Fluidity. Liquid and gaseous bodies differ from solid bodies in the yield. If small non-destructive forces act on the solid body, then they slightly change its shape, i.e. The relative position of its parts. If, under the action of an arbitrarily small external force, the body is deformed unlimited until the internal tangent stresses in it become equal zero, in this case, the property is realized called fluidity .

Many physical bodies are dual in nature. For example, glass that we used to consider as a fragile solid body, under the action of a long-term load can behave like a liquid. So the window windows, stood over 100 years at the bottom thick, than in the top, as under the action of gravity, the material "flows" down. On the other hand, such a typical liquid as water with rapid loading (impact) behaves like a solid body.

We will try to determine the nature of this duality at the molecular level. Due to the action of the attraction and repulsion forces, the location of molecules in space is ordered. The average characteristic distance between liquid and solid body molecules is approximately the same and equal to "(3¸4) 10 -8 cm. Under the action of heat, the molecule is moving (chaotic oscillates) near the position of the equilibrium, increasing with the temperature of the oscillation amplitude. In solid bodies, the amplitude is much less than the distance between molecules, in liquid - These are the values \u200b\u200bof one order. Therefore, fluctuations in molecules performed with the amplitude of the same order as the distance can lead to the fact that the molecules can jump from one place of the cell to another. In some liquids it happens more often, in others - less often.

The fluidity of the body is determined by the characteristic time T R of finding the molecule in each cell from the moment of entering it until the resettlement in another cell. If the time of finding the molecule in a cell is much less than the time of force, then for the period of the force of the molecule, it can be repeatedly changed its position in space, i.e. Allowing the strength continuously and irreversibly deform the body (i.e., behave like a fluid). We call such a body liquid . Otherwise, we are dealing with a solid body. With an increase in the temperature of the fluidity of the body increases.

For gaseous tel characteristic feature It is a chaotic movement and collision of molecules in space. Therefore, gases have not only fluidity, but also compressibility.

Compressibility of liquids and gases. We will apply the power of DF and increase the pressure in the volume V by the value of DP (Fig. 1.2). The solid medium will be frightened by reducing its volume by the value of DV. It is empirically obtained that the connection between the change in volume and the pressure of the linear, i.e. For each liquid and gas, you can enter a constant, which is called the voluminous expansion coefficient (at a constant temperature):

. (1.3.1)

The volume compression coefficient has dimension (PA) -1. The minus sign is introduced in order to reflect the reduction of the volume under the action of compression, but for practical calculations it is convenient to have it positive.

The modulus of volume elasticity e V is called the value inverse b V:

. (1.3.2)

Both of these values \u200b\u200bdepend on the temperature and type of fluid. Module of volumetric elasticity for water at T \u003d 293 ° K is equal to e v \u003d 2 × 10 9 Pa "20,000 kgf / cm 2.

Example. If in addition to the water in addition to atmospheric pressure (p and \u003d 101325 PA or 1.033 kgf / cm 2), the same pressure will additionally act, then the volume of water will decrease by approximately 1/20000, i.e. Almost this is impossible to notice. Consequently, water and other liquids can be considered incompressible and make their density constant (R \u003d const), independent of pressure.

For gas, it is possible to effectively use the model of the ideal gas characterized by the Klapairone equation. - Mendeleev

or , (1.3.3)

where R is a specific gas constant, independent of density and temperature, but different depending on the nature of the gas (for example, for air R \u003d 287J / kHk). With the help of equation (1.5.3), you can find air density at atmospheric pressure and ambient temperature of 20ºС:

.

From this law, the law of boil - Mariotta, establishing an isothermal connection between pressure and density:

for a given amount of gas at a constant temperature.

For the adiabatic process (when there is no heat exchange between the allocated gas and the environment), the following dependence is characteristic:

, (1.3.5)

where - adiabatic permanent gas; With V. - Gas heat capacity at a constant volume; with R. - The same at constant pressure.

The difference of liquid mechanics from gas mechanics. Despite the fact that the fluidity property is the main in the study of liquids and gas, however, in some cases it is necessary to distinguish fluids from gases.

· The main difference is that the gas is easily compressed and in it the speed of propagation of sound (and, therefore, all mechanical perturbations) are significantly less than in liquid. This feature of the gas should be taken into account when the speed of movement (or the speed of driving in it is solid) becomes commensurate at the speed of sound or exceed it.

· In contrast to the gas, the liquid has a boundary surface between it and its surrounding gas, which is called a free surface. In the field of gravity, the free surface of the fluid has a horizontal profile. Under conditions of weightlessness, due to surface tension, the free surface of the spheric. This property of fluid, as well as its small compressibility, is due to the constant interaction of adjacent molecules. In the gas, the molecules interact with each other only at the time of the collision, most of the time they move freely in space, therefore, due to the chaotic movement of the movement, the gas seeks evenly to host the entire closed part of the space. If the space is not closed, the volume of gas may increase indefinitely.

· Gaza can unlimited reduce pressure and increase the temperature, and the properties of the gas will change continuously. In fluid, pressure can decrease to a certain value below which the formation inside the gas bubbles begins, and phase transitions begin, which qualitatively change the properties of the fluid. The same can occur when the temperature of the fluid can occur.

Liquid viscosity and gases. Rheological properties of liquids. The viscosity is called the property of the fluid, which consists in the occurrence of internal forces that prevent its deformation, i.e. Change the relative position of its parts. Consider the private event of the molecular-kinetic theory of the perfect gas - a simple shift flow (Fig. 1.3).

Fig.1.3. Viscous stresses in liquids and gases

The elementary platform of the surface separating the layers 1 and 2 moves together with the liquid. With this layer of liquid 1 slides in a layer 2 at relative speed. Gas molecules are involved in the movements of two types:

· Ordered (longitudinal) with a speed U x or u x + d u x depending on which layer they are;

· Chaotic, disordered (including transverse) thermal motion, whose speed is usually two orders of magnitude higher than the speed of ordered movement.

The gas viscosity is due to the transfer of molecules with their thermal motion through the elementary area of \u200b\u200bthe DXDU, lying in the plane, which separates two layers having various longitudinal velocities U x and u x + du x, the amount of movement due to the difference in DU X velocities of these layers. Molecules are moving chaotic randomly, while they go out of one layer to another, crossing the DHD. Molecules having an ordered speed U x, go into a layer 2 and slow down his movement, and the same number of molecules falling in the layer 1 from the layer 2, accelerates the layer 1.

Introducing a solid medium (i.e., excluding the molecular structure of the substance), it is believed that there is a tangent voltage on the DHD site, compensating for the transfer of the amount of movement due to the thermal motion of molecules. According to molecular-kinetic theory of tangent

(1.3.6)

where h is dynamic viscosity coefficient , or simply the dynamic viscosity of the gas. This is a hydrodynamic characteristic, determined by the physical properties of the fluid. The voltage sign is as if it is "trying to" reduce the difference in the speeds of the layers. With increasing temperature, the rate of chaotic movement of molecules increases, which leads to an increase in the number of molecules crossing the DXDU platform per unit; Therefore, the amount of movement of one layer in the other increases and, respectively, the tangent voltage P ZX. According to (1.3.6), this means that with increasing temperature, the dynamic gas viscosity coefficient increases.

In the liquid, the main cause of the effect of one layer to another (that is, the transfer of the amount of movement) is the interaction of molecules located along different sides of the boundary between the layers, and not the transfer of molecules through this boundary. As already noted, the molecular-kinetic theory of fluid is still underdeveloped, therefore the mechanism of viscosity in the fluid is well worse than in gases. It is usually considered that in liquids are continuously formed and destroyed with relative slides of the layers of quasicrystalline structures, and the forces necessary for their destruction are caused by viscosity. Naturally, with an increase in the temperature of the fluid molecule becomes more movable and the destruction of the structures occurs with smaller values \u200b\u200bof the shifting forces. Thus, the dynamic viscosity coefficient of fluid with increasing temperature decreases (as opposed to gases - see above).

Despite the different molecular mechanism of the occurrence of stresses in liquids and in gases, in both of these environments, tangent stresses are associated with the variability of the velocity field of the same

bridge (1.3.6), which is called newton's law for viscous stresses.In contrast to the dry friction law, the shear tangent voltage in liquids and gases does not depend on normal voltage.

According to definition (1.3.6), the dynamic viscosity coefficient H has the following unit of measure:

.

The dimension H is expressed through the dimension of the voltage of the PA and time with. Sometimes as a unit H, r / cm × C, which is called PUAZ (in honor of the French doctor A. Poiseil, who fulfilled the fundamental studies of the movement of a viscous fluid) and is indicated by P:

PA × C \u003d 10 × p.

Dependence (1.3.6) characterizes the transfer of the flow of the flow of the fluid layer movement, which is proportional to both the velocity U x and the fluid density R. Having this in mind, Newton's law is advisable to submit in the form

,

. (1.3.7)

This value has dimension

.

In view of the fact that the dimension n only consists of meters and seconds (and does not include the dimension of the mass), this magnitude is called kinematic viscosity coefficient(or kinematic viscosity). The dimension of the CM 2 / C is called Stokes (in honor of the English hydromechanics J. Stokes, which formulated differential equations Movement of a viscous fluid), and is denoted by st:

1st \u003d 10 -4 m 2 / s.

In conclusion, we note that in gases and viscosity (characterizing the transfer of the amount of movement), and molecular diffusion (characterizing the transfer of foreign gas) are due to the thermal chaotic movement of molecules. Therefore, the viscosity N has one order of magnitude with a molecular diffusion coefficient in the law of the fic. In fluids, the viscosity (and associated with the transfer of the amount of movement) is due to the destruction of intermolecular bonds, and diffusion - the thermal motion of molecules, i.e. These phenomena have a different physical nature. As a result of this, the diffusion coefficient in the liquid is hundreds of times less viscosity coefficient N. in Table. 1.1 shows the values \u200b\u200bof H, R, N for some liquids and gases.

Table 1.1.

Values \u200b\u200bH, R, N for some liquids and gases

From the above values \u200b\u200bof viscosity coefficients, it follows that water viscosity decreases with increasing temperature from 0 to 100 ° to seven times, and air viscosity increases with increasing temperature from 20 to 50 ° C by 25% .

For settlements in engineering practice, they use the estimated value of the kinematic viscosity coefficient of the water n \u003d 0.01 cm 2 / s \u003d 0.01 art. The liquids for which the relationship is valid (1.3.6) are called Newtonian.

However, there are many liquids for which Newton's law is not executed. The science of the character of dependence is called a rheology (Greek. REO - flow, logos - doctrine). If you submit a dependence (1.3.6) as a graph (Fig.1.4), then it will have the kind of straight line 1.

With an experimental study of some liquids, there may be a form of curve 2. Such liquids that resist small (P ZX< ) сдвигающим напряжениям, как твердое тело, а при (p zx > ) behave like liquid bodies, called Bingama - Swedov liquids.

The fluid, the behavior of which is described by curves 3, 4, are called fluids of ostelald - Weyl. If they obey dependencies 3, then they are called pseudoplastic, and if they follow 4 - dilatated dependencies. Mechanics movement of such liquids (these are resin, petroleum products, polymer solutions, etc.) is very complex.

The liquid is called a substance that is in an aggregate state, which is intermediate between solid and gaseous. At the same time, its condition, as in the case of solid bodies, is condensed, that is, it assumes the relationship between particles (atoms, molecules, ions). The liquid has properties that radically distinguish it from substances that are in other aggregate states. The main thing is the ability to repeatedly change the form under the influence of mechanical stresses without loss of volume. Today we will find out what properties are fluid, and that they actually represent themselves.

general characteristics

Gas does not retain volume and shape, the solid preserves both, and the liquid is only the volume. That is why the liquid aggregate state is considered an intermediate. The surface of the liquid represents the similarity of the elastic membrane and determines its form. Molecules of such bodies, on the one hand, do not have a certain position, and on the other - they cannot receive complete freedom of movement. They can be collected in drops and flow under their own surface. There is an attraction between fluid molecules that is enough to hold them at close range.

The substance is in a liquid state at a certain temperature range. If the temperature falls below it, the transition to a solid shape (crystallization), and if rises above - into gaseous (evaporation). The boundaries of this interval for the same liquid can vary depending on the pressure. For example, in the mountains, where pressure is significantly lower than on the plains, water boils at a lower temperature.

Usually, the liquid has only one modification, therefore, it is both an aggregate state, and the thermodynamic phase. All fluids are divided into pure substances and mixtures. Some of these mixtures are determining in human life: Blood, sea \u200b\u200bwater and others.

Consider the basic properties of liquids.

Fluidity

From other substances, the liquid is different, first of all, fluidity. If there is an external force to it, in the direction of its application there is a flow of particles. Thus, when exposed to external unbalanced forces, the fluid is not capable of preserving the shape and relative location of the particles. For the same reason, it takes the shape of the vessel in which it falls. Unlike solid plastic bodies, fluids do not have a yield strength, that is, they flow with the slightest output from the equilibrium state.

Saving volume

One of the characteristic physical properties of liquids is the ability to preserve the volume under mechanical exposure. They are extremely difficult to compress due to the high density of molecules. According to the law of Pascal, the pressure that is produced on the liquid concluded in the vessel is changed to every point of its volume. Along with minimal compressibility, this feature is widely used in hydraulics. Most liquids during heating increases in volume, and when cooling, it decreases.

Viscosity

Among the main properties of liquids, as in the case of gases, it is worth noting viscosity. The viscosity is called the ability of particles to resist the movement relative to each other, that is, internal friction. When moving the adjacent layers of fluid relative to each other, there is an inevitable collision of molecules, and the forces arise that the ordered movement occur. The kinetic energy of an ordered movement is converted to thermal energy chaotic movement. If the liquid placed in the vessel is moved, and then leave alone, it will gradually stop, but its temperature will increase.

Free Surface and Surface Tension

If you look at a drop of water, which lies on a flat surface, then you can see that it is rounded. Conducted by these properties of liquids as the formation of a free surface and surface tension. The ability of fluids to preserve the volume causes the formation of a free surface, which is not different as the surface of the phase separation: liquid and gaseous. In contact with these phases of the same substance, forces arise aimed at reducing the area of \u200b\u200bthe section plane. They are called superficial tension. The border of the phase partition is an elastic membrane, striving for tightening.

Surface tension is also explained by attraction of fluid molecules to each other. Each molecule seeks to "surround" himself with other molecules and leave the border of the section. Because of this, the surface is rapidly reduced. This explains the fact that soap bubbles and bubbles formed when boiling seek to take a spherical shape. If only the strength of the surface tension is to be operated on the liquid, it will certainly take such a form.

Small objects whose density exceeds the density of the liquid, can remain on its surface due to the fact that the force that prevents an increase in the surface area is greater than the force of gravity.

Evaporation and condensation

Evaporation is called a gradual transition of a substance from liquid state in gaseous. In the process of thermal motion, part of the molecules leave the liquid, passing through its surface, and are converted to steam. In parallel with this, the other part of the molecules, on the contrary, passes from the vapor into the liquid. When the number of compounds that left the liquid exceeds the number of compounds that came to it, the evaporation process takes place.

Condensation is called the process, inverse evaporation. During condensation, the liquid receives more molecules from steam than gives.

Both described processes are nonequilibrium and can continue until local equilibrium is established. At the same time, the liquid can completely evaporate or enter into equilibrium.

Boiling

Boiling is called the process of internal fluid transformations. When the temperature is raised to a certain indicator, the pair pressure exceeds the pressure inside the substance, and bubbles begin to form in it. In the conditions of earthly attraction they pop up.

Wetting

Wetting is called a phenomenon that occurs when liquid contact with a solid in the presence of steam. Thus, it occurs on the border of the section of the three phases. This phenomenon characterizes the "sticking" of a liquid substance to solid, and its spreading on the surface of the solid. There are three types of wetting: limited, complete and non-wing.

Mixedness

Characterizes the ability of fluids to dissolve each other. An example of mixed liquids can be played by water and alcohol, and unsuccessful water and oil.

Diffusion

When the two mixed liquids are in one vessel, due to the thermal movement of the molecule begin to overcome the interface, and the liquids are gradually mixed. This process is called diffusion. It can occur in substances that are in other aggregate states.

Overheating and supercooling

Among the fascinating properties of fluids should be noted overheating and supercooling. These processes often form the basis of chemical focus. With uniform heating, without strong temperature differences and mechanical effects, the liquid can warm up the boiling point, not to type it. This process was called overheating. If there is an object into superheated liquid, it will scribe instantly.

Similarly, the supercooling of the liquid occurs, that is, its cooling to the temperature below the freezing point, bypassing the freezing itself. With a light blow, the hypochealed liquid instantly crystallizes and turns into ice.

Waves on the surface

If you break the balance of the surface of the liquid, then it, under the action of returning forces, will move back to equilibrium. This movement is not limited to one cycle, but turns into oscillations and applies to other sections. So the waves are obtained, which can be observed on the surface of any liquid.

When the strength of gravity is advantageous as a return force, the waves are called gravitational. They can be seen on the water everywhere. If the return force is formed mainly from the strength of the surface tension, then the waves are called capillary. Now you know which property of liquids causes a familiar to everyone's excitement.

Waves of density

The liquid is extremely squeezed, nevertheless, with a change in temperature, its volume and density are still changing. It does not happen instantly: when compressing one section, others are compressed with delay. Thus, within the liquid, elastic waves are applied, which received the name of the density wave. If, as the wave propagates, the density changes weakly, then it calls the sound, and if it is strong enough - shock.

We met with the common properties of liquids. All major characteristics depend on the type and composition of liquids.

Classification

Having considered the basic physical properties of liquids, let's find out how they are classified. The structure and properties of liquid substances depend on the individuality of the particles included in their composition, as well as the nature and depth of the interaction between them. Based on this, allocate:

1. Atomic fluids. Consist of atoms or spherical molecules that are interconnected by central Van der Wales forces. A bright example is liquid argon and liquid methane.
2. Liquids consisting of diatomic molecules with the same atoms whose ions are bound by Coulomb forces. As an example, you can call: liquid hydrogen, liquid sodium and liquid mercury.
3. Fluids that consist of polar molecules associated with dipole-dipole interaction, for example, liquid bromomarodes.
4. Associated fluids. Have hydrogen bonds (water, glycerin).
5. Fluids that consist of large molecules. For the latter, internal degrees of freedom play an important role.

The substances of the first two (less often three) groups are called simple. They are learned better than everyone else. Among the difficult liquids, the most studied water. This classification does not include liquid crystals and quantum fluids, as they are special cases and are considered separately.

From the point of view of hydrodynamic properties, fluids are divided into Newtonian and Nengeton. The current is subordinate to Newton's law. This means that their tangent stress linearly depends on the speed gradient. The proportionality coefficient between the specified values \u200b\u200bis called viscosity. In Nengeton liquids, the viscosity fluctuates depending on the gradient of the speed.

Study

The study of the movement and mechanical equilibrium of liquids and gases, as well as their interaction, including with solid bodies, is engaged in such a section of mechanics as a hydraulic member. It is also called hydrodynamics.

Unempatherable fluids are studied in the subsection of hydroameromechanics, which is called simply hydromechanics. Since the compressibility of liquids is very small, in many cases it is simply neglected. Compressible fluids studies gas dynamics.

Hydromechanics are additionally subdivided into hydrostatics and hydrodynamics (in a narrow sense). In the first case, the equilibrium of incompressible fluids is studied, and in the second - their movement.

Magnetic hydrodynamics is engaged in the study of magnetic and electrically conductive liquids, and hydraulics is engaged in applied tasks.

The main law of hydrostatics is the law of Pascal. The movement of ideal incompressible liquids is described by the Euler equation. For their stationary stream, Bernoulli law is performed. A Torrichell formula describes the leakage of liquid substances from the holes. The movement of viscous liquids is obeying the Navier-Stokes equation, which, among other things, can also take into account compressibility.

Elastic waves and fluctuations in the liquid (as, however, in other environments) such science is studied as acoustics. Hydroacience - subsection, which is devoted to the study of sound in an aquatic environment to solve the problems of underwater, location and other things.

Finally

Today we met with the common physical properties of liquids. We also learned that in general there are such substances, and how they are classified. Concerning chemical properties Liquids, then they directly depend on its composition. Therefore, they are considered separately for each substance. What a fluid property is important, and what is not - it is difficult to answer. It all depends on the problem, in the context of which this liquid is considered.

The liquid is the aggregate state of the substance that occupies an intermediate position between its solid and gaseous states.

The most common fluid on the ground is water. Its solid state - ice, and gaseous - steam.

In liquids, the molecules are located almost close to each other. They possess more freedom than solid molecules, although they cannot completely move freely. The attraction between them although weaker than in solid bodies, but it is still enough that the molecules have retained close to each other. Each liquid molecule can fluctuate near some kind of equilibrium center. But under the action of the outer force, the molecules can leap on the free place in the direction of the applied force. This is explained fluid flow .

Fluidity

The main physical property of fluid - fluidity . When the external force is applied to the liquid, the flow of particles occurs in it, the direction of which coincides with the direction of this force. Tilting the kettle with water, we will see how the water flows from its nose down under the action of gravity. Similarly, water flows from the watering can, when we water the plants in the garden. We observe such a phenomenon in waterfalls.

Due to the fluidity, the fluid is capable of changing the form in a small time under the action of even a small force. All fluids can pour jets, spray with drops. They are easy to pour out one vessel to another. At the same time they do not preserve the form , and take the form of the vessel in which there are. This property of fluid is used, for example, when casting metal parts. Melted liquid metal is bottled in the form of a specific configuration. Cooled, it turns into a solid body that saves this configuration.

The fluidity increases with increasing fluid temperature and decreases when it decreases. This is explained by the fact that with increasing temperature, the distance between the particles of the liquid also increases, and they become more movable. Depends on the fluidity and on the structure of molecules. The more complicated their form, the lower fluid has a fluid.

Viscosity

Various liquids have different fluidity. So, water from the bottle flows faster than vegetable oil. Honey from a glass pours slower than milk. These liquids operate the same gravity. So why are their turnover differ? The thing is that they possess different viscosity . The higher the viscosity of the fluid, the less its fluidity.

What is viscosity, and what is its nature? Viscosity is also called internal friction . This is the ability of the liquid to resist the movement of various layers of fluid relative to each other. Molecules located in one of the layers and encountered between themselves during the heat movement are also faced with molecules of neighboring layers. There are forces that slow down their movement. They are directed to the side opposite to the movement of the layer under consideration.

Viscosity is an important characteristic of liquids. She is taken into account in various technological processes, for example, when pipelines need to pump fluid.

The viscosity of the fluid is measured using the device called viscometer. The simplest is considered capillar Viscometer. The principle of its action is not complicated. The time is calculated for which the specified volume of the liquid flows through a thin tube (capillary) under the influence of pressure difference at its ends. Since the diameter and the length of the capillary are known, the pressure difference is known, then calculations can be made on the basis law of Poazoyl , Whereby passing per second volume of liquid (the second volume flow) is directly proportional to the pressure difference per unit of pipe length and the fourth degree of its radius and inversely proportional to the fluid viscosity coefficient .

where Q. - the second consumption of fluid, m 3 / s;

p 1 - P 2 = ΔР. - pressure drop at the ends of the capillary, PA;

R. - radius of the capillary, m;

d. - diameter of the capillary, m;

ƞ - dynamic viscosity coefficient, PA / C;

l. - Capillary length, m.

Volume

The distance between molecules inside the liquid is very small. It is less than the sizes of the molecules themselves. Therefore, the liquid is very difficult to squeeze mechanically. The pressure produced on the liquid enclosed in the vessel is transmitted to any point without changes in all directions. So formulated pascal law . On this feature of liquids, the work of brake systems, hydraulic presses and other hydraulic devices is based.

The fluid saves its volume if the external conditions (pressure, temperature) do not change. But when heated, the volume of the liquid increases, and during cooling decreases. However, there is an exception. Under normal pressure and increase temperatures from 0 to 4, the volume of water does not increase, but decreases.

Waves of density

Squeeze the liquid is very difficult. But when changing pressure is still possible. And in this case, its density changes and volume. If the compression occurs in one section of the fluid, then it will be transmitted to other sections gradually. This means that the fluid will spread elastic waves. If the density changes weakly, then we get a sound wave. And if it is strong enough, then a shock wave arises.

Surface liquid tension

We observe the phenomenon of surface tension every time the water slowly drips from the water tap. First we see a thin transparent film that is stretched under the weight of water. But it does not break, but covers a small amount of water and forms a droplet falling from the crane. It is created by the forces of surface tension, which tighten the water into a small semblance of a ball.

How do these forces arise? Unlike gas, the liquid fills only part of the volume of the vessel in which it is located. Its surface is the boundary of the section between the fluid itself and the gas (air or ferry). On all sides, the molecule inside the liquid surround other molecules of the same liquid. There are forces of intermolecular impact. They are barely balanced. Equality of these forces is zero.

And on the molecules that are in the surface layer of fluid, the forces of attraction from the molecules of the same fluid can only act on one side. On the other hand, they have the strength of attraction of air molecules. But since they are very small, they neglect.

The equally all forces acting on the molecule on the surface is directed inside the liquid. And in order not to be drawn into the liquid and stay on the surface, the molecule makes work against this force. As a result, the upper layer molecules receive an additional supply of potential energy. The greater the surface of the fluid, the greater the amount of molecules is there, and the greater the potential energy. But in nature everything is arranged in such a way that any system is trying to reduce its potential energy to a minimum. Investigator, there is a force that will strive to reduce the free surface of the fluid. This power is called surface tension power .

The tension of the liquid surface is very large. And to break it requires quite significant power. The undisturbed surface of the water can easily hold the coin, razor blade or steel needle, although these items are much heavier than water. The strength of gravity acting on them turns out to be less for the strength of the surface tension of water.

The smallest surface of all geometric volumetric bodies has a ball. Therefore, if only the forces of the surface tension apply to the liquid, it takes the form of the sphere. Such form have drops of water in weightlessness. Bubble Or boiling liquid bubbles also try to take a spherical shape.

Mixedness

Liquids can be dissolved in each other. This their ability is called mixedness . If you put two mixed fluids into one vessel, then as a result of the thermal motion, their molecule will gradually move over the interface. As a result, mixing will occur. But not all fluids can be mixed. For example, water and vegetable oil are never mixed. And water and alcohol mix very easily.

Adhesion

We all know that geese and ducks leave the water with dry. Why are their feathers wet? It turns out that they have a special iron that highlights fat, which waterfowl with beak lubricate their feathers. And they remain dry, because water flows from them droplets.

Let's place a drop of water on a polystyrene plate. It takes the shape of a split ball. We try to put the same drop on the glass plate. We will see that it spreads on the glass. What happens to water? The thing is that attraction forces act not only between the molecules of the fluid itself, but also between the molecules of different substances in the surface layer. These forces are called forces adhesion (from Latin adhasio. - sticking).

The interaction of fluid with solid body is called wetting . But the surface of the solid body is not always wetting. If it turns out that the molecules of the fluid itself are attracted to each other stronger than to a solid surface, the liquid will gather in the droplet. This is how water behaves on a polystyrene plate. She is does not wet This plate. In the same way, the droplets of the morning dew on the leaves of plants are not spread. And for the same reason, water flows with waterfowl feathers covered with fat.

And if the attraction of fluid molecules to a solid surface is stronger than the attraction forces between the molecules themselves, the liquid is blurred on the surface. Therefore, our droplet on the glass also spread. In this case, water wetting Glass surface.

Hall water in a polystyrene vessel. Looking at the surface of the water, we will see that it is not horizontal. At the edge of the vessel, it is twisted down. This is happening, because the forces of attraction between water molecules are greater than the forces of adhesion (adhesion). And in the glass vessel, the water surface of the edge is twisted upwards. In this case, the strength of adhesion is more intramolecular water forces. In wide vessels, this curvature is observed only at the walls of the vessels. And if the vessel is narrow, then this curvature is noticeably all over the surface of the water.

The adhesion phenomenon is widely used in various industries - paint, pharmaceutical, cosmetic, etc. Wetting is necessary when gluing, painting tissues, applying to the surfacepaints, varnishes. And in the construction of the pools of their walls, on the contrary, covered with a material that is not wetted with water. The same materials are used for umbrellas, raincoats, waterproof shoes, awnings.

Capillarity

One more interesting feature liquids - capillary effect . It is so called its ability to change its level in tubes, narrow vessels, porous bodies.

If you omit a narrow glass tube (capillary) into water, then you can see how the water column rises in it. The already tube, the higher the water column. If you lower the same tube into a liquid mercury, the height of the mercury column will be below the level of fluid in the vessel.

Liquid in capillaries can rise through the narrow channel (capillary) only if it wets its walls. This happens in the ground, sand, glass tubes, for which moisture is easily rising. For the same reason, kerosene wick in the kerosene lamp is impregnated, the towel absorbs moisture from wet hands, various chemical processes occur. In plants in capillaries come to the leaves nutrients and moisture. Thanks to the capillary effect there is a vital activity of living organisms.

We already know that fluids have a fixed volume and take the form of the vessel in which they are. We also know that liquids density is much larger than gases. In the general case, the density of liquids is values \u200b\u200bsimilar to the densities of solids. The squeezability of liquids is very small, since there is a very little free space between particles of the liquid.

Freely falling drop of water. Its spherical form is due to surface tension.

We have to consider another three other important properties of liquids. All these properties can be explained on the basis of the representations of the kinetic theory of fluids.

Fluidity and viscosity. Like liquid gases can flow, and this property is called fluidity. Resistance to flow is called viscosity. The fluidity and viscosity affects a number of factors. The most important of these are the forces of attraction between liquid molecules, as well as the form, structure and relative molecular weight of these molecules. The fluid flow consisting of large molecules is lower than the liquid from small molecules. Liquid viscosity is approximately 100 times more than gases.

Surface tension. On the molecule, which is in the depths of the fluid, the forces of intermolecular attraction are evenly operating on all sides. However, on the surface of the fluid, these forces are unbalanced, and as a result of this, surface molecules are experiencing the result of the resulting force directed inside the liquid. Therefore, the surface of the fluid turns out to be in the tension state, it all the time seeks to cut. The surface tension of the fluid is the minimum force required to overcome the aspiration of the particles of the liquid inside and thereby keep the surface of the fluid from the reduction. The existence of surface tension explains the spherical form of freely falling droplets of the liquid.

Diffusion. This is the name of the process by which the substance is redistributed from a region with a high concentration or high pressure into a region with a smaller concentration or less pressure. Diffusion in liquids is much slower than in gases, because the particles of the liquid are packaged much more dense than gas particles. A particle diffusing in a liquid is subjected to frequent collisions and therefore moves hard. In the gases between the particles a lot of free space, and they can redistribute much faster. Diffusion is carried out between mutually soluble, or mixing, liquids. It does not occur between unsuccessful fluids. Unlike liquids, all gases are mixed with each other and therefore can diffuse one to another.