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Introduction
Relevance of the topic: Knowledge of the natural sciences is necessary for people not only to explain the phenomena of nature, but also for use in practical activities. Showing interest in physics, I may not become a theoretical physicist, but I will be an engineer, a technician. The success of my work will be ensured not only by the ability to think, but also by the ability to do, and the topic I have chosen is not only relevant for study, it provides an opportunity for such successful activity. In the world around us, along with gravity and friction, there is another force that we pay little attention to. This force is comparatively small and never causes spectacular effects. However, we cannot pour water into a glass, we cannot do anything with any liquid at all, without setting in motion this force - the force of surface tension. It plays an important role in nature and technology, in the physiology of our organism and in the life of insects.
Field of study - Molecular physics
Subject of study liquid (water, soap solution, milk, vegetable oil.)
Target: the study of surface phenomena in liquids and the study of essential methods for determining the coefficient of surface tension at the "liquid - air" interface.
Tasks of this work:
The study of the fundamentals of molecular physics related to surface phenomena in liquids.
The study of the use of surface tension, its role in the reality around us.
Experimentally determine the coefficient of surface tension of a liquid by the method of droplet separation and wire frame tension.
Compare the received data with tabular values.
Research methods: theoretical collection of information, analysis, synthesis,
generalization; experimental- statement of a question; study design; data collection; analysis of results; conclusions on the experiment; publication of results.
In the theoretical part The work deals with the basic theoretical information from the field of molecular physics of the surface layer of a liquid.
In the experimental part results are given research work. The coefficients of surface tension of liquids (water, milk, vegetable oil, soap solution), and I found out how the surface tension of a liquid depends on temperature and the type of liquid.
2.Theoretical part 2.1. Interesting Facts about the shape of the liquid.
We tend to think that liquids have no shape of their own. This is not true. The natural shape of any liquid is a sphere. Usually, gravity prevents the liquid from taking this shape, and the liquid either spreads in a thin layer if poured without a vessel, or else takes the form of a vessel if poured into it.
The liquid (in the absence of gravity or in the case when it is balanced by the force of Archimedes) takes on a spherical shape, having a minimum surface with the same volume (see appendix fig. 1). Being inside another liquid of the same specific gravity, the liquid, according to the law of Archimedes, “loses” its weight: it seems to weigh nothing, gravity does not act on it - and then the liquid takes its natural, spherical shape. ..
It is known that Provence oil floats in water, but sinks in alcohol. Therefore, it is possible to prepare such a mixture of water and alcohol in which the oil does not sink and does not float. By injecting a little oil into this mixture with a syringe, you can do a strange thing : oil is collected in a large round drop, which does not float or sink, but hangs motionless (see appendix fig. 2).
2.2. Surface tension of a liquid.
Molecules of a substance in a liquid state are located almost close to each other. Unlike solid crystalline bodies, in which molecules form ordered structures throughout the volume of the crystal and can perform thermal vibrations around fixed centers, liquid molecules have greater freedom. Each molecule of a liquid, as well as in a solid body, is “clamped” on all sides by neighboring molecules and performs thermal vibrations around a certain equilibrium position. However, from time to time any molecule can move to a nearby vacancy. Such jumps in liquids occur quite often; therefore, the molecules are not tied to certain centers, as in crystals, and can move throughout the entire volume of the liquid. This explains the fluidity of liquids. Due to the strong interaction between closely spaced molecules, they can form local (unstable) ordered groups containing several molecules. This phenomenon is called short-range order (see app. fig. 3).
Liquid, unlike gases, does not fill the entire volume of the vessel into which it is poured. An interface is formed between the liquid and the gas (or vapor), which is in special conditions compared to the rest of the mass of the liquid. The molecules in the boundary layer of a liquid, in contrast to the molecules in its depth, are not surrounded by other molecules of the same liquid from all sides. The forces of intermolecular interaction acting on one of the molecules inside the liquid from the side of neighboring molecules are on average mutually compensated and inside the liquid the resulting force of attraction acting on the molecules from the side of neighboring molecules is equal to zero (see Appendix Fig. 4). The molecules of the surface layer of the liquid are attracted only by the molecules of the inner layers, and under the action of the resulting attractive force are drawn into the liquid. The number of molecules remains on the surface, at which the surface area of the liquid is minimal for a given volume.
|
A ext. =σS, |
The coefficient of proportionality σ is called the surface tension coefficient or simply surface tension (σ> 0) and is the main characteristic that depends on the nature of the media and their thermal state. A is work and it serves as a measure of the change in energy. This energy must be potential, since it is associated with the arrangement of molecules in the surface layer at a constant temperature and common property of such systems is a spontaneous change in the state of the system in the direction of reducing the stock of potential energy in order to bring the system into a state with the lowest potential energy. [7].
The direction of the processes to reduce the potential energy of the liquid determines the property of spontaneous reduction of the free surface of the liquid to a possible minimum value. The desire of liquids to contract their surface, to make it minimal, can be considered as a certain force acting along the surface. The presence of surface tension forces makes the liquid surface look like an elastic stretched film, with the only difference that the elastic forces in the film depend on its surface area (i.e., on how the film is deformed), and the surface tension forces do not depend on the surface area liquids. Some liquids, such as soapy water, have the ability to form thin films. Well-known soap bubbles have a regular spherical shape (see photo No. 5) - this also manifests the action of surface tension forces. If a wire frame is lowered into the soapy solution, one of the sides of which is movable, then the whole of it will be covered with a film of liquid (see appendix fig. 5). As a result, surface tension can define as the force that tightens the surface and related to the unit length.
, is the coefficient of surface tension. In the system of units of measurement - SI, the surface tension coefficient is measured in joules per square meter (J / m 2) or in newtons per meter (1N / m \u003d J / m 2). The surface tension coefficient is the most important quantity that characterizes the physical and chemical properties of a liquid, is used in technological processes and is taken into account in explaining many phenomena: wetting, boiling, flotation, cavitation. F - the surface tension force is directed tangentially to the liquid surface, perpendicular to the section of the contour on which it acts and is proportional to the length of this section.
The following simple experiments further clarify the essence of surface tension forces. A ring of wire with a freely suspended (not stretched) thread attached to it at two points (see appendix fig. 6) is immersed in a soapy solution. In this case, the ring is tightened by a thin film of liquid, and the thread is in equilibrium, taking on a random shape. If now the film is destroyed on one side of the thread by touching the film with a heated needle, then the thread will stretch, taking the form of an arc of a circle. The tension in the thread was due to the force of surface tension from the side of the shrinking film, the force applied to the thread, which in this case is the dividing line. This force, of course, is perpendicular to the thread at all points. This force acted on the thread and. until the destruction of the film, but at the same time, the same forces acted on it from both sides. After the breakthrough of one part of the film, the other got the opportunity to reduce its area and, as the shape on the stretching thread shows, this area became minimal.
2.3. The phenomenon of wetting and non-wetting
The behavior of a liquid at the boundary with a solid is closely related to surface phenomena. At the boundary of contact with a solid body, the surface of the liquid can rise above the horizontal surface of the liquid or fall below the horizontal surface. . A liquid that spreads over the surface of a solid is called wetting, and the liquid, which contracts into a drop, - non-wetting(See appendix fig. 7). The difference in contact angles in the phenomena of wetting and non-wetting is explained by the correspondence between the forces of attraction between the molecules of a solid and liquids and the forces of intermolecular attraction in liquids .. If the forces of attraction between the molecules of a solid and a liquid> F attraction between molecules liquid, the liquid will be wetting. If the liquid's molecular attraction (inside) > F of the attraction between the molecules of the solid and the liquid, then the liquid will be nonwetting.
2.4. Capillary phenomena
"Сapillaris" - hair (translated from Latin) - narrow cylindrical tubes with a diameter of about a millimeter or less are called capillaries. That is, capillary phenomena are phenomena in thin tubes (capillaries). In life, we often deal with bodies pierced by many small channels (paper, yarn, leather, various Construction Materials, soil, tree). Coming into contact with water or other liquids, such bodies very often absorb them. This is the basis of the action of a towel when wiping hands, the action of a wick in a kerosene lamp.
Very often, the liquid, being absorbed into the porous body, rises up. Capillarity - the phenomenon of the rise or fall of liquid in the capillaries [ 13] .In the case of a wetting liquid (A) (see appendix fig. 8), the attraction forces Fl-t between the molecules of the liquid and the solid (capillary walls) exceed the interaction forces Fl between the molecules of the liquid, therefore the liquid is drawn into the capillary, and the liquid rises into capillary occurs until the resulting force Fv, acting on the liquid upwards, is balanced by the gravity mg of the liquid column with height h: (see appendix. Fig. 8 - B) Fv = mg. A liquid that does not wet the walls of the capillaries (B) descends in it at a distance h (see appendix fig. 8). According to Newton's third law, the force Fv acting on the liquid is equal to the surface tension force Fs. acting on the wall along the line of contact with the liquid: Fv = Fs [ 8]
3. Practical work
3.1 Methods for determining surface tension. In the study of surface phenomena at the gas-liquid interface, the most commonly used method is based on measuring the surface tension of this interface, which, despite its simplicity, allows obtaining sufficiently reliable data. [ 15] . Existing methods for determining surface tension are divided into three groups: static, semi-static and dynamic.
static methods the surface tension of practically immobile surfaces formed long before the start of measurements and therefore in equilibrium with the liquid volume is determined. These methods include the capillary rise method and the sessile or hanging drop (bubble) method.
Dynamic Methods are based on the fact that some types of mechanical effects on a liquid are accompanied by periodic stretching and compression of its surface, which are affected by surface tension. These methods determine the non-equilibrium value . Dynamic methods include methods of capillary waves and an oscillating jet.
semi-static called methods for determining the surface tension of the phase boundary that arises and is periodically updated in the measurement process (the method of maximum bubble pressure and the stalagmometric method), as well as methods for tearing off the ring and retracting the plate. These methods make it possible to determine the equilibrium value of surface tension if the measurements are made under such conditions that the time during which the formation of the interface occurs is much longer than the time for equilibrium in the system to be established.
In this work, to determine the surface tension coefficient of a liquid, I use a semi-static method: the droplet separation method(stalagmometric ) and the wire frame method.(plate retraction).
3.2 Droplet separation method . Observing the detachment of a liquid drop from a vertical narrow tube, one can determine the coefficient of surface tension of the liquid . Consider how a drop of liquid grows (see appendix fig. 9). The size of the drop gradually increases, but it comes off only when it reaches a certain size (see appendix fig. 9, a). While the drop is not large enough, the surface forces the tensions are sufficient to resist gravity and prevent lift-off. Before separation, a narrowing is formed - the neck of the drop (see appendix fig. 9 b). As long as the drop is held at the end of the capillary tube, the following forces will act on it: (1) - gravity, directed vertically downward and tending to tear off the drop; surface tension forces directed tangentially to the liquid surface and perpendicular to the contour l neck drops. (see appendix fig. 10). These forces tend to hold the drop. The resulting surface tension force is directed upwards and is equal to (2), where l- the length of the contour of the neck of the drop. When the force of gravity becomes equal to the force of surface tension, the drop will detach: (3). For force modules: taking into account (2) and (3), we write: [ 11]
Since the length of the contour of the neck of the drop is d is the diameter of the neck of the drop. Therefore, whence (4), where m- mass of one drop . Formula (4) is a working calculation formula.
The described method of experimental determination of the surface tension coefficient of a liquid gives good results, despite the fact that in reality the drop does not detach exactly as described above. In reality, the drop does not detach along the neck circumference. At the moment when the drop size reaches the value determined by equation (3), the neck begins to narrow rapidly (see appendix fig. 9 b), and it is accompanied by another small drop (see appendix fig. 9 c). In addition, in calculations, the diameter of the drop neck at the moment of detachment can be taken equal to the inner diameter of the tube, since the tube is quite narrow and its diameter is comparable to the diameter of the drop neck. For calculation according to formula (4), it is necessary to monitor the purity of the capillary and water during the measurement. In addition, the surface tension coefficient depends on the temperature of the liquid under study: with increasing temperature, it decreases. At room temperature 20 С, the tabular value of the coefficient for water table = 72,510 3 N/m. [ 9][ 2] .
Equipment: a vessel with water, an empty glass, a micrometer, a balance with a weight, a thin glass tube (buret).
Progress of work: 1. Assemble the installation. Measure the temperature in the room and d.
2. Determine the mass of an empty glass m 1 and drip 30 drops of pure water. (see attached photo1).
3. Determine - m 2 - the mass of a glass with water droplets. (see attached photo 2).
4. Find the mass of one drop of water
6. Carry out the experiment 3 times using 40 and 50 drops.
7. Find δ cf. = = [ 11]
│Δδ│ 1 =│δav.-δ 1 │ │Δδ│ 2 =│δav.-δ 2 │Δδ│ 3 =│δav.-δ 3
Δδ cf. = and E = 100%
Enter the data in the table (see appendix table No. 1). 9. Compare the calculated value of the coefficient of surface tension of water with the table and determine the absolute and relative errors using the formulas: and Conclusion : in a research work, I determined the coefficient of surface tension of water at a temperature of 19 0 C using the droplet separation method and received δ = (74.33 + 0.89) mN/m, E = 1.2%. Comparing with the table value, we get absolute errorΔδ = 1.38 mN/m and relative error E = 1.9%.
Analyzing the results obtained, one can see the difference in the measurement error (the value of the physical quantity obtained experimentally and so close to the true value). Measurement error is a characteristic of measurement accuracy, and we have determined it in different ways). This can be explained:
The number of drops as a result of counting is an exact number, and if we take π = 3.14 and g = 9.81 m / s 2, then the relative errors of these quantities, as well as for the drop mass, will be too small compared to the relative error measuring the diameter of the tube channel).
The measurements were indirect (by formula);
The studies were carried out at liquid temperature t = 19 0 С;
Instrumental error (micrometer, scales);
experimenter action.
3.3 Wire frame method
In liquids, the average distances between molecules are much smaller than in gases. Therefore, interaction forces play an essential role in liquids. Excessive intermolecular bonds appear in the surface layer of the liquid: the molecules in this layer experience an inwardly directed force of attraction from the molecules of the rest of the liquid. The surface tension force is directed tangentially to the surface of the liquid, so it does not act on the walls of the vessel and the body immersed in the liquid. Consider a wire rectangular frame of length l touching the surface of the liquid (see app. fig. 11). When the frame is raised above the liquid surface, a film is formed between the frame and the surface, which pulls down. The force holding the frame is: (1) l- the length of the wire frame, σ - the coefficient of surface tension of the liquid. Knowing this force with the help of a dynamometer, we will find the coefficient of surface tension of any liquid σ = F / 2l (2).
Equipment: dynamometer, rectangular wire frame, vessel, ruler, test liquid.
Progress
1. Measure the length of the wireframe l
2. Pour the test liquid into the beaker, carefully lower the wire frame until it comes into contact with the liquid, set the dynamometer pointer to 0.
Note: make sure that the frame is in contact with the liquid evenly around its entire perimeter.
4. Gently lifting the dynamometer, raise the frame until it is separated from the liquid. Note and record the dynamometer readings in the table F at the moment of separation of the frame from the liquid. (see attached photo 3)
5. Conduct experiments for various liquids and calculate the value of the surface tension coefficient using formula (2).
6. Record the data in a table (see appendix table No. 2).
7. The obtained values of the surface tension of the studied liquids are compared with the tabular value at t = 20 0 С.
8. Determine experimentally the dependence of the coefficient of surface tension of water on the temperature of the liquid - t. Record the data in a table (see appendix table No. 3).
9. Present the results of the study in the form of graphs.
10. Determine the absolute and relative measurement errors.
Conclusion: Using the wire frame method, I determined the coefficient of surface tension of liquids. According to the results presented in the table and on the graph, it follows that the surface tension coefficient depends on the type of liquid and its temperature. The higher the temperature, the lower the surface tension. The results of the errors are presented in table No. 4.
Manifestations of surface tension forces
The concept of surface tension was first introduced by J. Segner (1752). In the 1st half of the 19th century. based on the concept of surface tension was developed mathematical theory capillary phenomena (P. Laplace, S. Poisson, K. Gauss, A.Yu. Davidov). In the 2nd half of the 19th century. J. Gibbs developed the thermodynamic theory of surface phenomena, in which surface tension plays a decisive role. In the 20th century methods are being developed for regulating surface tension with the help of surfactants and electrocapillary effects (I. Langmuir, P. A. Rehbinder, A. H. Frumkin).
Among the current topical problems is the development of the molecular theory of surface tension of various liquids, including molten metals. . The surface tension of the metal and the molten electrolyte should be taken into account for the following reasons. When molten metal is released, it is necessary that it wets the cathode well and is obtained in the form of a compact layer. The metal that does not wet the cathode forms small drops, which increases the surface of its contact with the electrolyte and its solubility. During the precipitation of a solid metal, its wettability with an electrolyte promotes the formation of a protective film and prevents oxidation. Oxygen reduces the surface tension of the metal , and therefore, with an increase in its content in an argon-based mixture, the critical current decreases. . Nitrogen increases the surface tension of the metal; therefore, with an increase in the nitrogen content in argon at the same current strength, the droplet size increases. When welding in a nitrogen atmosphere, large-drop metal transfer occurs with intense spatter.
The methods and technical means of collecting oil products from the surface of the water are considered. Surface tension is a determining factor in many technological processes: flotation, impregnation of porous materials, coating, washing action, powder metallurgy, soldering. The role of surface tension in the processes occurring in weightlessness is great [ 3] .
Surface tension forces play an essential role in natural phenomena, biology, medicine, in various modern technologies, polygraphy, technology, in the physiology of our body.
Without these powers, we would not be able to write with ink. An ordinary pen would not scoop up ink from an inkwell, but an automatic one would immediately put a large inkblot, emptying its entire reservoir (see appendix fig. 12). .
Carefully place the needle on the surface of the water (see photo 4 attached). The surface film will flex and prevent the needle from sinking. For the same reason, light water striders can quickly glide over the surface of the water (see appendix fig. 13), like skaters on ice.
In medicine, the dynamic and equilibrium surface tension of venous blood serum is measured, which can be used to diagnose a disease and control the treatment (see Appendix Fig. 14). It has been found that water with low surface tension is biologically more accessible. It enters into molecular interactions more easily, then the cells will not have to spend energy to overcome surface tension.
The volume of printing on polymer films is constantly growing due to the rapid development of the packaging industry, high demand for consumer goods in colorful polymer packaging. An important condition for the competent implementation of such technologies is precise definition conditions of their application in printing processes.
In printing, processing plastic before printing is necessary so that the paint falls on the material. The reason is the surface tension of the material. The result is determined by how the liquid wets the surface of the product. Wetting is considered optimal when a drop of liquid remains where it was applied. In other cases, the liquid may roll into a drop, or, conversely, spread. Both cases equally lead to negative results during ink transfer.
Conclusion At the beginning of my work, I set the goal of studying surface phenomena in liquids and studying essential methods for determining the coefficient of surface tension of a liquid at the “liquid-air” boundary. During my research work, I learned:
1 ) about essential experimental methods for measuring the coefficient of surface tension of a liquid;
2 ) using the method of detachment of drops and a wire frame, determined the coefficient of surface tension of the liquid at the “liquid-air” boundary; 3 ) surface tension forces are small and appear at small volumes of liquid;
4 ) the surface energy of a liquid depends on the type of liquid, on the medium with which it borders, and also on the temperature of the liquid.
5 ) as the temperature increases, the internal energy increases and, naturally, the stress in the boundary layer of the liquid decreases and, consequently, the surface tension forces decrease.
6) soapy water, has the ability to form thin films. The liquid film turns into an elastic surface, which tends to minimize its area, and, consequently, to minimize the tension energy per unit area (see attached photo No. 6); (this is the shape of the ball).
7 ) surface tension forces exist, play an important role in nature, technology and human life. It would be impossible to soap your hands: the foam would not form. The water regime of the soil would be disturbed, which would be disastrous for plants. would suffer important features our body. The manifestations of surface tension forces are so diverse.
6. Literature
1. Detlaf, A.A., Yavorsky B.M. Physics course. M.: graduate School, 2002. 718 p.
2. Kasyanov V.A. Physics. Grade 10: Textbook for general images. institutions. - 6th ed., stereotype. - M.: Bustard, 2008 .
3. Kuhling, H. Handbook of Physics. - M., 1982. - 520s
4. Landsberga G.S. Elementary textbook of physics. Volume 1: Mechanics. Heat. Molecular physics. - M., Book on Demand, 2012. - 618 p.
5. Myakishev G.Ya., Sinyakov A.Z. "Physics: Molecular Physics and Thermodynamics". Textbook for grade 10 profile level. Moscow, 2012.
6. Matveev, A.N. Molecular physics. M.: Higher school, 1987. 360s.
7. Pinsky A.A. Kabardin O.F. Textbook on physics 10 cells. Profile level. 13th ed. - M.: Enlightenment, 2011
8. Perelman Ya.I. Entertaining physics. In two books. Book. 1. -20th ed., stereotype. - M.: Nauka, 1979
9. Trofimova, T.I. Physics course. - M: Academy, 2007. - 560 p.
10. https://ru.wikipedia.org/wiki/Surface_tension
11. Formulas http://studyport.ru/referaty/tochnyje-nauki/3948
12. Properties of liquids. Surface Tension http://www.physics.ru/courses/op25part1/content/chapter3/section/paragraph5/theory.html#.Vo9nifmLTcc
13. Wetting, capillary http://phys-bsu.narod.ru/lib/mkt/mkt/207.htm
14. Wire frame method http://allrefs.net/c12/3smth/p5/
15. Surface tension of a liquid http://physflash.narod.ru/Search/mechanics/24.htm
16. Interesting facts about the shape of the liquid http://www.afizika.ru/svojstvazhidkostejgazov/95-estestvennayaformazidkosti
17. http://www.ngpedia.ru/id181006p1.html
Application
Picture 1. [ 6] Cross section of a spherical liquid drop
Figure 2.swimming oil drop
Figure 3 [ 2] An example of the short-range order of liquid molecules and the long-range order of molecules of a crystalline substance: 1 - water; 2 - ice.
Figure 4 Molecular mechanism of surface tension
Figure 5 [ 10] Movable side of a wire frame in equilibrium under the action of an external force and the resulting surface tension force
Figure 6 [ 2][ 0] Surface tension of soap film
Figure 7 [ 14] Terms equilibrium at the liquid-solid interface
Q90° - non-wetting
Q - Contact angle
Q = 0 ° - perfect non-wetting
Q=180 ° - perfect wetting
Figure 8capillaries [ 13]
A B C.
Figure 9Formation of a liquid drop [ 10]
Figure 10. [ 12]
Figure 11.
wire frame [ 14]
Figure 12. Surface tension forces play a significant role in natural phenomena, biology, medicine, various modern technologies, printing, and engineering.
Figure 13.
Figure 14. Surface tension forces play an essential role in the physiology of our body.
Table No. 1 Coefficient of surface tension of water at the border with air.
|
Δ δavg. (mN/m) |
|||||||||
Table No. 2 Coefficient of surface tension of liquids at the border with air
Table No. 3 Coefficient of surface tension of water at the border with air at different temperatures
Table No. 4 Absolute and relative error in measuring the surface tension coefficient of various types of liquids
Schedule #1. Dependence of the coefficient of surface tension of a liquid on the type of liquid, and comparison of the results of the experiment with the table.
Graph No. 2. The dependence of the coefficient of surface tension of water on temperature
Photo #1
Photo #2
Photo #3
Photo #4
Photo #5
Photo #6
A liquid is an aggregate state of matter, 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 interconnected by the forces of intermolecular interaction. In this case, the average energy of the thermal motion of gas molecules is much greater than the average potential energy due to the forces of attraction between them, so the gas molecules scatter in different directions, and the gas occupies the entire volume provided to it.
In solid and liquid bodies, 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, so 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 randomly, so there is no pattern in their relative position. For solids, the so-called long range order in the arrangement of particles, i.e. their orderly arrangement, repeating over long distances. In liquids, the so-called short range order in the arrangement of particles, i.e. their ordered arrangement, repeating at distances comparable to interatomic ones.
The theory of fluid has not been fully developed to date. 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, which is at a distance of the order of the interatomic distance from the initial one. Thus, the molecules of a liquid move quite slowly throughout the mass of the liquid, and diffusion occurs much more slowly than in gases. With an increase in the temperature of the liquid, the frequency of the oscillatory motion increases sharply, the mobility of the molecules increases, which is the reason for the decrease in the viscosity of the liquid.
Attractive forces act on each molecule of the liquid from the side of the surrounding molecules, rapidly decreasing 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 molecular action radius r , and a sphere of radius r-sphere of molecular action.
Select a molecule inside the liquid BUT and draw a sphere of radius around it r(fig.10.1). It is sufficient, according to the definition, to take into account the action on a given molecule of only those molecules that are inside the sphere
Fig.10.1. molecular action. The forces with which these molecules act on the molecule BUT, are directed in different directions and, on average, are compensated, so the resulting force acting on a molecule inside the liquid from other molecules is equal to zero. The situation is different if the molecule, for example the molecule AT, located at a distance from the surface 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 inside the liquid. Thus, the resulting forces of all the molecules of the surface layer exert pressure on the liquid, called molecular(or internal). Molecular pressure does not act on a body placed in a liquid, since it is due to forces acting only between the molecules of the liquid itself.
The total energy of liquid particles is the sum of the energy of their chaotic thermal motion and the potential energy due to the forces of intermolecular interaction. To move a molecule from the depth of the liquid to the surface layer, work must be expended. This work is done at the expense of the kinetic energy of the molecules and goes to increase their potential energy. Therefore, the molecules of the surface layer of the liquid have a greater potential energy than the molecules inside the liquid. This extra energy possessed by molecules in the surface layer of a liquid is called surface energy, is proportional to the layer area Δ S:
Δ W=σ Δ S,(10.1)
where σ – coefficient of surface tension, defined as the surface energy density.
Because equilibrium state is characterized by a minimum of potential energy, then the liquid in the absence of external forces will take such a shape that for a given volume it has a minimum surface, 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 the forces 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 the liquid for a given volume should have the smallest surface area, i.e. 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.
Consider the surface of a liquid bounded by a closed contour. Under the action 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 considered contour moved. The forces acting from the selected area to the adjacent areas do the work:
Δ A=fΔ lΔ x,
where f=F/Δ l -surface tension force, acting per unit length of the liquid surface contour. It can be seen that Δ lΔ x= Δ S, those.
Δ A=f∆S.
This work is done by reducing the surface energy, i.e.
Δ Α =Δ W.
From the comparison of the expressions, it can be seen that
i.e., the surface tension coefficient σ is equal to the surface tension force per unit length of the contour that bounds the surface. The unit of surface tension is newton per meter (N/m) or joule per square meter(J / m 2). 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 essentially depends on the impurities present in liquids. Substances , liquids that reduce surface tension are called surface-active substances (surfactants). Soap is the best known surfactant for water. It greatly reduces its surface tension (from about 7.5 10 -2 up to 4.5 10 -2 N/m). Surfactants that lower 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 the molecules of the liquid more strongly than the molecules of the liquid interact with each other.
In construction, surfactants are used to prepare solutions used in the processing of parts and structures operating in adverse 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 form shown in Fig. 10.2, while mercury on the same surface turns into a somewhat flattened drop. In the first case, it is said 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 the media in contact. For a wetting liquid, the attractive forces between the molecules of the liquid and the solid are greater than between the molecules of the liquid itself, and the liquid tends to increase
surface of contact with a solid body. For a nonwetting liquid, the forces of attraction between the molecules of the liquid and the solid are less than those 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 three media (point 0 is its intersection with the plane of the drawing), which are directed tangentially into 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 the tangents to the surface of a liquid and a solid is called edge angle. The condition for the equilibrium of a drop is the equality to zero of the sum of the projections of the surface tension forces on the direction of the tangent to the surface of the solid, 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 a σ 13 >σ 12 , then cos θ >0 and angle θ sharp, i.e. liquid wets a solid surface. If a σ 13 <σ 12 , then cos θ <0 и угол θ – blunt, i.e., the liquid does not wet the hard surface.
The contact angle satisfies condition (10.3) if
(σ 13 - σ 12)/σ 23 ≤1.
If the condition is not met, then the drop of liquid for any values θ cannot be in balance. If a 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), - we have complete wetting(in this case θ =0).
If a σ 12 >σ 13 +σ 23 , then the liquid shrinks 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), - we have 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 enrichment of ore (separation of ore from waste rock), finely crushed ore 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. At the same time, rock particles wetted 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 wetted with special liquids, which facilitates and accelerates surface treatment.
In construction, the phenomenon of wetting is important for the preparation of liquid mixtures (putties, putties, mortars for laying bricks and preparing concrete). It is necessary that these liquid mixtures wet well 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 liquid components.
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More about surface tension
General information
Surface tension is the property of a liquid to resist the force that acts on it. Compared to other liquids, surface tension water one of the highest. This property of water is due to its molecular structure, due to which the bonds between molecules are much stronger than those of other liquids.
Surface tension depends on the liquid itself and its molecular structure, but also on what material this liquid is in contact with. When it comes to surface tension in the animal kingdom and in many other examples below, either the water-air system or aqueous solutions of various substances are usually considered, since these are the most common systems that occur in nature.
Surface Tension Calculations
To increase the surface area of water, that is, to stretch this surface, it is necessary to perform mechanical work to overcome the forces of surface tension. If no other external forces are applied to the fluid, it tends to assume a shape in which the surface area of this fluid is minimal. As we will see below, the most optimal shape is a sphere. In zero gravity, the liquid really takes the form of a ball. The potential energy of surface tension is found by the formula:
Ε surf = σ S
Here σ is the coefficient of surface tension, and S - total area liquids. This formula can also be expressed as:
σ = surf/S
As can be seen from this formula, the surface tension coefficient σ is expressed in joules per square meter (J/m² = N/m). That is, the coefficient of surface tension at a constant temperature of the liquid is equal to the work that must be done to increase the surface of the liquid per unit area. Recall that a joule is equal to a newton multiplied by a meter, and we get another unit for measuring surface tension - newton per meter (N / m).
About terminology
Surface tension does not only occur in air-liquid systems. Most often, when people talk about force in length, they mean surface tension in liquid-gas systems. Sometimes we are talking about liquid-liquid systems, which also have surface tension. An example of a liquid-liquid system in which we can talk about surface tension is lava lamps. When the lamp is turned off, the paraffin in it is in a solid state, but when it is turned on, it heats up, melts, and rises, since in the heated state the paraffin is lighter than the liquid in which it is located, and in the cold state it is heavier.
Surface Tension Mechanism
Each molecule in a liquid acts on the surrounding molecules with a certain force. Accordingly, a number of forces from different directions from the sides of other molecules also act on each molecule. The action of these forces between molecules is shown in the illustration. These forces arise due to the fact that the hydrogen and oxygen atoms that make up water are attracted to each other due to the difference in charges (the negative charge of oxygen is attracted to the positive charge of hydrogen). These forces pull the molecules in different directions, towards each other.
The situation with molecules on the surface of a substance is a little different, since the magnitude of the force with which air molecules act on water molecules is much less than the force with which water molecules act on each other. As shown in the illustration, the forces acting on molecules on the surface of a liquid are less than the forces acting on all other molecules inside the substance. The forces acting on these molecules act on them from the sides from which they are surrounded by other water molecules, but not from the surface. Due to this, the molecules on the surface are attracted into the liquid with a greater force than they are attracted towards the surface. Because of this, a much more “durable” layer of water forms on the surface. The forces acting on the molecules on the surface cause the surface to contract in order to reduce the surface area as much as possible. Compared to other bonds, these bonds are much harder to break.
The forces that act on water molecules determine the presence of two properties of water - adhesion and cohesion. Cohesion is the property of molecules of the same substance to attract each other. As we have seen from previous examples, water molecules are highly cohesive. It is thanks to cohesion that surface tension is possible.
Adhesion, on the contrary, is the property of molecules of different substances or materials to be attracted to each other. For example, if the adhesion between the liquid and the vessel is high, then the liquid "climbs" on the surface of the vessel, while the area in the center of the liquid remains in place. This is clearly seen in the example of water in a glass vessel - water forms concave meniscus if you pour it into a narrow vessel.
Of course, a concave meniscus will form in any glass vessel if it is not too full, but this effect is much easier to see in a narrow vessel, such as a pipe. It is worth noting that in the illustration of a full glass, the meniscus convex. This is because the water has nothing to "hook" on other than other water molecules. The convex shape of the meniscus is caused by cohesion between water molecules. The process of formation of a convex meniscus is similar to the process of formation of water droplets, which is described below.
If the adhesion between the surface of the substance and the liquid is small, then the meniscus will be convex. This is because the molecules of the liquid are attracted to other molecules of the liquid more than they are attracted to the surface of the vessel. A good example of such a meniscus is mercury. If you have a measuring device with mercury inside, such as a thermometer, then you can easily see this meniscus.
Examples of surface tension at work
Examples of surface tension in everyday life and technology surround us everywhere. The effect of surface tension is easiest to see in water-air systems.
Water drops
The formation of spherical droplets also occurs due to the forces that attract the molecules of the surface of the liquid inward. Imagine a drop, as children often draw it - its shape is not at all spherical, but oblong, elongated at the top and rounded at the bottom. The most common image of a drop has this shape because we most often see drops like this when various forces act on them. For example, this is how drops look that roll down the surface of leaves and tree branches, and then flow down.
When a drop is not yet glassed from the surface on which it is located, several forces act on it, including the force of attraction. Water easily changes shape, and a drop, before falling down, is stretched and represents hanging drop. We are familiar with this shape, since such drops, unlike spherical ones, move rather slowly and are easy to see.
As the drop stretches, it reaches a point of maximum stretch, after which surface tension forces can no longer hold the drop molecules together. The drop breaks off from other water molecules and falls down. As it flies downwards, the influence of surrounding forces on it decreases, and due to surface tension, its shape becomes spherical, as we discussed above.
As you can see in the photo of a coffee drop falling into a cup from an espresso coffee machine, the shape of this drop is very close to spherical, although it is slightly deformed by the force of gravity that acts on it.
To understand the mechanism behind the formation of a spherical drop, one can also consider surface tension in terms of energy, as in the definition of this phenomenon above. Particles are attracted to other particles with opposite charges, so we can say that these particles have a potential energy that depends on how these molecules interact with the surrounding molecules. Molecules on the surface of a liquid are not surrounded by other molecules on the surface side, so their potential energy is higher. Such a system tends to reduce the potential energy, according to principle of minimum potential energy. This means that molecules with a higher potential energy tend to reduce it, for example, by changing their shape. In our case, this is achieved by changing the form that water takes.
With constant surface tension, the potential energy can be reduced by decreasing the area. It is important to remember that we are talking about the area between molecules. Having considered the formulas for calculating the area of various geometric shapes, we note that the ball is best suited to reduce the area between molecules, that is, this area for molecules on the outer surface of the ball is minimal compared to other geometric shapes. This relationship can be proven using Euler-Lagrange equation.
Change in surface tension with a change in temperature and chemical composition of a substance
It should be noted that as the temperature increases, the surface tension decreases. This is because as the temperature increases, the molecules become more active and the intensity of their vibrations increases. As a result, the distance between molecules increases and the bonds between molecules weaken. Some substances added to water, such as soap, also reduce surface tension, and this allows the water to better adhere to other surfaces.
The reduced surface tension allows water to penetrate into pores and hard-to-reach holes, such as between fabric fibers. This is possible due to the fact that water molecules are easily separated from each other at low surface tension. That is why fabrics, dishes, and other objects and surfaces are most often washed with hot water. Detergents have the same effect of reducing surface tension as heating, so they are also often used to clean surfaces, often in combination with hot water.
Surface tension in capillaries
Above, we looked at the formation of a meniscus due to adhesion, but this is not the only example of how liquids behave in narrow tubes and capillaries. Liquids rise up the capillary or tube due to adhesion, but in order for the liquid to rise through the tube as a whole without breaking apart, cohesion is also needed in addition to adhesion. The narrower the capillary, the higher the liquid can rise, since in a wider tube there may not be enough surface tension to lift a large amount of water up.
Examples of this phenomenon in capillaries are paper towels, which absorb spilled liquid, sportswear made of fabric that absorbs sweat, and roots that absorb water from the ground and move it along the trunk, to branches and leaves. It is worth noting that such fluid movement can be caused not only by surface tension, but also by osmosis. An interesting phenomenon in Hindu temples known as milk miracle also explained by the work of capillaries. The milk miracle was as follows. Visitors to one of the Hindu temples in India noticed that the statues of the gods on the territory of the temple "drank" the milk that believers left on plates in front of them. This phenomenon has been seen in some other temples in India as well as outside the country. Scientists explain this phenomenon by the work of capillaries: the stone from which the statues were carved was porous, and milk rose through the capillaries inside the statues.
As can be seen from these examples, without surface tension there would be no phenomena of liquid movement through capillaries. The liquid can stick to the walls of the vessel if the adhesion between the liquid and the material of the vessel is high, but without surface tension, it cannot creep up, since it cannot move as a whole.
Objects floating on the surface of a liquid
Objects that do not get wet in a liquid and have a density greater than that of water can float on the surface of the water due to the balance between the forces that create surface tension and the forces that pull the body down, such as body weight. Here we are talking only about bodies made of waterproof materials. If water penetrates into the material or sticks to the shell, then the picture becomes much more complicated. This property of the body to remain on the surface is easily demonstrated by the example of a paper clip or a needle floating on the surface of water. Carefully lower the paperclip into the water, trying not to apply force, a large force of surface tension. To reduce the amount of water that sticks to the surface of the paper clip and causes it to sink under the water, cover the paper clip with oil. If we put the paperclip on the water gently enough, it will stay on the surface of the water.
The shape of droplets that stick to a hard surface
In the examples described earlier, we saw that the water droplets tend to become spherical in order to reduce the potential energy in the system. Sometimes it is impossible to achieve the shape of a ball, so the drops take on a shape that is closest to it. If a drop of water falls on a solid surface and sticks to it, then the lower part of the drop, which is in contact with this surface, will take the form of this surface, for example, it will become flat. This is because the force of attraction pulls the drop towards the surface. The surface of the drop, which is in contact only with air, will, on the contrary, be close to the shape of a ball. As a result, drops on flat surfaces, such as on a sheet or on glass, acquire the shape of a hemisphere.
When drops fall on a solid surface, they assume a shape that allows for a reduction in area, and remain in this shape until the balance between forces is so disturbed that surface tension can no longer hold the drop on the surface in this shape. For example, dew drops remain on the fabric of the tent until they come into contact with another surface. When the drops have formed on the outside, if you touch the fabric of the tent from the inside and remove your hand, the surface tension will break so much that the drops will penetrate the fabric of the tent and the water will remain on your fingers.
An interesting phenomenon can be seen when an alcoholic beverage, such as wine, is poured into a glass, especially when it is wine with a high alcohol content. Drops of water form on the walls of this glass, known as "tears of wine".
This phenomenon is caused by a number of factors, including the difference in surface tension between ethanol and water. As we mentioned above, the surface tension of water is high compared to other liquids. It is many times greater than the surface tension of ethyl alcohol. In mixtures of water and alcohol, as, for example, in wine, water molecules are attracted to each other more than to alcohol molecules. Because of this, the water "runs away" from the alcohol molecules, up the walls of the glass. In other words, water moves from ethanol molecules towards water molecules.
Of course, there is ethanol in wine in a glass, but it is not on the surface of the glass above the level of the wine, so the water moves precisely up the walls of the glass. At the same time, drops similar to tears form on the walls above the level of the wine. Hence the name of this phenomenon.
The more water collects in a drop, and the higher it rises, the more difficult it is for it to stay on the glass only due to surface tension. Eventually, the drop flows back into the glass. The higher the alcohol content of the wine, the more pronounced this effect.
Surface tension in medical diagnostics
Doctors use information about the surface tension of a substance to determine its content in a mixture. For example, some forms of jaundice are characterized by a high content of bile salts in the urine. The presence of these salts lowers the surface tension of the urine, and therefore their content can be determined by checking whether a certain substance floats or sinks in the urine, in our case sulfur powder. It does not sink in the urine of a healthy patient, but if there is an admixture of bile salts in it, then the surface tension is not enough, and the sulfur powder sinks. This test is called Hay's test.
In nature
Surface tension measurement
There are several ways to find surface tension using various measuring instruments. Below we consider several well-known measuring systems.
In devices of the first type, the force applied to the measuring device as a result of surface tension is measured. When measured by the du Nouy ring tear-off method and du Nuy-Padey method the force required to lift the ring or needle from the surface of the liquid, respectively, is estimated. According to Newton's third law, the force applied to a ring or needle due to surface tension when we lift it from the surface of a liquid is equal in magnitude to the force that is needed to lift these objects from the surface of the water. That is, by measuring the force that is needed to lift these objects, we also get the amount of force that prevents them from lifting.
Wilhelmy method measures the force that acts on a metal plate immersed in a liquid whose surface tension is being measured. Liquid adheres to a plate, a ring, or a needle (as in previous measurement methods), and surface tension holds the molecules of the liquid adhering to the surface, as well as the rest of the molecules, together as a whole. That is, the liquid "does not let go" of the plate, ring or needle. The material from which the plate is made is known, as well as how strongly water adheres to this material, and this is taken into account when calculating the force.
Surface tension can also be found using the weight of water droplets that fall from a vertical tube or capillary. This method is called stalagmometric, and the device that measures surface tension is a stalagmometer. The surface tension of a liquid can be easily calculated from the weight of a drop, since weight and surface tension are related. If the diameter of the tube is known, then the weight of a drop can be determined from the number of drops in a certain amount of liquid.
Method for determination by the shape of a hanging drop similar to the previous one in that it also uses a drop to determine the surface tension force. In this case, it is measured how much the drop can lengthen before it separates from the rest of the liquid and falls down.
There are also measuring devices that spin liquid and gas (for liquid-gas systems) until the system reaches equilibrium and the shape of the substance becomes constant. In this case, the surface tension is determined by the shape of a substance with a lower density. This method of measuring surface tension is called rotating drop method.
Do you find it difficult to translate units of measurement from one language to another? Colleagues are ready to help you. Post a question to TCTerms and within a few minutes you will receive an answer.
In this lesson, we will talk about liquids and their properties. From the point of view of modern physics, liquids are the most difficult subject of research, because, compared to gases, it is no longer possible to speak of a negligibly small interaction energy between molecules, and compared to solid bodies one cannot speak of an ordered arrangement of liquid molecules (there is no long-range order in a liquid). This leads to the fact that liquids have a number of interesting properties and their manifestations. One such property will be discussed in this lesson.
First, let's discuss the special properties that the molecules of the near-surface layer of a liquid have in comparison with the molecules in the bulk.
Rice. 1. The difference between the molecules of the near-surface layer and the molecules in the bulk of the liquid
Consider two molecules A and B. Molecule A is inside the liquid, molecule B is on its surface (Fig. 1). Molecule A is surrounded by other liquid molecules evenly, so the forces acting on molecule A from molecules falling into the sphere of intermolecular interaction are compensated, or their resultant is zero.
What happens to the molecule B, which is located at the surface of the liquid? Recall that the concentration of gas molecules that is above the liquid is much less than the concentration of liquid molecules. Molecule B is surrounded on one side by liquid molecules, and on the other side by highly rarefied gas molecules. Since many more molecules act on it from the side of the liquid, the resultant of all intermolecular forces will be directed inside the liquid.
Thus, in order for a molecule to get from the depth of the liquid to the surface layer, it is necessary to perform work against uncompensated intermolecular forces.
Recall that work is the change in potential energy, taken with a minus sign.
This means that the molecules of the near-surface layer, in comparison with the molecules inside the liquid, have excess potential energy.
This excess energy is a component of the internal energy of the fluid and is called surface energy. It is designated as, and is measured, like any other energy, in joules.
Obviously, the larger the surface area of the liquid, the more such molecules that have excess potential energy, and hence the greater the surface energy. This fact can be written as the following relation:
,
where is the surface area, and is the proportionality factor, which we will call surface tension, this coefficient characterizes one or another liquid. Let us write down a rigorous definition of this quantity.
The surface tension of a liquid (coefficient of surface tension of a liquid) is a physical quantity that characterizes a given liquid and is equal to the ratio of surface energy to the surface area of the liquid
The coefficient of surface tension is measured in newtons divided by a meter.
Let us discuss what the coefficient of surface tension of a liquid depends on. To begin with, let us recall that the surface tension coefficient characterizes the specific energy of the interaction of molecules, which means that the factors that change this energy will also change the surface tension coefficient of the liquid.
So, the surface tension coefficient depends on:
1. The nature of the liquid (for "volatile" liquids, such as ether, alcohol and gasoline, the surface tension is less than that of "non-volatile" - water, mercury and liquid metals).
2. Temperature (the higher the temperature, the lower the surface tension).
3. Presence superficially active substances, reducing surface tension (surfactant), such as soap or washing powder.
4. Properties of a gas adjoining a liquid.
Note that the surface tension coefficient does not depend on the surface area, since for one individual near-surface molecule it is absolutely unimportant how many of the same molecules are around. Pay attention to the table, which shows the surface tension coefficients of various substances, at a temperature:
Table 1. Coefficients of surface tension of liquids at the boundary with air, at
So, the molecules of the near-surface layer have excess potential energy compared to the molecules in the bulk of the liquid. In the course of mechanics, it was shown that any system tends to a minimum of potential energy. For example, a body thrown from a certain height will tend to fall down. In addition, you feel much more comfortable lying down, because in this case the center of mass of your body is located as low as possible. What does the desire to reduce its potential energy in the case of a liquid lead to? Since the surface energy depends on the surface area, it means that it is energetically unfavorable for any liquid to have a large surface area. In other words, in the free state, the liquid will tend to minimize its surface.
This is easy to verify by experimenting with a soap film. If a wire frame is dipped into a soapy solution, then a soap film is formed on it, and the film acquires such a shape that its surface area is minimal (Fig. 2).
Rice. 2. Figures from a soapy solution
You can verify the existence of surface tension forces using a simple experiment. If a thread is tied to the wire ring in two places, and in such a way that the length of the thread is somewhat greater than the length of the chord connecting the points of attachment of the thread, and the wire ring is dipped in soap solution (Fig. 3a), the soap film will tighten the entire surface of the ring and the thread will lie on soap film. If now the film is broken on one side of the thread, the soap film remaining on the other side of the thread will shrink and stretch the thread (Fig. 3b).
Rice. 3. Experiment to detect surface tension forces
Why did this happen? The fact is that the soap solution remaining on top, that is, the liquid, tends to reduce its surface area. Thus, the thread is pulled up.
So, we are convinced of the existence of the surface tension force. Now let's learn how to calculate it. To do this, let's do a thought experiment. Let us lower a wire frame, one of the sides of which is movable, into the soapy solution (Fig. 4). We will stretch the soap film, acting on the movable side of the frame with force . There are thus three forces acting on the crossbar - an external force and two surface tension forces acting along each surface of the film. Using Newton's second law, we can write that
Rice. 4. Calculation of the surface tension force
If, under the action of an external force, the crossbar moves a distance , then this external force will do work
Naturally, due to the performance of this work, the surface area of the film will increase, which means that the surface energy will also increase, which we can determine through the surface tension coefficient:
The change in area, in turn, can be determined as follows:
where is the length of the movable part of the wire frame. Given this, we can write that the work of the external force is equal to
Equating the right parts in (*) and (**), we obtain an expression for the surface tension force:
Thus, the surface tension coefficient is numerically equal to the surface tension force that acts per unit length of the line that bounds the surface
So, we have once again seen that the liquid tends to take such a shape that its surface area is minimal. It can be shown that for a given volume, the surface area will be minimal for a sphere. Thus, if no other forces act on the fluid or their action is small, the fluid will tend to take on a spherical shape. So, for example, water will behave in weightlessness (Fig. 5) or soap bubbles (Fig. 6).
Rice. 5. Water in zero gravity
Rice. 6. Soap bubbles
The presence of surface tension forces can also explain why a metal needle "lies" on the surface of the water (Fig. 7). The needle, which is carefully placed on the surface, deforms it, thereby increasing the area of this surface. Thus, a surface tension force arises, which tends to reduce such a change in area. The resultant force of surface tension will be directed upward, and it will compensate for the force of gravity.
Rice. 7. Needle on the surface of the water
The principle of operation of the pipette can be explained in the same way. The droplet, on which the force of gravity acts, is pulled down, thereby increasing its surface area. Naturally, surface tension forces arise, the resultant of which is opposite to the direction of gravity, and which do not allow the droplet to stretch (Fig. 8). When you press down on the pipette's rubber cap, you create extra pressure that helps with gravity, causing the drop to fall down.
Rice. 8. How the pipette works
Let's take another example from everyday life. If you dip a paint brush into a glass of water, its hairs will fluff up. If you now take this brush out of the water, you will notice that all the hairs are stuck to each other. This is due to the fact that the surface area of the water adhering to the brush will then be minimal.
And one more example. If you want to build a dry sand castle, you are unlikely to succeed, since the sand will crumble under the influence of gravity. However, if you wet the sand, it will retain its shape due to the surface tension of the water between the sand grains.
Finally, we note that the theory of surface tension helps to find beautiful and simple analogies when solving more complex physical problems. For example, when you need to build a light and at the same time strong structure, the physics of what happens in soap bubbles comes to the rescue. And it was possible to build the first adequate model of the atomic nucleus by likening this atomic nucleus to a drop of charged liquid.
Bibliography
- G. Ya. Myakishev, B. B. Bukhovtsev, N. N. Sotsky. "Physics 10". - M.: Education, 2008.
- Ya. E. Geguzin "Bubbles", Kvant Library. - M.: Nauka, 1985.
- B. M. Yavorsky, A. A. Pinsky "Fundamentals of Physics" vol. 1.
- G. S. Landsberg "Elementary textbook of physics" vol. 1.
- Nkj.ru ().
- Youtube.com().
- Youtube.com().
- Youtube.com().
Homework
- Having solved the tasks for this lesson, you will be able to prepare for questions 7,8,9 of the GIA and questions A8, A9, A10 of the Unified State Examination.
- Gelfgat I.M., Nenashev I.Yu. "Physics. Collection of problems grade 10 "5.34, 5.43, 5.44, 5.47 ()
- Based on problem 5.47, determine the coefficient of surface tension of water and soap solution.
List of questions and answers
Question: Why does surface tension change with temperature?
Answer: As the temperature increases, the molecules of the liquid begin to move faster, and therefore, the molecules more easily overcome the potential forces of attraction. This leads to a decrease in the surface tension forces, which are potential forces that bind the molecules of the near-surface layer of the liquid.
Question: Does the coefficient of surface tension depend on the density of the liquid?
Answer: Yes, it does, because the energy of the molecules of the near-surface layer of the liquid depends on the density of the liquid.
Question: What are the ways to determine the surface tension coefficient of a liquid?
Answer: AT school course study two ways to determine the coefficient of surface tension of a liquid. The first is the wire tearing method, its principle is described in problem 5.44 from homework, the second is the drop counting method described in Problem 5.47.
Question: Why do soap bubbles collapse after a while?
Answer: The fact is that after a while, under the action of gravity, the bubble becomes thicker at the bottom than at the top, and then, under the influence of evaporation, collapses at some point. This results in the whole bubble being like balloon, collapses under the action of uncompensated surface tension forces.
