Drip, drip... Another drop collected on the spout of the faucet, swelled and fell down. This picture is familiar to everyone. Or a warm summer rain waters the soil yearning for moisture - and again there are drops. Why drops? What is the reason here? It's very simple: the reason for this is the surface tension of water.
This is one of the properties of water or, more generally, all liquids. As you know, gas fills the entire volume into which it enters, but liquid cannot do this. Molecules located inside a volume of water are surrounded by the same molecules on all sides. But those on the surface, at the boundary of liquid and gas, are not affected from all sides, but only from those molecules that are located inside the volume; they are not affected by the gas.
In this case, a force will act on the surface of the liquid, directed along it perpendicular to the portion of the surface on which it acts. As a result of this force, surface tension of water arises. Its external manifestation will be the formation of something like an invisible, elastic film at the interface. Due to the influence of surface tension, a drop of water will take the shape of a sphere as a body that has the smallest area for a given volume.
Now we can define that surface tension is the work done to change the surface of a liquid. On the other hand, it can be defined as the energy required to break a unit of surface. Surface tension is possible at the interface between liquid and gas. It is determined by the force acting between molecules, and therefore responsible for volatility (evaporation). The lower the surface tension, the more volatile the liquid will be.
You can determine what it is equal to. The formula for calculating it includes the surface area and As mentioned earlier, the coefficient does not depend on the shape and size of the surface, but is determined by the strength of intermolecular interaction, i.e. type of liquid. For different liquids its value will be different.
The surface tension of water can be changed. This is achieved by heating, adding biologically active substances - such as soap, powder, paste. Its value depends on the degree of purity of the water. The purer the water, the greater the surface tension, and its value is second only to mercury.
A curious effect is observed when a liquid comes into contact with both a solid and a gas. If we apply a drop of water to the surface of paraffin, it will take the shape of a ball. This is due to the fact that the forces acting between the paraffin and the drop are less than the interaction between themselves, as a result of which a ball appears. When the forces acting between the surface and the drop are greater than the forces of intermolecular interaction, the water will spread evenly over the surface. This phenomenon is called wetting.
The wettability effect can to some extent characterize the degree of surface cleanliness. On a clean surface, the drop spreads evenly, and if the surface is dirty or covered with a substance that is not wetted by water, the latter collects into balls.
An example of the use of surface tension in industry is the casting of spherical parts, such as shotgun pellets. Drops of molten metal simply freeze on the fly, taking on a spherical shape.
The 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.
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.
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.
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.
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 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.
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.
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.
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.
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.
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.
Liquid is a state of aggregation of a substance, intermediate between gaseous and solid, therefore it has the properties of both gaseous and solid substances. Liquids, like solids, have a certain volume, and like gases, they take the shape of the vessel in which they are located. Gas molecules are practically not connected to each other by intermolecular interaction forces. In this case, the average energy of thermal motion of gas molecules is much greater than the average potential energy caused by the forces of attraction between them, so the gas molecules fly apart in different directions, and the gas occupies the entire volume provided to it.
In solids and liquids, the forces of attraction between molecules are already significant and keep the molecules at a certain distance from each other. In this case, the average energy of the chaotic thermal motion of molecules is less than the average potential energy due to the forces of intermolecular interaction, and it is not enough to overcome the forces of attraction between molecules, therefore solids and liquids have a certain volume.
X-ray diffraction analysis of liquids showed that the nature of the arrangement of liquid particles is intermediate between a gas and a solid. In gases, molecules move chaotically, so there is no pattern in their relative arrangement. For solids, the so-called long range order in the arrangement of particles, i.e. their ordered arrangement, repeating over large distances. In liquids there is a so-called close order in the arrangement of particles, i.e. their ordered arrangement, repeating at distances comparable to interatomic ones.
The theory of liquids has not yet been fully developed. Thermal motion in a liquid is explained by the fact that each molecule oscillates for some time around a certain equilibrium position, after which it jumps to a new position, separated from the original one at a distance of the order of interatomic. Thus, the molecules of the liquid move rather slowly throughout the mass of the liquid, and diffusion occurs much more slowly than in gases. With increasing temperature of the liquid, the frequency of vibrational motion increases sharply, the mobility of molecules increases, which causes a decrease in the viscosity of the liquid.
Each molecule of a liquid is subject to attractive forces from surrounding molecules, which quickly decrease with distance; therefore, starting from a certain minimum distance, the forces of attraction between molecules can be neglected. This distance (approximately 10 -9 m) is called radius of molecular action r , and the sphere of radius r-sphere of molecular action.
Let us isolate a molecule inside the liquid A and draw a sphere of radius around it r(Fig. 10.1). It is enough, according to the definition, to take into account the effect on a given molecule only of those molecules that are inside the sphere
Fig. 10.1. molecular action. The forces with which these molecules act on the molecule A, are directed in different directions and are compensated on average, so the resulting force acting on a molecule inside the liquid from other molecules is zero. The situation is different if the molecule, e.g. IN, located from the surface at a distance less than r. In this case, the sphere of molecular action is only partially located inside the liquid. Since the concentration of molecules in the gas located above the liquid is small compared to their concentration in the liquid, the resultant force F, applied to each molecule of the surface layer, is not equal to zero and is directed into the liquid. Thus, the resulting forces of all molecules of the surface layer exert a pressure on the liquid, called molecular(or internal). Molecular pressure does not act on a body placed in a liquid, since it is caused by forces acting only between the molecules of the liquid itself.
The total energy of liquid particles consists of the energy of their chaotic thermal motion and potential energy due to the forces of intermolecular interaction. To move a molecule from the depths of the liquid to the surface layer, work must be expended. This work is done due to the kinetic energy of the molecules and goes to increase their potential energy. Therefore, the molecules in the surface layer of a liquid have greater potential energy than the molecules inside the liquid. This additional energy possessed by molecules in the surface layer of a liquid, called surface energy, proportional to the layer area Δ S:
Δ W=σ Δ S,(10.1)
Where σ – surface tension coefficient, defined as the surface energy density.
Since the equilibrium state is characterized by a minimum potential energy, the liquid, in the absence of external forces, will take such a shape that for a given volume it has a minimum surface area, i.e. ball shape. Observing the smallest droplets suspended in the air, we can see that they really have the shape of balls, but somewhat distorted due to the action of gravity. Under conditions of weightlessness, a drop of any liquid (regardless of its size) has a spherical shape, which has been proven experimentally on spacecraft.
So, the condition for stable equilibrium of a liquid is a minimum of surface energy. This means that a liquid for a given volume should have the smallest surface area, i.e. the liquid tends to reduce the free surface area. In this case, the surface layer of the liquid can be likened to a stretched elastic film in which tension forces act.
Let us consider the surface of a liquid bounded by a closed contour. Under the influence of surface tension forces (they are directed tangentially to the surface of the liquid and perpendicular to the section of the contour on which they act), the surface of the liquid contracted and the contour in question moved. The forces acting from the selected area on the areas bordering it do work:
Δ A=fΔ lΔ x,
Where f=F/Δ l –surface tension force, acting per unit length of the liquid surface contour. It is clear that Δ lΔ x= Δ S, those.
Δ A=fΔS.
This work is done by reducing the surface energy, i.e.
Δ Α =Δ W.
From a comparison of expressions it is clear that
i.e. the surface tension coefficient σ is equal to the surface tension force per unit length of the contour delimiting the surface. The unit of surface tension is newton per meter (N/m) or joule per square meter (J/m2). Most liquids at a temperature of 300K have a surface tension of the order of 10 -2 –10 -1 N/m. Surface tension decreases with increasing temperature, as the average distances between liquid molecules increase.
Surface tension significantly depends on the impurities present in liquids. Substances , liquids that weaken the surface tension are called surfactants (surfactants). The most well-known surfactant in relation to water is soap. It greatly reduces its surface tension (from about 7.5 10 -2 up to 4.5·10 -2 N/m). Surfactants that reduce the surface tension of water are also alcohols, ethers, oil, etc.
There are substances (sugar, salt) that increase the surface tension of a liquid due to the fact that their molecules interact with liquid molecules more strongly than liquid molecules interact with each other.
In construction, surfactants are used to prepare solutions used in the processing of parts and structures operating in unfavorable atmospheric conditions (high humidity, elevated temperatures, exposure to solar radiation, etc.).
Wetting phenomenon
It is known from practice that a drop of water spreads on glass and takes the shape shown in Fig. 10.2, while mercury on the same surface turns into a slightly flattened drop. In the first case they say that the liquid wets hard surface, in the second - does not wet her. Wetting depends on the nature of the forces acting between the molecules of the surface layers of contacting media. For a wetting liquid, the force of attraction between the molecules of the liquid and the solid is greater than between the molecules of the liquid itself, and the liquid tends to increase
surface of contact with a solid body. For a non-wetting liquid, the force of attraction between the molecules of the liquid and the solid is less than between the molecules of the liquid, and the liquid tends to reduce the surface of its contact with the solid.
Three surface tension forces are applied to the line of contact of the three media (point 0 is its intersection with the plane of the drawing), which are directed tangentially inside the contact surface of the corresponding two media. These forces, per unit length of the line of contact, are equal to the corresponding surface tensions σ 12 , σ 13 , σ 23 . Corner θ between tangents to the surface of a liquid and a solid is called edge angle. The condition for equilibrium of a drop is that the sum of the projections of surface tension forces on the direction of the tangent to the surface of the solid body is equal to zero, i.e.
–σ 13 + σ 12 + σ 23 cos θ =0 (10.2)
cos θ =(σ 13 - σ 12)/σ 23 . (10.3)
It follows from the condition that the contact angle can be acute or obtuse depending on the values σ 13 and σ 12. If σ 13 >σ 12 then cos θ >0 and angle θ spicy, i.e. liquid wets a solid surface. If σ 13 <σ 12 then cos θ <0 и угол θ – dull, i.e. the liquid does not wet the solid surface.
The contact angle satisfies condition (10.3) if
(σ 13 - σ 12)/σ 23 ≤1.
If the condition is not met, then a drop of liquid at any value θ cannot be in balance. If σ 13 >σ 12 +σ 23, then the liquid spreads over the surface of the solid, covering it with a thin film (for example, kerosene on the surface of glass), – this occurs complete wetting(in this case θ =0).
If σ 12 >σ 13 +σ 23, then the liquid contracts into a spherical drop, in the limit having only one point of contact with it (for example, a drop of water on the surface of paraffin), - complete non-wetting(in this case θ =π).
Wetting and non-wetting are relative concepts, i.e. a liquid that wets one solid surface does not wet another. For example, water wets glass, but does not wet paraffin; Mercury does not wet glass, but it does wet clean metal surfaces.
The phenomena of wetting and non-wetting are of great importance in technology. For example, in the method of flotation beneficiation of ore (separation of ore from waste rock), it, finely crushed, is shaken in a liquid that wets the waste rock and does not wet the ore. Air is blown through this mixture and then it settles. In this case, rock particles moistened with liquid sink to the bottom, and grains of minerals “stick” to air bubbles and float to the surface of the liquid. When machining metals, they are moistened with special liquids, which facilitates and speeds up surface treatment.
In construction, the phenomenon of wetting is important for the preparation of liquid mixtures (putty, putty, mortars for bricklaying and concrete preparation). It is necessary that these liquid mixtures well wet the surfaces of the building structures to which they are applied. When selecting mixture components, not only the contact angles for mixture-surface pairs are taken into account, but also the surface-active properties of the liquid components.
“We came across such a phenomenon as a drop of water (in the articles “A drop of water - as it is” and “How much does a drop of water weigh”). Surface tension is responsible for the spherical shape of water. Let's try to talk about water filters today, surface tension and health. Let's see if there is any important (or useful) relationship here. And at the same time we’ll watch a video of water in zero gravity.
Water surface tension and health rarely go together. Usually there are “minerals and health”, “living and dead water”, “and”, “oxidation-reduction potential and health” and so on. Which in our opinion is a bit strange :)
There is an opinion: reduced surface tension of water has a worse (better) effect on humans. And the reason is water filters. Because they change it.
Tension is the application of force to something in different directions. For example, ten people pull a sheet in different directions. The tension of the sheet increases. You can even try to jump from a height onto a sheet and not hit yourself too hard :)
Surface tension of water - forces pull the surface in different directions.
It turns out that the surface of the water is stretched? How is it stretched, what, so to speak, “pulls the sheet”? Due to the structure of the water molecule. As you remember, a water molecule has positive and negative poles. Which form hydrogen bonds with each other.
In the volume of liquid, molecules are attracted from everywhere, the forces of attraction are balanced. And on the surface, the tension comes only from “bottom”. The forces are not balanced, the surface pulls on itself. And when gravity does not interfere with it (for example, in zero gravity), this force achieves its goal, water in zero gravity turns into a ball.
Otherwise: the molecules in the boundary layer, unlike the molecules in its depth, are only half surrounded. Hydrogen bonds pull them inward and tighten the surface. It would be approximately the same if our 10 people wrapped themselves in a sheet and pulled it in with all their might. They would form something like a ball. But between people there are voids where a sheet can fit. But water has no voids. So we get the perfect ball :)
If we dig really deep: if a molecule moves from the surface into the liquid, the forces of intermolecular interaction will do positive work. On the contrary, in order to pull a certain number of molecules from the depths of the liquid to the surface (that is, to increase the surface area of the liquid), it is necessary to expend positive work of external forces, proportional to the change in surface area. So, the force of surface tension is equal to the force that must be applied to increase the surface area per unit area. For reference: the surface tension of water is 0.07286 N/m.
Examples of surface tension from Wikipedia:
Could they have anything to do with surface tension?
Let's go all the way through the water.
At what stages does something happen to water that changes its ability to hold onto itself? That is, it changes surface tension? If this happens, it is at the stage of reverse osmosis, because the water is squeezed through very small fibers and in some way swirls.
Approximately the same thing happens during boiling (also water purification) - the volume of water is split into smaller, relatively stationary parts. By the way, the result is temperature-activated water. Which, according to a number of researchers, has a surface tension less than that of the original water.
Unfortunately, we did not find accurate data on how much surface tension decreases during boiling or purification by reverse osmosis.
Another example is electromagnetic water treatment. Here the decrease in surface tension is confirmed by an interesting experiment. Thus, plants that are watered with brackish water do not grow well. The reason is that it is difficult for them to draw in water with salts; salts do not release water into the plant well. However, brackish water after electromagnetic treatment passes more easily into the plants, and they are not so strongly suppressed.
However, there are no numerical data or experiments here either.
Now back to the main question:
Again, there is no experimental data. But it can be assumed theoretically, based on our knowledge of the surface tension of water.
So, the lower the surface tension of water, the better it is absorbed into cells (since it does not resist or interfere with surface tension). Consequently, metabolic products and other harmful substances will be removed from the cells faster. In general, the body will be more healthy than one in which metabolic products and toxic substances are eliminated more slowly.
So the conclusion is simple:
Based on materials from http://voda.blox.ua/
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 in part 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 liquids are of great importance for life: blood, sea water, etc. Liquids can act as solvents.
Physical properties of 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). Liquids are extremely difficult to compress mechanically because, unlike gases, there is very little free space between the molecules. 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 a 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
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) find 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 determining the shape of the liquid surface becomes extremely difficult.
A 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 .
