid, the strength of the surface tension does not change (it does not depend on the surface area).
see also
- Kikoin A.K. About the forces of surface tension // Quant. - 1983. - № 12. - P. 27-28
Wetting
In case of contact with the solid body of the clutch force of fluid molecules with solid body molecules start playing significant role. The behavior of the fluid will depend on the fact that more: the grip between the fluid molecules or the grip of the fluid molecules with solid molecules.
Wetting - phenomenon resulting from the interaction of fluid molecules with solid molecules. If the forces of attraction between liquid and solid molecules are more attraction forces between liquid molecules, then the liquid is called wetting; If the forces of attraction of the fluid and solid body are less than the forces of attraction between fluid molecules, then the liquid is called non-parallery This is the body.
The same liquid can be wetting and unmatuting in relation to different bodies. So, the water wets the glass and does not wet the fat surface, the mercury does not wet the glass, and wets the copper.
Wetting or non-seating with a liquid of the vessel walls in which it is located, affects the shape of a free surface of the fluid in the vessel. If a a large number of The fluid is poured into the vessel, the shape of its surface is determined by the force of gravity, which provides a flat and horizontal surface. However, the very walls of wetting and non-waging phenomenon lead to the curvature of the surface of the liquid, the so-called regional effects.
Quantitative characteristic of edge effects regional corner θ is the angle between the plane tangent to the surface of the liquid and the surface of the solid body. Inside the edge angle is always fluid (Fig. 4, a, b). When wetting it, it will be sharp (Fig. 4, a), and when non-inspired - blunt (Fig. 4, b). In the school course, physicists consider only complete wetting (θ \u003d 0º) or complete non-charging (θ \u003d 180º).
The forces associated with the presence of surface tension and aimed at the surface tangent to the surface of the liquid, in the case of a convex surface, they give the resulting, directed into the liquid (Fig. 5, a). In the case of a concave surface, the resulting force is directed, on the contrary, towards the gas bounding with the liquid (Fig. 5, b).
If the wetting fluid is on the open surface of the solid (Fig. 6, a), it occurs its spreading on this surface. If an open liquid is on the open surface of the solid body, it takes a shape close to the ball (Fig. 6, b).
Wetting is essential both in everyday life and in industry. Good wetting is necessary when painted, washing, processing photographic materials, applying paintwork coatings, when gluing materials, when soldering, in flotation processes (enrichment of ores valuable rock). Conversely, when building waterproofing devices, materials that are not wetted by water are necessary.
Capillary phenomena
The curvature of the surface of the liquid at the edges of the vessel is particularly clearly visible in narrow tubes, where the entire free surface of the fluid is curved. In the tubes with a narrow section, this surface is part of the sphere, it is called menysky. A wetting fluid is formed a concave meniscus (Fig. 7, a), and at the non-paralible - convex (Fig. 7, b). Since the surface area of \u200b\u200bthe meniscus is greater than the cross-sectional area of \u200b\u200bthe tube, then under the action of molecular forces, the curved surface of the fluid seeks to straighten up.
Surface tension forces create additional (Laplasovo) Pressure under the curved surface of the liquid.
If the surface of the liquid concave, The strength of the surface tension is directed from the liquid (Fig. 8, a), and the pressure under the concave surface of the fluid is less than under the flat, on \\ (~ p \u003d \\ dfrac (2 \\ Sigma) (R) \\). If the surface of the liquid convex, The strength of the surface tension is directed inside the liquid (Fig. 8, b), and the pressure under the convex surface of the liquid is greater than under the flat, on the same value.
Fig. eight - This formula is a special case of the formula of the Laplace, which determines the excess pressure for the arbitrary surface of the liquid of the twofold curvature:
where R. 1 I. R. 2 - the radii of curvature of two any mutually perpendicular normal cross sections of the liquid surface. The radius of curvature is positive if the curvature center of the corresponding section is inside the liquid, and is negative if the curvature center is outside the liquid. For a cylindrical surface ( R. 1 = l.; R. 2 \u003d ∞) overpressure \\ (~ p \u003d \\ dfrac (\\ sigma) (R) \\).
If you put a narrow tube ( capillary) in one end to the liquid, poured into a wide vessel, then due to the presence of Pressure Laplace Pressure, the liquid in the capillary rises (if the liquid is wetting) or omit (if the liquid is unmatuting) (Fig. 9, a, b), since under the flat surface of the fluid in There is no excess pressure vessel.
The phenomena of changes in the height of the level of fluid in the capillaries compared to the level of fluid in wide vessels are called capillary phenomena.
Liquid in the capillary rises or falls on such height h., in which the power of the hydrostatic pressure of the fluid column is equalized by the force of overpressure, i.e.
\\ (~ \\ dfrac (2 \\ Sigma) (R) \u003d \\ rho \\ cdot g \\ cdot h. \\)
Where \\ (~ H \u003d \\ DFRAC (2 \\ Sigma) (\\ rho \\ cdot g \\ cdot r) \\). If wetting is not complete θ ≠ 0 (θ ≠ 180 °), then the calculations show, \\ (~ H \u003d \\ DFRAC (2 \\ Sigma) (\\ rho \\ cdot g \\ cdot r) \\ CDOT \\ COS \\ THATA \\).
Capillary phenomena are very common. Raising water in the soil, the system of blood vessels in the lungs, root system in plants, wick and clock paper - capillary systems.
Literature
- Aksenovich L. A. Physics in high school: Theory. Tasks. Tests: studies. Manual for institutions ensuring the production of total. media, education / L. A. Aksenovich, N.N.Rakina, K. S. Farino; Ed. K. S. Fyrino. - MN: Adukatsya I Vikhavanna, 2004. - C. 178-184.
The forces of attraction between molecules on the surface of the liquid hold them from movement beyond its limits.
Liquid molecules experience mutual attraction forces - in fact, it is due to this that the fluid instantly does not disappear. On the molecules inside the liquid, the force of attraction of other molecules act on all sides and therefore mutually balancing each other. The molecules on the surface of the fluid do not have neighbors outside, and the resulting force of attraction is directed inside the liquid. As a result, the entire surface of the water is striving to shit under the influence of these forces. By the aggregate, this effect leads to the formation of the so-called surface tension force, which acts along the surface of the liquid and leads to the formation of the similarity of the invisible, fine and elastic film.
One of the consequences of the surface tension effect is that to increase the surface area of \u200b\u200bthe liquid - its stretching - it is necessary to do a mechanical work to overcome the forces of the surface tension. Consequently, if the liquid is left alone, it seeks to take the form at which its surface area will be minimal. Such a form, naturally, is the sphere - that's why raindrops in flight take almost spherical shape (I say "almost", because the drops in flight are slightly pulled out due to air resistance). For the same reason, the water drops on the body covered with fresh wax car are going to beads.
Surface tension forces are used in industry - in particular, when casting spherical forms, such as a rifle fraction. The drops of molten metal simply give to stick on the fly when falling with sufficient for this height, and they themselves freeze in the shape of the balls before they fall into the receiving container.
You can bring a lot of examples of surface tension forces in action from our everyday life. Under the influence of wind on the surface of the oceans, seas and lakes, ripples are formed, and this ripple is a wave in which the internal pressure force of water is equalized by acting down the surface tension force. The two of these forces alternate, and ripples are formed on the water, just as a wave in the string of the musical instrument is formed due to alternate stretching and compression.
There will be a liquid to be assembled into "beads" or a smooth layer on a solid surface, depends on the ratio of the forces of intermolecular interaction in liquids that cause surface tension, and the attraction forces between liquid molecules and solid surface. In liquid water, for example, the strength of the surface tension is due to hydrogen bonds between molecules ( cm. Chemical bonds). The glass surface with water is wetted, since the glass contains quite a few oxygen atoms, and water easily forms hydrogen bonds not only with other water molecules, but also with oxygen atoms. If you lubricate the surface of the glass with fat, the hydrogen bonds will not be formed, and the water will be collected in the droplets under the influence of internal hydrogen bonds that determine the surface tension.
In the chemical industry in the water, special sequel reagents are often added - surfactants- Without giving water to gather in drops on any surface. They are added, for example, in liquid detergents for dishwashers. Finding into the surface layer of water, the molecules of such reagents are noticeably weakened by the forces of the surface tension, the water is not assembled into the drops and does not leave on the surface of dirty speck after drying ( cm.
Liquid– The substance located in a liquid aggregate state occupies an intermediate position between solid and gaseous states. The main property of a fluid that distinguishes it from substances in other aggregate states is the ability to unlimitedly change the form under the action of tangent mechanical stresses, even as small, practically preserving the volume.
general information About liquid state
The liquid state is usually considered an intermediate between the solid body and gas: the gas does not retain neither the volume or the form, and the solid body maintains both.
The form of liquid bodies can be fully or partly to be determined by the fact that their surface behaves like an elastic membrane. So, water can be collected in drops. But the fluid is capable of flowing even under its fixed surface, and this also means disbusting the shape (internal parts of the liquid body).
Liquid molecules do not have a certain position, but at the same time, complete freedom of displacements are not available. Between them there is an attraction, strong enough to keep them at close range.
The substance in the liquid state exists in a certain temperature range, below which goes into a solid state (crystallization or conversion to solid-state amorphous-glass), above, gaseous (evaporation). The boundaries of this interval depend on pressure.
As a rule, a substance in a liquid state has only one modification. (The most important exceptions are quantum fluids and liquid crystals.) Therefore, in most cases, the liquid is not only an aggregate state, but also a thermodynamic phase (liquid phase).
All fluids are customized on clean liquids and mixtures. Some liquids mixtures are of great importance for life: blood, sea water and other liquids can perform solvent function.
Physical properties of liquids
1 ).Fluidity
The main property of fluids is fluidity. If there is an external force to the area of \u200b\u200bliquid in equilibrium, then the flow of the fluid particles in the direction in which this force is applied is: liquid flows. Thus, under the action of unbalanced external forces, the liquid does not retain the form and the relative location of the parts, and therefore takes the shape of the vessel in which it is located.
Unlike plastic solid bodies, the fluid does not have a yield strength: it is sufficient to attach an arbitrarily small force so that the liquid flows.
2).Saving volume
One of the characteristic properties of the fluid is that it has a certain amount (with unchanged external conditions). The liquid is extremely difficult to squeeze mechanically, since, in contrast to gas, there is very little free space between molecules. The pressure produced on the liquid concluded in the vessel is transmitted unchanged into each point of the volume of this liquid (Pascal's law, also for gases). This feature, along with very low compressibility, is used in hydraulic machines.
Liquids usually increase the volume (expand) when heated and reduced volume (compress) when cooling. However, exceptions are also found, for example, water is compressed when heated, under normal pressure and temperature from to approximately.
3).Viscosity
In addition, liquids (as well as gases) are characterized by viscosity. It is defined as the ability to resist the movement of one of the parts relative to another, that is, as inner friction.
When the adjacent layers of the fluid move relative to each other, the molecules collide inevitably occur to that due to the thermal motion. There are forces to brake ordered movement. In this case, the kinetic energy of an ordered movement moves into the thermal energy of the chaotic movement of molecules.
The liquid in the vessel, which is moved and provided to itself, will gradually stop, but its temperature will increase.
4).Mixedness
The mixability of liquids is dissolved in each other. Example of mixed liquids: water and ethyl alcohol, an example of non-disheveled: water and liquid oil.
5).Formation of free surface and surface tension
Due to the preservation of volume, the liquid is capable of forming a free surface. Such a surface is the surface of the phase partition of this substance: on one side there is a liquid phase, according to another-gaseous (pairs), and possibly other gases, for example, air.
If the liquid and gaseous phase of the same substance contacts, the forces arise, which seek to reduce the surface area of \u200b\u200bthe surface tension separation. The surface of the section behaves like an elastic membrane, which seeks to push.
6).Waves of density
Although the liquid is extremely difficult to compress, nevertheless, when the pressure changes, its volume and density are still changing. This does not happen instantly; So, if one plot is compressed, then such compression is transmitted to the other sections. This means that elastic waves are capable of inside the liquid, more specifically, density waves. Along with the density, other physical quantities are changed, for example, temperature.
If during the spread of the wave the density changes sufficiently slightly, such a wave is called sound wave, or sound.
If the density changes strong enough, then such a wave is called a shock wave. The shock wave is described by other equations.
The density waves in the liquid are longitudinal, that is, the density changes along the direction of the wave propagation. The transverse elastic waves in the liquid are absent due to failure of the form.
Elastic waves In the fluid, they fade with time, their energy is gradually moving into thermal energy. Causes of attenuation - viscosity, "classical absorption", molecular relaxation and others. In this case, the so-called second, or volume viscosity - internal friction when changing density. The shock wave as a result of attenuation after a while goes into sound.
The elastic waves in the liquid are also exposed to scattering on heterogeneities arising from the chaotic heat movement of molecules.
Liquid structure
Experimental studies of a liquid state of a substance based on the observation of X-ray diffraction and neutron fluxes during passing them through liquid media, found in liquid middle-order. The presence of some orderliness in the location of the particles is only at a small distance from any dedicated position (Fig. 140).
The mutual arrangement of neighboring particles in liquids is similar to the ordered arrangement of adjacent particles in crystals. However, this ordering in liquids is observed only inside small volumes. At distances: from some selected "central" molecules, an orderly is broken (-effective diameter of the molecule). Similar orderliness in the location of particles in liquids and is called near-order .
Because of the lack of long-range fluid, for a few exceptions, anisotropy characteristic of crystals is not detected. For this reason, the structure of the fluid is sometimes called a quasicrystalline or crystalline .
For the first time, the idea of \u200b\u200bthe proximity of some properties of liquids (especially metal melts) and crystalline solids was expressed and then developed in the works of the Soviet physics Ya.I.Frenkel 1930-1940. According to the views of Frankel, which now obtained universal recognition, the thermal motion of atoms and molecules in the liquid consists of irregular oscillations with an average frequency close to the frequency of oscillations of atoms in crystalline bodies. The oscillation center is determined by the field of forces of neighboring particles and shifts together with the displacements of these particles.
It is simplified by such a heat movement as an overlapping of relatively rare jumps of particles from one side of the same time of equilibrium to other and thermal oscillations in the intervals between jumps. The average time of the "settled" stay of the fluid molecule near some equilibrium position is called relaxation time.After the time of time, the molecule changes the place of equilibrium, a jump moving into a new position, located from the previous one for the distance of the sizes of the molecules themselves. Thus, the molecule slowly moves inside the liquid. With increasing temperature, the time decreases, the molecules of the molecules will increase, which changes the viscosity of fluids (increases fluidity). According to the figurative expression, Ya.I.Frenkel, the molecules are watched throughout the volume of fluid, leading a nomadic lifestyle, in which short-term moving is replaced by relatively long periods of settling life.
Amorphous solid bodies (glass, resin, bitumen, etc.) can be considered as supercooled liquids whose particles due to highly increased viscosity have limited mobility.
Due to the small ordering of the liquid state, the theory of fluid is less developed than the theory of gases and crystalline solids. There is no complete fluid theory.
A special type of liquids is some organic compoundsconsisting of an elongated or disk-shaped molecules, or so-called liquid crystals. The interaction between molecules in such liquids seeks to build long axes of molecules in a certain order. At high temperatures, the thermal movement prevents this, and the substance is a conventional liquid. At temperatures below the critical liquid, a highlighted direction appears, a long-range orientation order occurs. Keeping the main features of the liquid, for example, fluidity, liquid crystals have the characteristic properties of solid crystals - anisotropy of magnetic, electrical and optical properties. These properties (along with the fluidity) find numerous technical applications, for example, in electronic watches, calculators, mobile phones, as well as in monitors of personal computers, TVs, as indicators, scoreboard and screens for displaying digital, letter and analog information.
Surface tension
The most interesting feature of liquids is the presence free surface. With the surface of the liquid connected free energyproportional to the area of \u200b\u200bthe free surface of the fluid :. Since the free energy of an isolated system is committed to a minimum, the liquid (in the absence of external fields) seeks to take a shape having a minimal surface area. Thus, the problem of the form of a fluid is reduced to an isoperimetric problem under the specified additional conditions (the initial distribution, the volume of IT.P.). The free drop takes the shape of the ball, however, with more complex conditions, the problem of the form of the surface of the liquid becomes extremely complex.
Liquid, unlike gases, does not fill the entire volume of the vessel in which it is nanitis. Between the liquid and gas (or ferry), the boundary of the partition is formed, which is under special conditions compared to the rest of the fluid. Molecules in the border layer of fluid, in contrast to molecules in its depth, are surrounded by other molecules of the same fluid not from all sides. The forces of intermolecular interaction acting on one of the molecules inside the liquid from the neighboring molecules, on average, mutually compensated (Fig. 141).
But all molecules, including the boundary layer molecules, should be in a state of equilibrium. This equilibrium is achieved due to a certain decrease in the distance between the surface layer molecules and their nearest neighbors inside the liquid. With a decrease in distance between molecules, repulsion force arise. The surface layer molecules are packed somewhat more tight, and therefore they have an additional reserve of potential energy compared to internal molecules. Hence, the molecules of the surface layer of fluid have redundant compared to molecules inside the liquid potential energy equal to free energy. In order, the potential energy of the surface of the liquid is proportional to its area :.From the mechanics it is known that the equilibrium states of the system corresponds to the minimum value of its potential energy, i.e. The free surface of the fluid seeks to reduce its area. The fluid behaves as if for the tangent of its surface there are forces that reduce (tightening) this surface. These forces are called forces of surface tension .
We choose some closed circuit on the surface of the liquid. For all molecules lying inside this contour, all forces mutually balanced. However, for molecules located along the contour, the forces are directed outward, are external forces; They are perpendicular to the perimeter and tangents to the surface of the liquid. These forces stretching the film, and are the surface tension (Fig. 143).
 Fig. 143. |
The presence of surface tension forces makes the surface of the liquid similar to the elastic stretched film, with the only difference that the elastic force in the film depends on its surface area (that is, on how the film is deformed), and the surface tension forces do not depend From the surface of the surface of the liquid.
