Surface tension symbol. Start in science
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Introduction
In the world around us, along with gravity, elasticity and friction, there is another force that we usually do not pay attention to. This force acts along the tangent to the surfaces of all liquids. The force that acts along the surface of a liquid perpendicular to the line limiting this surface, tends to reduce it to a minimum, is called surface tension force. It is relatively small, its action never causes powerful effects. However, we cannot pour water into a glass, nor can we do anything with any liquid at all, without bringing into play the forces of surface tension. We are so accustomed to the effects called surface tension that we do not notice them. The manifestations of surface tension of liquids in nature and technology are surprisingly diverse. They play an important role in nature and in our lives. Without them, we would not be able to write with helium pens; printer cartridges would immediately make a large blot, emptying their entire reservoir. It would be impossible to soap your hands - foam would not form. A light rain would have soaked us through, and the rainbow would have been impossible to see no matter the weather. Surface tension collects water into droplets and, thanks to surface tension, a soap bubble can be blown. Using the rule “Be surprised in time” by the Belgian professor Plateau for researchers, we will consider unusual experiments in our work.
Purpose of the work: experimentally test the manifestations of surface tension of liquids, determine the coefficient of surface tension of liquids using the drop separation method
Study educational, popular science literature, use materials on the Internet on the topic “Surface Tension”;
carry out experiments to prove that the proper shape of a liquid is a sphere;
conduct experiments with decreasing and increasing surface tension;
design and assemble an experimental setup with which to determine the coefficient of surface tension of some liquids by the drop separation method.
process the data received and draw a conclusion.
Object of study: liquids.
Main part. Surface tension
Fig 1. G. Galileo
Numerous observations and experiments show that a liquid can take a form in which its free surface has the smallest area. In its desire to contract, the surface film would give the liquid a spherical shape if not for the attraction to the Earth. The smaller the drop, the greater the role played by surface tension forces. Therefore, small drops of dew on the leaves of trees and on the grass are close in shape to a ball; when falling in free fall, raindrops are almost strictly spherical. The tendency of a liquid to contract to the minimum possible can be observed in many phenomena that seem surprising. Galileo also thought about the question: why do the drops of dew that he saw on cabbage leaves in the morning take on a spherical shape? The statement that a liquid does not have its own shape turns out to be not entirely accurate. The proper form of a liquid is a sphere, as the most capacious form. The 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 entire volume of the crystal and can perform thermal vibrations around fixed centers, liquid molecules have greater freedom. Each molecule of a liquid, just like in a solid, is “sandwiched” on all sides by neighboring molecules and undergoes thermal vibrations around a certain equilibrium position. However, from time to time, any molecule may move to a nearby vacant location. Such jumps in liquids occur quite often; therefore, the molecules are not tied to specific 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 located molecules, they can form local (unstable) ordered groups containing several molecules. 1
Figure 2. An example of short-range order of liquid molecules and long-range order of molecules of a crystalline substance: 1 - water; 2 - ice
How can one explain the spontaneous contraction of the surface of a liquid? Molecules on the surface and deep in the liquid are in different conditions. Each molecule inside a liquid is subject to attractive forces from neighboring molecules surrounding it on all sides. The resultant of these forces is zero. Above the surface of the liquid there is vapor, the density of which is many times less than the density of the liquid, and the interaction of vapor molecules with liquid molecules can be neglected. Molecules that are on the surface of a liquid are attracted only by molecules that are inside the liquid. Under the influence of these forces, the molecules of the surface layer are drawn inward, the number of molecules on the surface decreases, and the surface area decreases. But not all molecules can move from the surface into the liquid; this is prevented by the repulsive forces that arise when the distances between the molecules decrease. At certain distances between the molecules drawn inward and the molecules located under the surface, the interaction forces become equal to zero, and the process of surface contraction stops. The number of molecules remaining on the surface is such that its area is minimal for a given volume of liquid. Since the liquid is fluid, it takes a form in which the number of molecules on the surface is minimal, and a sphere has the minimum surface for a given volume, that is, a drop of liquid takes a shape close to a spherical one. The easiest way to grasp the nature of surface tension forces is by observing the formation of a drop. Look closely at how the drop gradually grows, a narrowing forms - a neck - and the drop breaks off. 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. The film of a soap bubble produces the same impression.
Experience No. 1
The friction of a liquid towards a minimum of potential energy can be observed using soap bubbles. Soap film is a double surface layer. If you blow out a soap bubble and then stop inflating, it will begin to decrease in volume, squeezing out a stream of air.
Surface tension is a phenomenon of molecular pressure on a liquid caused by the attraction of molecules of the surface layer to molecules inside the liquid 5
The Plateau Experience (1849)
Rice. 4. J.Plateau
The gadfly that prompted the Belgian professor to experiment was chance. He accidentally poured a small amount of oil into a mixture of alcohol and water, and it took the shape of a ball. Reflecting on this fact, Plato outlined a series of experiments that were later brilliantly performed by his friends and students. In his diary, he wrote a rule for researchers: “It’s time to be surprised.” I decided to explore the Plateau experience, but in a different way: to use sunflower oil and tinted manganese water in the experiment.
Experiment proving that a homogeneous liquid takes shape with a minimum free surface
Plateau experience option #2
1) Sunflower oil was poured into a beaker.
2) Using an eye dropper, drop a drop of tinted manganese water with a diameter of approximately 5 mm into the sunflower oil.
) We observed water balls of different sizes slowly falling to the bottom and taking on a flattened oval shape (Photo 2).
5) We observed how the drop took the correct shape of a ball (Photo 2).
Conclusion: The liquid, attracting molecules of the surface layer, compresses itself. The oval flattened shape is explained by the fact that the weight of the drop, which does not mix with the oil, is greater than the buoyant force. The correct shape of the ball is explained by the fact that the drop floats inside the oil: the weight of the drop is balanced by the buoyant force.
When falling freely, in a state of weightlessness, raindrops practically have the shape of a ball. In a spaceship, a fairly large mass of liquid also takes on a spherical shape.
Surface tension coefficient
In the absence of an external force, a surface tension force acts along the surface of the liquid, which reduces the surface area of the film to a minimum. Surface tension force is a force directed tangentially to the surface of a liquid, perpendicular to the section of the contour that bounds the surface, in the direction of its contraction.
Ơ - surface tension coefficient - this is the ratio of the modulus F of the surface tension force acting on the boundary of the surface layer ℓ to this length, a constant value that does not depend on the length ℓ. The surface tension coefficient depends on the nature of the surrounding media and temperature. It is expressed in newtons per meter (N/m).
Experiments with reduction and increase
Photo 3
surface tensionExperience No. 3
Touch the center of the surface of the water with a piece of soap.
The foam pieces begin to move from the center to the edges of the vessel (Photo 3).
Dropped gasoline, alcohol, detergent into the center of the vessel "Fairy"
Conclusion: The surface tension of these substances is less than that of water.
These substances are used to remove dirt, grease stains, soot, i.e. substances that are insoluble in water. Due to the fairly high surface tension, water itself does not have a very good cleaning effect. For example, when water molecules come into contact with a stain, they are attracted to each other more than to particles of insoluble dirt. Soaps and synthetic detergents (SDCs) contain substances that reduce the surface tension of water. The first soap, the simplest detergent, was obtained in the Middle East more than 5,000 years ago. At first it was used mainly for washing and treating ulcers and wounds. And only in the 1st century AD. the man began to wash himself with soap.
At the beginning of the 1st century, soap was born.
It saved a person from dirt and he became clean from a young age.
I'm telling you about soap, which soon gave birth to: shampoo, gel, powder.
The world has become clean, how good it is!
Fig 5. F. Gunter
Detergents are natural and synthetic substances with a cleansing effect, especially soap and washing powders used in everyday life, industry and the service sector. Soap is obtained as a result of the chemical interaction of fat and alkali. Most likely, it was discovered by pure chance when meat was fried over a fire, and the fat flowed onto the ashes, which have alkaline properties. Soap production has a long history, but the first synthetic detergent (SDC) appeared in 1916, it was invented by a German chemist Fritz Gunther for industrial purposes. Household SMS, more or less harmless to the hands, began to be issued in 1933. Since then, a number of synthetic detergents (SDCs) for narrow purposes have been developed, and their production has become an important branch of the chemical industry.
It is because of surface tension that water by itself does not have a sufficient cleaning effect. When water molecules come into contact with a stain, they are attracted to each other instead of trapping dirt particles, in other words, they do not wet the dirt.
Soaps and synthetic detergents contain substances that increase the wetting properties of water by reducing surface tension. These substances are called surface-active agents (surfactants) because they act on the surface of the liquid.
Nowadays, the production of SMS has become an important branch of the chemical industry. These substances are called surfactant(surfactants) because they act on the surface of the liquid. Surfactant molecules can be represented as tadpoles. They “cling” to the water with their heads, and the fat with their “tails”. When surfactants are mixed with water, their molecules on the surface face their “heads” down and their “tails” out. By breaking up the surface of the water in this way, these molecules significantly reduce the effect of surface tension, thereby helping water penetrate the tissue. With these same “tails” the surfactant molecules (Fig. 6) capture the fat molecules they come across. 2
Experience No. 4
1.Pour milk into the saucer so that it covers the bottom (Photo 4)
2. Drop 2 drops of brilliant green onto the surface of the milk
3. We observed how the brilliant green was “carried away” from the center to the edges. Two drops of brilliant green cover most of the surface of the milk! (Photo 5)
Conclusion: the surface tension of brilliant green is much less than that of milk.
4. “Fairy” dishwashing liquid was dropped onto the surface of the brilliant green, we saw how this liquid spread over the entire surface. (Photo 6)
Conclusion: The surface tension of the detergent is less than that of brilliant green.
Experience No. 5
Water was poured into a wide glass vessel.
Pieces of foam were thrown onto the surface.
Touch the center of the surface of the water with a piece of sugar.
The styrofoam tendrils begin to move from the edges of the vessel towards the center (Photo 7).
Conclusion: The surface tension of an aqueous sugar solution is greater than that of pure water.
Experience No. 6
Removing fat stains from the surface of fabric
We moistened a cotton wool with gasoline and moistened the edges of the stain with this cotton wool (not the stain itself). Gasoline reduces surface tension, so fat accumulates in the center of the stain and can be removed from there; if you wet the stain itself with the same cotton wool, it can increase in size due to a decrease in surface tension.
To experimentally determine the value of the surface tension of a liquid, the process of formation and separation of droplets flowing from a dropper can be used.
Brief theory of the drop separation method
A small volume of liquid itself takes on a shape close to a sphere, since due to the small mass of the liquid, the force of gravity acting on it is also small. This explains the spherical shape of small drops of liquid. Figure 1 shows photographs showing various stages of the process of droplet formation and detachment. The photograph was taken using high-speed filming; the drop grows slowly; we can assume that at each moment of time it is in equilibrium. Surface tension causes contraction of the surface of the drop, it tends to give the drop a spherical shape. Gravity places the drop's center of gravity as low as possible. As a result, the drop appears elongated (Fig. 7a).
Rice. 7. a B C D
The process of formation and separation of droplets
The larger the drop, the greater the role played by the potential energy of gravity. As the drop grows, the bulk of the mass collects at the bottom and a neck is formed on the drop (Fig. 7b). The surface tension force is directed vertically tangentially to the neck and it balances the force of gravity acting on the drop. Now it is enough for the drop to increase quite a bit and the forces of surface tension no longer balance the force of gravity. The neck of the drop quickly narrows (Fig. 7c) and as a result the drop breaks off (Fig. 7d).
The method for measuring the surface tension coefficient of some liquids is based on the weighing of droplets. In the case of a slow flow of liquid from a small hole, the size of the droplets formed depends on the density of the liquid, the coefficient of surface tension, the size and shape of the hole, as well as the flow rate . When a wetting liquid slowly flows out of a vertical cylindrical tube, the resulting drop has the shape shown in Figure 8. The radius r of the drop neck is related to the outer radius of the tube R by the relation r = kR (1)
where k is a coefficient depending on the size of the tube and the flow rate.
The moment of separation, the weight of the drop must be equal to the resultant of the surface tension forces acting along a length equal to the length of the neck contour in its narrowest part. Thus, we can write
Mg = 2πrơ (2)
Substituting the value of the neck radius r from equality (1) and solving it, we obtain
Ơ =mg/2πkR (3)
To determine the mass of a drop, a certain number n of drops is weighed in a glass of known weight. If the mass of a cup without drops and with drops is M 0 and M, respectively, then the mass of one drop
Substituting the last expression into formula (3) and introducing its diameter d instead of the radius of the tube, we obtain the calculation formula
ơ = ((M-M0)g)/πkdn 3 (4)
Research work “Determination of the surface tension coefficient of some liquids by the drop separation method”
Purpose of the study: determine the coefficient of surface tension of a liquid by tearing off drops of some liquids. Devices: installation for measuring the coefficient of surface tension, scales, weight, cup, caliper, stopwatch. Materials: detergents: “Fairy”, “Aos”, milk, alcohol, gasoline, powder solutions: “Myth”, “Persil”, shampoos "Fruttis", « Pantene», "Schauma" And " Fruttis", shower gels " Sensen», "Monpensier" And " Discover».
Description of the device.
To determine the surface tension coefficient, a setup was assembled, consisting of a tripod on which a burette with the liquid being tested was installed. At the end of the burette, a tube tip is attached, at the end of which a drop is formed. The drops were weighed in a special cup.
Progress of the study
Using a caliper, the diameter of the tip-tube was measured three times and the average value d was calculated.
Weighed a clean, dry glass (M 0) on the scales.
Using a burette tap, we achieved the rate of drop flow
15 drops per minute.
60 drops of liquid were poured from a burette into a glass, counting exactly the number of drops cast.
We weighed a glass of liquid. (M)
Substituted the obtained values into the formula ơ = ((M-M0)g)/πkdn
The surface tension coefficient was calculated.
The experiment was carried out three times
The average value of the surface tension coefficient was calculated.
The coefficient of surface tension in the SI system is measured in N/m.
Table No. 1
Results of determining the surface tension coefficient (N/m)
Liquid |
Surface tension coefficient |
|
Measured |
Tabular |
|
Ethanol |
||
Milk (2.5) |
||
Milk (homemade cow) |
||
“Myth” powder solution |
||
Persil powder solution |
||
Detergent "Fairy" |
||
Detergent "Aos" |
Conclusion: Of the kitchen detergents studied, with all other parameters that affect the quality of “washing” being the same, it is better to use the product “ Fairy" Of the washing powders studied " Myth", because It is their solutions that have the lowest surface tension. Therefore, the first remedy (“ Fairy") better helps to wash off water-insoluble fats from dishes, being an emulsifier - a means that facilitates the production of emulsions (suspensions of the smallest particles of a liquid substance in water). Second (“ Myth") washes laundry better, penetrating into the pores between the fibers of the fabrics. Note that when using kitchen detergents, we force the substance (in particular fat) to dissolve in water at least for a while, because it is “crushed” into tiny particles. During this time, it is recommended to rinse off the applied detergent with a stream of clean water, rather than rinsing the dishes after some time in a container. In addition, the surface tension of shampoos and shower gels was studied. Due to the fairly high viscosity of these liquids, it is difficult to accurately determine their surface tension coefficient, but it can be compared. Shampoos were studied (by the method of tearing off drops) "Pantene», "Schauma" And " Fruttis", as well as shower gels " Sensen», "Monpensier" And " Discover».
Conclusion:
Surface tension decreases in shampoos on a range "Fruttis" - "Schauma" - "Pantene" in gels - in a row "Monpensier" - "Discover" - "Senses".
The surface tension of shampoos is less than the surface tension of gels (For example, " Pantene» < «Senses"by 65 mN/m), which justifies their purpose: shampoos - for washing hair, gels - for washing the body.
With all other identical characteristics affecting the quality of washing, it is better to use the studied shampoos. "Pantene" (Fig. 9), of the studied shower gels - “Senses” (Fig. 10).
The method of tearing off drops, although not very accurate, is, however, used in medical practice. This method determines the surface tension of cerebrospinal fluid, bile, etc. for diagnostic purposes.
Conclusion
1. Experimental confirmation of theoretical conclusions was obtained , proving that a homogeneous liquid takes shape with a minimum free surface
2. Experiments were carried out with a decrease and increase in surface tension, the results of which proved that soap and synthetic detergents contain substances that increase the wetting properties of water by reducing the force of surface tension.
3. To determine the surface tension coefficient of liquids
a) a brief theory of the droplet separation method was studied;
b) an experimental setup was designed and assembled;
c) the average values of the surface tension coefficient of various liquids were calculated and conclusions were drawn.
4. The results of experiments and research are presented in the form of tables and photographs.
Working on the project allowed me to acquire broader knowledge in the section of physics “Surface Tension”.
I would like to finish my project with the words of the great physicist
A. Einstein:
“It is enough for me to experience the feeling of the eternal mystery of life, to realize and intuitively comprehend the wonderful structure of all things and to actively strive to grasp even the smallest grain of intelligence that manifests itself in Nature.”
List of sources and literature used
http://www.physics.ru/
http://greenfuture.ru/
http://www.agym.spbu.ru/
Bukhovtsev B.B., Klimontovich Yu.L., Myakishev G.Ya., Physics, textbook for 9th grade of secondary school - 4th edition - M.: Education, 1988 - 271 p.
Kasyanov V.A., Physics, 10th grade, textbook for general education institutions, M.: Bustard, 2001. - 410 s.
Pinsky A.A. Physics: textbook. A manual for 10 grades with in-depth study of physics. M.: Education, 1993. - 416 s.
Yufanova I.L. Entertaining evenings in physics in high school: a book for teachers. - M.: Education, 1990. -215s
Chuyanov V.Ya., Encyclopedic Dictionary of Young Physicist, M.: Pedagogika, 1984. - 350 s.
1 1 http://www.physics.ru/
2 http://greenfuture.ru
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 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.
Molecules of a liquid interact with each other by forces of attraction and repulsion, which manifest themselves noticeably within a distance r, called the radius of molecular action (on the order of several molecular diameters). Radius Sphere r called the sphere of molecular action. If the molecule is in the surface layer, that is, less than r from the surface, then the resultant of the attractive forces from the surrounding molecules is directed into the liquid. Therefore, to move a molecule from the inside of a liquid to its surface, work must be done, as a result, the free energy of the surface increases. The free surface energy per unit surface of the liquid is called the surface tension coefficient:
where A is the work that needs to be done to increase the surface area by S. In the SI system, the coefficient of surface tension (measured in J/m2.
At equilibrium, the free energy of the system is minimal, so the liquid, left to its own devices, tends to reduce its surface area. Let's mentally limit some area of the surface layer with a closed contour. It contains forces called surface tension forces, directed tangentially to the surface and perpendicular to the section of the contour on which they act. Surface tension coefficient (can also be defined as the force per unit length of the contour limiting the surface:
Its unit of measurement in the SI system is 1N/m (newtons per meter = 1 J/m2, or millingtons per meter.
The surface tension coefficient depends on the chemical composition of the liquid, the environment with which it borders, and temperature. With increasing temperature (it decreases and at the critical temperature it becomes zero.
Depending on the strength of interaction between the molecules of the liquid and the particles of the solid body in contact with it, it is possible that the solid body may or may not be wetted by the liquid. In both cases, the surface of the liquid near the boundary with the solid body is curved...
Surface tension of water at different temperatures
Surface tension (at 20°C)
Surface tension of liquids
Substance | q, mN/m |
Molten aluminum (at t=7000 0 C, v) | 840 |
Liquid nitrogen (at t=-183 0 C, p) | 6,2 |
Acetone (p) | 24 |
Water (at t=0 0 С,в) | 75,6 |
Water (at t=20 0 С,в) | 72,8 |
Water (at t=100 0 С,в) | 58,8 |
Water (at t=374.15 0 С,в) | 0 |
Molten gold (at t=1130 0 C, v) | 1102 |
Glycerin (c) | 63 |
Kerosene (at t=0 0 С,в) | 29 |
Kerosene (c) | 24 |
Liquid oxygen (at t=-183 0 C, v) | 13,1 |
Milk (in) | 46 |
Oil (in) | 30 |
Soap solution (in) | 40 |
Mercury (p) | 472 |
Molten lead (at t=350 0 C, v) | 442 |
Molten silver (at t=970 0 C, at) | 930 |
Alcohol (at t=0 0 С,в) | 22 |
Ether (p) | 17 |
Surface tension of aqueous solutions (in dynes/cm)
Conversion to SI: 1 dyne/cm = 10 - 3 N/m
Solute | t, °C | Content, wt.% | |||
5 | 10 | 20 | 50 | ||
H2SO4 | 18 | - | 74,1 | 75,2 | 77,3 |
HNO3 | 20 | - | 72,7 | 71,1 | 65,4 |
NaOH | 20 | 74,6 | 77,3 | 85,8 | - |
NaCl | 18 | 74,0 | 75,5 | - | - |
Na2SO4 | 18 | 73,8 | 75,2 | - | - |
NaNO3 | 30 | 72,1 | 72,8 | 74,4 | 79,8 |
KC1 | 18 | 73,6 | 74,8 | 77,3 | - |
KNO 3 | 18 | 73,0 | 73,6 | 75,0 | - |
K2CO3 | 10 | 75,8 | 77,0 | 79,2 | 106,4 |
NH 3 | 18 | 66,5 | 63,5 | 59,3 | - |
NH 4 C1 | 18 | 73,3 | 74,5 | - | - |
NH4NO3 | 100 | 59,2 | 60,1 | 61,6 | 67,5 |
MgCl2 | 18 | 73,8 | - | - | - |
CaCl2 | 18 | 73,7 | - | - | - |
Liquid is a state of aggregation of a substance, intermediate between gaseous and solid, therefore it has the properties of both gaseous and solid substances. Liquids, like solids, have a certain volume, and like gases, they take the shape of the container in which they are located. Gas molecules are practically not connected to each other by intermolecular interaction forces. In this case, the average energy of thermal motion of gas molecules is much greater than the average potential energy caused by the forces of attraction between them, so the gas molecules fly apart in different directions, and the gas occupies the entire volume provided to it.
In solids and liquids, the forces of attraction between molecules are already significant and keep the molecules at a certain distance from each other. In this case, the average energy of the chaotic thermal motion of molecules is less than the average potential energy due to the forces of intermolecular interaction, and it is not enough to overcome the forces of attraction between molecules, therefore solids and liquids have a certain volume.
X-ray diffraction analysis of liquids showed that the nature of the arrangement of liquid particles is intermediate between a gas and a solid. In gases, molecules move chaotically, so there is no pattern in their relative arrangement. For solids, the so-called long range order in the arrangement of particles, i.e. their ordered arrangement, repeating over large distances. In liquids there is a so-called close order in the arrangement of particles, i.e. their ordered arrangement, repeating at distances comparable to interatomic ones.
The theory of liquids has not yet been fully developed. Thermal motion in a liquid is explained by the fact that each molecule oscillates for some time around a certain equilibrium position, after which it abruptly moves to a new position, separated from the original one at a distance of the order of interatomic. Thus, the molecules of the liquid move rather slowly throughout the mass of the liquid, and diffusion occurs much more slowly than in gases. With increasing temperature of the liquid, the frequency of vibrational motion increases sharply, the mobility of molecules increases, which causes a decrease in the viscosity of the liquid.
Each molecule of a liquid is subject to attractive forces from surrounding molecules, which quickly decrease with distance; therefore, starting from a certain minimum distance, the forces of attraction between molecules can be neglected. This distance (approximately 10 -9 m) is called radius of molecular action r , and the sphere of radius r-sphere of molecular action.
Let us isolate a molecule inside the liquid A and draw a sphere of radius around it r(Fig. 10.1). It is sufficient, according to the definition, to take into account the effect on a given molecule only of those molecules that are inside the sphere
Fig. 10.1. molecular action. The forces with which these molecules act on the molecule A, are directed in different directions and are compensated on average, so the resulting force acting on a molecule inside the liquid from other molecules is zero. The situation is different if the molecule, e.g. IN, located from the surface at a distance less than r. In this case, the sphere of molecular action is only partially located inside the liquid. Since the concentration of molecules in the gas located above the liquid is small compared to their concentration in the liquid, the resultant force F, applied to each molecule of the surface layer, is not equal to zero and is directed into the liquid. Thus, the resulting forces of all molecules of the surface layer exert a pressure on the liquid, called molecular(or internal). Molecular pressure does not act on a body placed in a liquid, since it is caused by forces acting only between the molecules of the liquid itself.
The total energy of liquid particles consists of the energy of their chaotic thermal motion and potential energy due to the forces of intermolecular interaction. To move a molecule from the depths of the liquid to the surface layer, work must be expended. This work is done due to the kinetic energy of the molecules and goes to increase their potential energy. Therefore, the molecules in the surface layer of a liquid have greater potential energy than the molecules inside the liquid. This additional energy possessed by molecules in the surface layer of a liquid, called surface energy, proportional to the layer area Δ S:
Δ W=σ Δ S,(10.1)
Where σ – surface tension coefficient, defined as the surface energy density.
Since the equilibrium state is characterized by a minimum potential energy, the liquid, in the absence of external forces, will take such a shape that for a given volume it has a minimum surface area, i.e. ball shape. Observing the smallest droplets suspended in the air, we can see that they really have the shape of balls, but somewhat distorted due to the action of gravity. Under conditions of weightlessness, a drop of any liquid (regardless of its size) has a spherical shape, which has been proven experimentally on spacecraft.
So, the condition for stable equilibrium of a liquid is a minimum of surface energy. This means that a liquid for a given volume should have the smallest surface area, i.e. the liquid tends to reduce the free surface area. In this case, the surface layer of the liquid can be likened to a stretched elastic film in which tension forces act.
Let us consider the surface of a liquid bounded by a closed contour. Under the 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 contour in question moved. The forces acting from the selected area on the areas bordering it do work:
Δ A=fΔ lΔ x,
Where f=F/Δ l –surface tension force, acting per unit length of the liquid surface contour. It is clear that Δ lΔ x= Δ S, those.
Δ A=fΔS.
This work is done by reducing the surface energy, i.e.
Δ Α =Δ W.
From a comparison of expressions it is clear that
i.e. the surface tension coefficient σ is equal to the surface tension force per unit length of the contour delimiting the surface. The unit of surface tension is newton per meter (N/m) or joule per square meter (J/m2). Most liquids at a temperature of 300K have a surface tension of the order of 10 -2 –10 -1 N/m. Surface tension decreases with increasing temperature, as the average distances between liquid molecules increase.
Surface tension significantly depends on the impurities present in liquids. Substances , liquids that weaken the surface tension are called surfactants (surfactants). The most well-known surfactant in relation to water is soap. It greatly reduces its surface tension (from about 7.5 10 -2 up to 4.5·10 -2 N/m). Surfactants that reduce the surface tension of water are also alcohols, ethers, oil, etc.
There are substances (sugar, salt) that increase the surface tension of a liquid due to the fact that their molecules interact with liquid molecules more strongly than liquid molecules interact with each other.
In construction, surfactants are used to prepare solutions used in the processing of parts and structures operating in unfavorable atmospheric conditions (high humidity, elevated temperatures, exposure to solar radiation, etc.).
Wetting phenomenon
It is known from practice that a drop of water spreads on glass and takes the shape shown in Fig. 10.2, while mercury on the same surface turns into a slightly flattened drop. In the first case they say that the liquid wets hard surface, in the second - does not wet her. Wetting depends on the nature of the forces acting between the molecules of the surface layers of contacting media. For a wetting liquid, the force of attraction between the molecules of the liquid and the solid is greater than between the molecules of the liquid itself, and the liquid tends to increase
surface of contact with a solid body. For a non-wetting liquid, the force of attraction between the molecules of the liquid and the solid is less than between the molecules of the liquid, and the liquid tends to reduce the surface of its contact with the solid.
Three surface tension forces are applied to the line of contact of the three media (point 0 is its intersection with the plane of the drawing), which are directed tangentially inside the contact surface of the corresponding two media. These forces, per unit length of the line of contact, are equal to the corresponding surface tensions σ 12 , σ 13 , σ 23 . Corner θ between tangents to the surface of a liquid and a solid is called edge angle. The condition for equilibrium of a drop is that the sum of the projections of surface tension forces on the direction of the tangent to the surface of the solid body is equal to zero, i.e.
–σ 13 + σ 12 + σ 23 cos θ =0 (10.2)
cos θ =(σ 13 - σ 12)/σ 23 . (10.3)
It follows from the condition that the contact angle can be acute or obtuse depending on the values σ 13 and σ 12 . If σ 13 >σ 12 then cos θ >0 and angle θ spicy, i.e. liquid wets a solid surface. If σ 13 <σ 12 then cos θ <0 и угол θ – blunt, i.e. the liquid does not wet the solid surface.
The contact angle satisfies condition (10.3) if
(σ 13 - σ 12)/σ 23 ≤1.
If the condition is not met, then a drop of liquid at any value θ cannot be in balance. If σ 13 >σ 12 +σ 23, then the liquid spreads over the surface of the solid, covering it with a thin film (for example, kerosene on the surface of glass), – this occurs complete wetting(in this case θ =0).
If σ 12 >σ 13 +σ 23, then the liquid contracts into a spherical drop, in the limit having only one point of contact with it (for example, a drop of water on the surface of paraffin), - complete non-wetting(in this case θ =π).
Wetting and non-wetting are relative concepts, i.e. a liquid that wets one solid surface does not wet another. For example, water wets glass, but does not wet paraffin; Mercury does not wet glass, but it does wet clean metal surfaces.
The phenomena of wetting and non-wetting are of great importance in technology. For example, in the method of flotation beneficiation of ore (separation of ore from waste rock), it, finely crushed, is shaken in a liquid that wets the waste rock and does not wet the ore. Air is blown through this mixture and then it settles. In this case, rock particles moistened with liquid sink to the bottom, and grains of minerals “stick” to air bubbles and float to the surface of the liquid. When machining metals, they are moistened with special liquids, which facilitates and speeds up surface treatment.
In construction, the phenomenon of wetting is important for the preparation of liquid mixtures (putty, putty, mortars for bricklaying and concrete preparation). It is necessary that these liquid mixtures well wet the surfaces of the building structures to which they are applied. When selecting mixture components, not only the contact angles for mixture-surface pairs are taken into account, but also the surface-active properties of the liquid components.
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