Surface tension, the desire of a substance (liquid or solid phase) to reduce its excess potential energy at the interface with another phase (surface energy). Defined as the work spent on creating a unit of interface area (dimension J/m 2). According to another definition, surface tension- force per unit length of the contour limiting the phase interface (dimension N/m); this force acts tangentially to the surface and prevents its spontaneous increase.
Surface tension- the main thermodynamic characteristic of the surface layer of a liquid at the boundary with the gas phase or other liquid. Surface tension of various liquids at the boundary with their own vapor varies widely: from units for liquefied low-boiling gases to several thousand mN/m for molten refractory substances. Surface tension depends on temperature. For many one-component non-associated liquids (water, molten salts, liquid metals) far from the critical temperature, the linear dependence holds well:
where s and s 0 are surface tension at temperatures T And T 0 accordingly, α≈0.1 mN/(m K) - temperature coefficient surface tension. The main method of regulation surface tension consists in the use of surfactants (surfactants).
Surface tension is included in many equations of physics, physical and colloid chemistry, electrochemistry.
It defines the following quantities:
1. capillary pressure, where r 1 and r 2 - main radii of curvature of the surface, and saturated vapor pressure r over a curved liquid surface: , where r- radius of curvature of the surface, R- gas constant, Vn- molar volume of liquid, p 0 - pressure above a flat surface (Lapplace and Kelvin's laws, see Capillary phenomena).
2. Contact angle θ in contact of a liquid with the surface of a solid: cos, where is the specific free surface energy of the solid at the interface with gas and liquid, - surface tension liquids (Young's law, see Wetting).
3. Surfactant adsorption 
where μ is the chemical potential of the adsorbed substance (Gibbs equation, see Adsorption). For dilute solutions where With- molar concentration of surfactant.
4. State of the surfactant adsorption layer on the liquid surface: (p s + a/A 2)·( A- b)=k T, where p s=(s 0 -s) - two-dimensional pressure, s 0 and s - respectively surface tension pure liquid and the same liquid in the presence of an adsorption layer, A- constant (analogous to the van der Waals constant), A- surface layer area per adsorbed molecule, b- area occupied by 1 molecule of liquid, k- Boltzmann constant (Frumkin-Volmer equation, see Surface activity).
5. Electrocapillary effect: - d s/ d f = r s, where r s is the surface charge density, f is the electrode potential (Lipman equation, see Electrocapillary phenomena).
6. The work of formation of the critical nucleus of a new phase W c. For example, during homogeneous condensation of steam at pressure, where p 0 - vapor pressure over a flat liquid surface (Gibbs equation, see Origin of a new phase).
7. Length l of capillary waves on the surface of the liquid: , where ρ is the density of the liquid, τ is the oscillation period, g- acceleration of gravity.
8. Elasticity of liquid films with a surfactant layer: elastic modulus, where s- film area (Gibbs equation, see Thin films).
Surface tension measured for many pure substances and mixtures (solutions, melts) over a wide range of temperatures and compositions. Because the surface tension is very sensitive to the presence of impurities; measurements using different methods do not always give the same values.
The main measurement methods are as follows:
1. rise of wetting liquids in capillaries. Lifting height, where 
- difference in density of liquid and displaced gas, ρ
- radius of the capillary. Determination accuracy surface tension increases with decreasing ratio ρ/α
(α
- capillary constant of the liquid).
2. Measuring the maximum pressure in a gas bubble (Rebinder method); The calculation is based on Laplace's equation. When a bubble is squeezed into a liquid through a calibrated capillary of radius r before the moment of separation, the pressure p m = 2σ/r
3. Method of weighing drops (stalagmometry): (Tate equation), where G- total weight n drops separated under the influence of gravity from the cut of a capillary tube with a radius r. To improve accuracy, the right-hand side is multiplied by a correction factor depending on r and the volume of the drop.
4. Plate balancing method (Wilhelmy method). When immersing a plate with a cross-sectional perimeter L into the wetting fluid is the weight of the plate, where G 0 - dry plate weight.
5. Ring tear-off method (Du Nouy method). To tear off a wire ring with a radius R force is required from the surface of the liquid
6. Sessile drop method. The profile of a drop on a non-wettable substrate is determined from the condition that the sum of hydrostatic and capillary pressures is constant. The differential equation of the drop profile is solved by numerical integration (Bashforth-Adams method). By measuring the geometric parameters of the drop profile using the appropriate tables, find surface tension.
7. Rotating drop method. A drop of liquid with density r 1 is placed in a tube with a heavier (density r 2) liquid. When the tube rotates with an angular velocity ω, the droplet is stretched along the axis, approximately taking the shape of a cylinder of radius r. Design equation: . The method is used to measure small surface tension at the interface of two liquids.
Surface tension is a determining factor in many technological processes: flotation, impregnation of porous materials, coating, detergent action, powder metallurgy, soldering, etc. The role is great surface tension in processes occurring in zero gravity.
Concept surface tension first introduced by J. Segner (1752). In the first half of the 19th century. based on the idea of surface tension the mathematical theory of capillary phenomena was developed (P. Laplace, S. Poisson, K. Gauss, A.Yu. Davidov). In the second half of the 19th century. J. Gibbs developed a thermodynamic theory of surface phenomena, in which the decisive role is played surface tension. In the 20th century regulation methods are being developed surface tension using surfactants and electrocapillary effects (I. Langmuir, P.A. Rebinder, A.H. Frumknn). Among modern topical problems is the development of molecular theory surface tension various liquids (including molten metals), the effect of surface curvature on surface tension.
Municipal educational institution
“Secondary school No. 24 with in-depth study of artistic and aesthetic subjects”
School scientific and practical conference
Abstract on the topic: “The role of surface tension forces in physics”
Completed:
Onokhin Dmitry Alekseevich, student of class 10 “A”, Municipal Educational Institution “Secondary School No. 24 with in-depth study of artistic and aesthetic subjects.”
Scientific adviser:
Volkhin Nikolay Ivanovich, physics teacher, Municipal Educational Institution “Secondary School No. 24 with in-depth study of artistic and aesthetic subjects.”
Arkhangelsk, 2009
Introduction
Bubble method
Wire method
Drop method
Test tube experiment
Experience "Plateau"
The role of surface tension in life
Conclusion
Bibliography
Applications
Introduction.
Forces such as gravity, elasticity and friction are obvious; we experience them directly every day. But in the world of everyday phenomena around us there is another force at work, to which we usually do not pay any attention. This power is relatively small, its actions never cause powerful effects. It has even recently been excluded from entrance examination programs for applicants to universities. Nevertheless, we cannot pour water into a glass, we cannot do anything at all with any liquid without bringing into action the forces that we will now discuss. These are surface tension forces.
Surface tension is a force caused by the mutual attraction of liquid molecules, directed tangentially to its surface.
The action of surface tension forces leads to the fact that the liquid in equilibrium has the minimum possible surface area. When a liquid comes into contact with other bodies, the liquid has a surface corresponding to the minimum of its surface energy.
The concept of “surface tension” was first introduced by J. Segner (1752).
We are so accustomed to the effects caused by surface tension that we do not notice them unless we have fun blowing soap bubbles. However, in nature and our lives they play a significant role.
There are quite a few different methods for determining surface tension: the drop method, the wire frame method, the ring method, the capillary wave method, the drop and bubble method, etc. The wire frame method and the ring method are used for rough measurements of surface tension.
1. Bubble method.
“Blow a soap bubble and look at it: you can study it all your life without ceasing to learn physics lessons from it,” wrote the great English physicist Lord Kelvin.
In particular, soap film is an excellent object for studying surface tension. Gravity plays virtually no role here, since soap films are extremely thin and their mass is completely negligible. Therefore, the main role is played by surface tension forces, due to which the shape of the film always turns out to be such that its area is the minimum possible under given conditions. Why does the film have to be soapy? It's all about the structure of the soap film. Soap is rich in so-called surfactants, the ends of long molecules of which have different relationships with water: one end willingly combines with a water molecule, the other is indifferent to water. Therefore, the soap film has a complex structure: the soap solution that forms it is, as it were, “reinforced” by a palisade of orderly located molecules of the surfactant that is part of the soap.
Let's return to soap bubbles. Probably everyone has had the opportunity not only to observe these amazingly beautiful creations, but also to let them go. They are spherical in shape and can float freely in the air for a long time. The pressure inside the bubble is greater than atmospheric pressure. The excess pressure is due to the fact that the soap film, trying to further reduce its surface, compresses the air inside the bubble, and the smaller its radius, the greater the excess pressure inside the bubble.
The free surface of the liquid tends to contract. This can be observed when the liquid is in the form of a thin film. An example of this condition would be soap films, like the ones you got from blowing soap bubbles as a child. Since the thickness of soap films is very small, the liquid in the film can be considered as two surface layers, without taking into account the influence of molecules located between the layers. Having received a soap bubble from the tube with which it was obtained. You will notice that the bubble is getting smaller. This indicates a reduction in the surface area of the soap film.
2. Wire frame method.
Take a wire quadrangular frame and connect its opposite vertices with a thin, loose thread. Having lowered the frame into soapy water, you will notice that the frame pulled out of the water is covered with a soap film. By piercing the film on one side of the thread, you will see that the thread will take the shape of an arc. Experience shows that the surface of the soap film shrinks.
The property of a liquid surface contracting can be interpreted as the existence of forces tending to contract this surface. These forces are called surface tension forces.
Using the experiment described below, you can find a way to measure surface tension forces. If you immerse a wire frame in soapy water and remove it from the water, you will easily notice that the upper part of the frame (all the way) is covered with soap film. If you pull the moving side of this frame down, the film will stretch, and if you release the moving side, the film will shrink.
The film formed on the frame is a thin layer of liquid and has two free surfaces.
Surface tension is measured by the force with which the surface layer acts per unit length of a particular contour on the free surface of a liquid tangential to this surface. In the International System of Units, this value is measured in newtons per meter (1 N/m).
3. Drop method.
The easiest way to grasp the nature of surface tension forces is to observe the formation of a drop at a poorly closed or faulty tap. While the drop is small, it does not come off: it is held by surface tension forces (the surface layer acts as a kind of bag). The larger the drop, the greater the role played by the potential energy of gravity. Look closely at how the drop gradually grows, a narrowing forms - a neck, and the drop breaks off.
The detachment of a drop occurs at the moment when its weight becomes equal to the resultant surface tension forces acting along the circumference of the drop neck. It doesn't take much imagination to imagine that the water is enclosed in an elastic bag, and this bag breaks when the weight exceeds its strength.
In reality, of course, there is nothing but water in the drop, but the surface layer of water itself behaves like a stretched elastic film.
Have you ever seen very large drops?
Under normal conditions there are no such drops. And this is no coincidence - drops of large diameter are unstable and break into small ones.
4. Test tube experiment.
The first glance at tea poured into a cup confirms the well-known position that the liquid does not have its own shape, but takes the shape of the vessel into which it is poured. Let's take a test tube filled with water. Let's turn it over onto a book or postcard and gradually pull out the postcard. Not a single drop was spilled, but the surface of the water swelled, forming a “slide.” All systems strive to reduce their energy. In the same way, the force of surface tension tends to reduce the surface area of a liquid to a minimum. Of all geometric shapes, a ball has the smallest surface area for a given volume. So the proper shape of a liquid is a sphere. A large amount of liquid cannot maintain a spherical shape; it changes under the influence of gravity. If the effect of gravity is eliminated, then under the influence of molecular forces the liquid will take the shape of a ball.
5. “Plateau” experience
If you take a mixture of water and alcohol and place a drop of liquid oil in it, then at some point the force of gravity will be balanced by the Archimedes force and the resulting oil ball will rest freely in the mixture. This ball is kept from scattering into molecules by the force of surface tension. The Belgian scientist J. Plato first thought of eliminating the effect of gravity when studying the surface tension of liquids in the middle of the last century; Plato used his method to study various phenomena.
6. The role of surface tension in life.
The role of surface tension in life is very diverse. Carefully place the needle on the surface of the water. The surface film will bend and prevent the needle from sinking. For the same reason, lightweight water striders can quickly glide across the surface of the water, like speed skaters on ice.
The deflection of the film will not allow water to pour out, carefully poured into a fairly thick sieve. So you can “carry water in a sieve.” This shows how difficult it is sometimes, even if you want, to say real nonsense. Fabric is the same sieve formed by interlacing threads. Surface tension makes it very difficult for water to seep through, so it doesn't get wet instantly.
In its desire to contract, the surface film would give the liquid a spherical shape if not for its heaviness. The smaller the droplet, the greater the role played by surface forces compared to volumetric forces (gravity). Therefore, small dew drops are close in shape to a ball. In free fall, a state of weightlessness occurs, and therefore raindrops are almost strictly spherical. A light rain would have soaked us through. Due to the refraction of the sun's rays, a rainbow appears in these drops. If the drops were not spherical, as the theory shows, there would be no rainbow.
The attractive forces between molecules on the surface of a liquid keep them from moving beyond it.
The molecules of a liquid experience forces of mutual attraction - in fact, it is precisely because of this that the liquid does not immediately evaporate. On the molecules inside a liquid, the attractive forces of other molecules act on all sides and therefore mutually balance each other. Molecules on the surface of a liquid have no neighbors on the outside, and the resulting force of attraction is directed inside the liquid. As a result, the entire surface of the water tends to contract under the influence of these forces. Taken together, 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 a kind of invisible, thin and elastic film on it.
One consequence of the surface tension effect is that to increase the surface area of a liquid—its stretching—mechanical work must be done to overcome the forces of surface tension. Consequently, if a liquid is left alone, it tends to take a shape in which its surface area is minimal. This shape, of course, is a sphere - which is why raindrops in flight take on an almost spherical shape (I say "almost" because in flight the drops are slightly stretched due to air resistance). For the same reason, drops of water on the body of a freshly waxed car collect in beads.
Surface tension forces are used in industry, particularly in the casting of spherical shapes such as shotgun pellets. Drops of molten metal are simply allowed to solidify in flight when dropped from a sufficient height, and they themselves solidify into the form of balls before falling into the receiving container.
We can give many examples of surface tension forces in action from our everyday life. Under the influence of wind, ripples are formed on the surface of oceans, seas and lakes, and these ripples are waves in which the upward force of internal water pressure is balanced by the downward force of surface tension. These two forces alternate, and ripples are formed on the water, just as a wave is formed due to alternate stretching and compression in the string of a musical instrument.
Whether the liquid will collect in “beads” or spread in an even layer over a solid surface depends on the ratio of the forces of intermolecular interaction in the liquid, causing surface tension, and the forces of attraction between the molecules of the liquid and the solid surface. In liquid water, for example, surface tension forces are caused by hydrogen bonds between molecules ( cm. Chemical bonds). The surface of the glass is wetted by water, since glass contains quite a lot of 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, hydrogen bonds will not form with the surface, and the water will gather into droplets under the influence of internal hydrogen bonds, which determine surface tension.
In the chemical industry, special wetting agents are often added to water - surfactants, - preventing water from collecting drops on any surface. They are added, for example, to liquid dishwasher detergents. Getting into the surface layer of water, the molecules of such reagents noticeably weaken the forces of surface tension, the water does not collect in drops and does not leave dirty specks on the surface after drying ( cm.
The molecules of the liquid are located so close to each other that the attractive forces between them are significant; these forces create surface tension acting in the plane of the free surface. Surface tension is most clearly demonstrated in experiments with liquid films. Some liquids, such as soapy water, can form thin films. If you lower a wire frame, one of whose sides is movable (Fig. 8.3), into a soap solution and then remove it, it will be covered with a soap film. The force generated by surface tension and applied to the crossbar causes the crossbar to rise upward. To keep the crossbar stationary, you need to hang a load from it
Let us select a unit segment on an arbitrary surface of the liquid (Fig. 8.4, a). A selected unit element of length is subjected to a force normal to it and tangential to the surface, which is called the coefficient of surface tension or simply surface tension. Surface tension is equal to the force acting per unit length and directed normal to the element of length and tangential to the surface of the liquid. The value is measured in dyn/cm and
Surface tension decreases with increasing temperature and is zero at the critical point. Impurities greatly affect the value
surface tension. So, for example, for pure water at room temperature dyne/cm, dissolving soap reduces this value to 45 dyne/cm, and dissolving table salt, on the contrary, leads to its increase.
Figure 8.4, b explains the origin of surface tension. Molecules are indicated by dots on a unit segment. The forces characterize the average interaction of selected molecules with other molecules of the liquid surface. Obviously, if there are X particles per unit length, then
Let's return to Figure 8.3. The surface tension forces acting on the movable crossbar are determined by the product:
where I is the length of the crossbar, and coefficient 2 takes into account the double surface of the film. If, at a constant temperature, the film is stretched by external forces so that the crossbar moves a distance (position in Fig. 8.3), then the work done by the surface tension forces will be equal to:
where is the change in the surface of the liquid (on both sides of the film). The resulting expression allows us to give a different definition
The coefficient of surface tension is numerically equal to the work required to form a unit area of a new surface at a constant temperature.
The molecules of the surface layer, in addition to forces acting along the surface, are affected by forces directed normal to the surface into the liquid (Fig. 8.5); the latter are the result of attraction from the molecules of the deep layers of the medium.
As the surface of a liquid increases, some of its molecules move from the depth to the surface, while work is done against the forces and the potential energy of the surface layer increases.
The first law of thermodynamics, taking into account the work of surface tension forces, can be written in the form
If the isothermal change in the state of a liquid consists only of a decrease in its surface area, then the work of external pressure can be neglected and written down.
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