Surface tension temperature. Surface liquid layer

Surface tension describes the ability of a fluid to resist the force of gravity. For example, water on the surface of a table forms drops as the water molecules are attracted to each other, which counteracts the force of gravity. It is thanks to surface tension that heavier objects, such as insects, can be held on the surface of the water. Surface tension is measured in force (N) divided by unit length (m), or the amount of energy per unit area. The force with which water molecules interact (cohesive force) causes tension, resulting in droplets of water (or other liquids). Surface tension can be measured with a few simple items found in almost every home and a calculator.

Steps

With the help of a rocker

Write down the equation for surface tension. In this experiment, the equation for determining surface tension is as follows: F = 2Sd, where F- force in newtons (N), S- surface tension in newtons per meter (N/m), d is the length of the needle used in the experiment. We express the surface tension from this equation: S = F/2d.

• The force will be calculated at the end of the experiment.
• Before starting the experiment, use a ruler to measure the length of the needle in meters.

1. Build a small rocker. In this experiment, a rocker and a small needle that floats on the surface of the water are used to determine surface tension. It is necessary to carefully consider the construction of the rocker arm, since the accuracy of the result depends on it. You can use various materials, the main thing is to make a horizontal bar out of something hard: wood, plastic or thick cardboard.

   • Determine the center of the rod (for example, a straw or plastic ruler) that you are going to use as a crossbar, and drill or poke a hole in this place; this will be the fulcrum of the crossbar, on which it will rotate freely. If you are using a plastic straw, just pierce it with a pin or nail.
   • Drill or poke holes in the ends of the crossbar so that they are the same distance from the center. Thread the threads through the holes on which you will hang the weight cup and needle.
   • If necessary, support the rocker with books or other hard objects to keep the crossbar in a horizontal position. It is necessary that the crossbar rotate freely around a nail or rod stuck in its middle.

2. Take a piece of aluminum foil and fold it into a box or saucer shape. It is not at all necessary that this saucer has the correct square or round shape. You will fill it with water or other weight, so make sure it can support the weight.

   • Hang the foil box or saucer from one end of the bar. Make small holes along the edges of the saucer and thread a thread through them so that the saucer hangs on the crossbar.

3. Hang a needle or paperclip from the other end of the crossbar so that it is horizontal. Tie a needle or paperclip horizontally to a thread that hangs from the other end of the crossbar. For the experiment to succeed, it is necessary to position the needle or paperclip exactly horizontally.

4. Place something on the bar, such as plasticine, to balance the aluminum foil container. Before proceeding with the experiment, it is necessary to ensure that the crossbar is located horizontally. The foil saucer is heavier than the needle, so the bar will drop down on its side. Attach enough plasticine to the opposite side of the crossbar so that it is horizontal.

   • This is called balancing.

5. Place a hanging needle or paperclip in a container of water. This step will require extra effort to position the needle on the surface of the water. Make sure the needle is not immersed in water. Fill a container with water (or another liquid of unknown surface tension) and place it under the hanging needle so that the needle is directly on the surface of the liquid.

   • At the same time, make sure that the rope holding the needle remains in place and is sufficiently taut.

6. Weigh a few pins or a small amount of measured drops of water on a small scale. You will add one pin or a drop of water to the aluminum saucer on the rocker. In this case, it is necessary to know the exact weight at which the needle will come off the surface of the water.

   • Count the number of pins or drops of water and weigh them.
   • Determine the weight of one pin or drop of water. To do this, divide the total weight by the number of pins or drops.
   • Suppose 30 pins weigh 15 grams, then 15/30 = 0.5, that is, one pin weighs 0.5 grams.

7. Add pins or drops of water one at a time in an aluminum foil saucer until the needle comes off the surface of the water. Gradually add one pin or drop of water. Watch the needle carefully so as not to miss the moment when, after the next increase in the load, it will come off the water. Once the needle comes off the surface of the liquid, stop adding pins or drops of water.

   • Count the number of pins or drops of water that took the needle at the opposite end of the crossbar to come off the surface of the water.
   • Record the result.
   • Repeat the experiment several (5 or 6) times to get more accurate results.
   • Calculate the average value of the results obtained. To do this, add up the number of pins or drops in all experiments and divide the sum by the number of experiments.

8. Convert the number of pins to strength. To do this, multiply the number of grams by 0.00981 N/g. To calculate surface tension, you need to know the force required to lift the needle from the surface of the water. Since you calculated the weight of the pins in the previous step, to determine the strength, it is enough to multiply this weight by 0.00981 N/g.

   • Multiply the number of pins placed in the saucer by the weight of one pin. For example, if you put in 5 pins weighing 0.5 grams each, their total weight would be 0.5 grams/pin = 5 x 0.5 = 2.5 grams.
   • Multiply the number of grams by the factor 0.00981 N/g: 2.5 x 0.00981 = 0.025 N.

9. Substitute the obtained values ​​into the equation and find the desired value. With the help of the results obtained during the experiment, the surface tension can be determined. Just plug in the found values ​​and calculate the result.

   • Let's say that in the example above, the length of the needle is 0.025 meters. Plugging the values ​​into the equation, we get: S = F/2d = 0.025 N/(2 x 0.025) = 0.05 N/m. Thus, the surface tension of the liquid is 0.05 N/m.

surface layer,

a thin layer of matter near the contact surface of two phases (bodies, media), which differs in properties from substances in the bulk of the phases. Special properties of P. with. due to the excess of free energy concentrated in it (see Surface energy, Surface tension), as well as the features of its structure and composition. P. s. at the boundary of condensed phases is often called the interfacial layer. P.'s thickness with. depends on the difference in phase densities, the intensity and type of intermolecular interactions in the boundary zone, temperature, pressure, chemical potentials, and other thermodynamic parameters of the system. In some cases, it does not exceed the thickness of the monomolecular layer, in others it reaches tens and hundreds of molecular sizes. So, P. s. liquids near critical mixing temperatures can have a thickness of 1000 (100 nm) or more. A surface layer formed by molecules (or ions) of an adsorbed substance is called an adsorption layer. The composition and properties of P. s change especially sharply. during the adsorption of surfactants. Adsorption, chemisorption and chemical effects on P. s. solid body can cause its lyophilization or lyophobization (see Lyophilicity and lyophobicity), lead to a decrease in its strength (see Rebinder effect) or, conversely, increase mechanical characteristics. P.'s condition with. various structural, radio engineering, and other materials are strongly reflected in their operational, technical and technological characteristics. With the properties of P. s. connected diverse superficial phenomena in the world around us.

Surface tension is a thermodynamic characteristic of the interface between two phases in equilibrium, determined by the work of reversible isothermokinetic formation of a unit area of ​​this interface, provided that the temperature, system volume and chemical potentials of all components in both phases remain constant.

The surface tension has a double physical meaning- energy (thermodynamic) and power (mechanical). Energy (thermodynamic) definition: surface tension is the specific work of increasing the surface when it is stretched, provided that the temperature is constant. Force (mechanical) definition: Surface tension is the force per unit length of a line that limits the surface of a liquid.

Static Methods:

1. Capillary rise method

2. Wilhelmy method

3. The sessile drop method

4. Method for determination by the shape of a hanging drop.

5. Rotating drop method

Dynamic Methods:

1. Du Nuy method (ring tearing method).

2. Stalagmometric, or drop counting method.

3. Maximum bubble pressure method.

4. Oscillating jet method

5. Method standing waves

6. Traveling wave method

Surface tension, the desire of a substance (liquid or solid phase) to reduce the excess of its potential energy at the interface with another phase (surface energy). It is defined as the work expended on creating a unit area of ​​the phase interface (the dimension of J / m 2). According to another definition, surface tension is the force per unit length of the contour that limits the interface (dimension N/m); this force acts tangentially to the surface and prevents its spontaneous increase.

Surface tension is the main thermodynamic characteristic of the surface layer of a liquid at the interface with the gas phase or another liquid. The surface tension of various liquids at the boundary with their own vapor varies over a wide range: from units for liquefied low-boiling gases to several thousand mN/m for molten refractory substances. Surface tension depends on temperature. For many one-component non-associated liquids (water, molten salts, liquid metals) far from the critical temperature, the linear dependence:

Surfactants (surfactants) - chemical compounds, which, concentrating on the interface, cause a decrease in surface tension.

The main quantitative characteristic of surfactants is surface activity - the ability of a substance to reduce surface tension at the phase boundary - this is the derivative of surface tension with respect to surfactant concentration as C tends to zero. However, the surfactant has a solubility limit (the so-called critical micelle concentration or CMC), reaching which, when the surfactant is added to the solution, the concentration at the phase boundary remains constant, but at the same time, the surfactant molecules self-organize in the bulk solution (micelle formation or aggregation). As a result of this aggregation, so-called micelles are formed. A distinctive feature of micelle formation is the turbidity of the surfactant solution. Aqueous solutions of surfactants, during micelle formation, also acquire a bluish tint (gelatinous tint) due to the refraction of light by micelles.

On this lesson will go about liquids and their properties. Liquids have a number of interesting properties and their manifestations. One such property will be discussed in this lesson.

In the world around us, along with gravity, elasticity and friction, there is another force that we usually pay little or no attention to. This force is relatively small, its action never causes impressive effects. However, we cannot pour water into a glass, we cannot do anything at all with any liquid without setting in motion the forces that we will talk about. These are surface tension forces.

The ability of a liquid to contract its surface is called surface tension.

surface tension force called the force that acts along the surface of the liquid perpendicular to the line limiting this surface, and tends to reduce it to a minimum.

The surface tension force is determined by the formula, the product of sigma and el. Where sigma is the surface tension coefficient, el is the length of the wetting perimeter.

Let us dwell on the concept of “coefficient of surface tension” in more detail.

Surface tension coefficient numerically equal to strength acting per unit length of the wetting perimeter and directed perpendicular to this perimeter.

Also, the coefficient of surface tension of a liquid is physical quantity, which characterizes a given liquid and is equal to the ratio of the surface energy to the surface area of ​​the liquid.

The molecules of the surface layer of a liquid have an excess of potential energy compared to the energy that these molecules would have if they were inside the liquid.

surface energy is the excess potential energy possessed by molecules on the liquid surface.

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" ones - water, mercury and liquid metals).

2. Temperature (the higher the temperature, the lower the surface tension).

3. The presence of surfactants that reduce surface tension (surfactants), such as soap or washing powder.

4. Properties of a gas adjoining a liquid.

Surface tension forces determine the shape and properties of liquid droplets, a soap bubble. These forces keep a steel needle and a water strider insect on the surface of the water, and keep moisture on the surface of the fabric.

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 so that the length of the thread is somewhat greater than the length of the chord connecting the points of attachment of the thread, and dip the wire ring in soapy water, the soap film will tighten the entire surface of the ring and the thread will lie on the soap film. If the film is now torn on one side of the thread, the soapy film remaining on the other side of the thread will shrink and stretch the thread. 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.

Consider an experiment confirming the desire of a liquid to reduce the surface of contact with air or vapor of this liquid.

An interesting experiment was carried out by the Belgian physicist Joseph Plateau. He argues that if a drop is in conditions where the main influence on its shape is exerted by surface tension forces, it takes the form with the smallest surface, that is, spherical.

The text of the work is placed without images and formulas.
Full version work is available in the "Files of work" tab in PDF format

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 the 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 can't pour water into a glass, we can't do anything with any liquid at all without bringing the surface tension forces into play. We are so used to effects called surface tension that we do not notice them. Surprisingly diverse are the manifestations of the surface tension of a liquid in nature and technology. They play an important role in nature and in our life. Without them, we could not write with helium pens, cartridge printers would immediately put a big blot, emptying their entire tank. It would be impossible to soap your hands - the foam would not form. A light rain would have soaked us through, and the rainbow would not have been visible in any weather. Surface tension collects water into droplets and thanks to surface tension, a soap bubble can be blown out. Using the Belgian professor Plato's rule for researchers to be surprised in time, we will consider unusual experiments in the work.

The purpose of the work: to experimentally check the manifestations of the surface tension of a liquid, to determine the coefficient of surface tension of liquids by the method of drop separation

Study educational, popular science literature, use materials on the Internet on the topic "Surface tension";

do experiments proving that the proper form of a liquid is a ball;

conduct experiments with decreasing and increasing surface tension;

to design and assemble an experimental setup with which to determine the surface tension coefficient of some liquids by the droplet separation method.

process the received data 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 such a form in which its free surface has the smallest area. In its tendency to shrink, the surface film would give the liquid 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 dew drops on the leaves of trees, on the grass are close in shape to a ball, with free fall raindrops are almost strictly spherical. The tendency of the liquid to shrink to the minimum possible can be observed in many phenomena that seem surprising. Even Galileo thought about the question: why do the dew drops that he saw in the morning on cabbage leaves take on a spherical shape? The statement that a liquid has no shape of its own turns out to be not entirely accurate. The proper form of a liquid is a sphere, as the most capacious form. Molecules of matter in liquid state located almost close to each other. Unlike solid crystalline bodies, in which the molecules form ordered structures throughout the volume of the crystal and can perform thermal vibrations around fixed centers, the 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 an adjacent 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. Because of strong interaction between closely spaced molecules, they can form local (unstable) ordered groups containing several molecules. one

Figure 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

And how can one explain the spontaneous contraction of the surface of a liquid? Molecules on the surface and in the depth of the liquid are in different conditions. Each molecule inside the liquid is affected by attractive forces from neighboring molecules surrounding it from 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 the 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 go 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, 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 on a shape in which the number of molecules on the surface is minimal, and the ball has the minimum surface for a given volume, that is, a liquid drop takes a shape close to spherical. The easiest way to catch the nature of surface tension forces is to observe the formation of a drop. Look carefully at how the drop gradually grows, a narrowing is formed - a neck - and the drop comes off. It doesn't take much imagination to imagine that the water is, as it were, 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 makes the same impression.

Experience #1

Friction of a liquid to a minimum of potential energy can be observed using soap bubbles. The 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 - the phenomenon of molecular pressure on a liquid, caused by the attraction of surface layer molecules to molecules inside the liquid 5

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 form of a ball. Reflecting on this fact, Plato outlined a series of experiments, which were subsequently brilliantly performed by his friends and students. In his diary, he wrote a rule for researchers: "Be surprised in time." I decided to explore the Plateau experiment, but in a different version: to use sunflower oil and tinted manganese water in the experiment.

An experiment proving that a homogeneous liquid takes a form with a minimum free surface

Plateau experience option #2

1) Pour sunflower oil into a beaker.

2) With an eyedropper, a drop of tinted manganese water with a diameter of approximately 5 mm was dropped into sunflower oil.

) Observed balls of water different size, slowly falling to the bottom and taking an oval flattened shape (Photo 2).

5) Observe how the drop takes the correct shape of the ball (Photo 2).

Output: The liquid, by attracting the molecules of the surface layer, compresses itself. The oval flattened shape is explained by the fact that the weight of a drop that does not mix with oil is greater than the buoyancy 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 buoyancy force.

In free fall, in a state of weightlessness, raindrops practically have the shape of a ball. IN spaceship a sufficiently large mass of liquid also assumes 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 - a force directed tangentially to the surface of the liquid, perpendicular to the section of the contour that bounds the surface, in the direction of its contraction.

Ơ - coefficient of surface tension - this is the ratio of the module F of the surface tension force acting on the boundary of the surface layer ℓ, to this length there is a constant value that does not depend on the length ℓ. The surface tension coefficient depends on the nature of the adjacent media and on the temperature. It is expressed in newtons per meter (N/m).

Experiments with decrease and increase

Photo 3

surface tension

Experience #3

Touch the center of the surface of the water with a bar of soap.

The foam pieces begin to move from the center to the edges of the vessel (Photo 3).

They dripped 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, greasy stains, soot, i.e. substances insoluble in water. Because of the rather high surface tension, water does not have a very good cleaning effect on its own. For example, when coming into contact with a stain, water molecules are attracted to each other more than to particles of insoluble dirt. Soaps and synthetic detergents(SMS) contain substances that reduce the surface tension of water. The first soap, the simplest detergent, was made in the Middle East over 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.

A man was saved from dirt and he became clean from a young age.

I'm telling you about the soap that soon gave birth: shampoo, gel, powder.

The world has become clean, how good!

Figure 5. F. Günther

Detergents are natural and synthetic substances with a cleansing effect, in particular soaps 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 roasted over a fire, and the fat flowed onto the ash, which has alkaline properties. Soap production has long history, but the first synthetic detergent (SMC) appeared in 1916, it was invented by a German chemist Fritz Günther for industrial purposes. Household SMS, more or less harmless to hands, began to be produced in 1933. Since then, a number of synthetic detergents (SMC) have been developed for narrow purposes, and their production has become an important branch of the chemical industry.

It is because of the surface tension that water does not have a sufficient cleaning effect on its own. When coming into contact with the stain, the water molecules 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.

Now the production of SMS has become an important branch of the chemical industry. These substances are called surfactant(surfactants), since they act on the surface of the liquid. Surfactant molecules can be represented as tadpoles. With their heads they "cling" to water, and with their "tails" for fat. When surfactants are mixed with water, their molecules on the surface are turned "heads" down, and "tails" out. By crushing the surface of the water in this way, these molecules greatly reduce the effect of surface tension, thereby helping the water to penetrate the tissue. With the same "tails" of surfactant molecules (Fig. 6) they capture fat molecules that come across to them. 2

Experience No. 4

1. Pour milk into a saucer so that it covers the bottom (Photo 4)

2. Dropped 2 drops of brilliant green on 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)

Output: the surface tension of the detergent is less than the brilliant green.

Experience No. 5

Water was poured into a wide glass vessel.

Styrofoam pieces were thrown on the surface.

Touched the center of the surface of the water with a piece of sugar.

Styrofoam tendrils begin to move from the edges of the vessel to the center (Photo 7).

Output: The surface tension of an aqueous solution of sugar is greater than clean water.

Experience No. 6

Removal of fatty stains from the surface of the tissue

We moistened a cotton wool with gasoline and moistened the edges of the stain (and not the stain itself) with this cotton wool. Gasoline reduces surface tension, so the fat is collected to the center of the stain and from there it can be removed, if the same cotton wool is moistened, the stain itself can increase in size due to a decrease in surface tension.

For experimental definition values ​​of the surface tension of the liquid, you can use the process of formation and detachment of drops flowing from the dropper.

Brief theory of the droplet 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 liquid droplets. Figure 1 shows photographs showing different stages of the process of drop formation and detachment. The photo 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 the surface of the drop to shrink, it tends to give the drop a spherical shape. Gravity forces the center of gravity of the drop as low as possible. As a result, the drop is elongated (Fig. 7a).

Rice. 7. a B C D

The process of formation and detachment of drops

The larger the drop, the greater the role played by the potential energy of gravity. As the drop grows, the main mass is collected at the bottom and a neck is formed near 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 a drop to increase quite a bit and the surface tension forces no longer balance the force of gravity. The neck of the drop rapidly 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 droplet weighing. 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 surface tension coefficient, the size and shape of the hole, and also on the outflow velocity. . With a slow outflow of the wetting liquid from 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 dimensions of the tube and the flow rate.

The moment of detachment, 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, one can write

Mg = 2πrơ (2)

Substituting the neck radius r from equality (1) and solving it, we obtain

Ơ=mg/2πkR (3)

To determine the mass of a drop, a number n of drops are weighed into a beaker of known weight. If the mass of the glass 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 method of drop separation"

Purpose of the study: to determine the coefficient of surface tension of a liquid by the method of detachment of 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 Frutis, « Pantene», "Schauma" And " frutis», shower gels Sensen», "Montpensier" 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 under study was installed. At the end of the burette, a tip-tube was fixed, at the end of which a drop forms. The drops were weighed in a special beaker.

Research progress

Using a caliper, the diameter of the tip-tube was measured three times and the average value of d was calculated.

A clean, dry glass (M 0) was weighed on a balance.

With the help of a burette tap, we achieved the speed of dripping

15 drops per minute.

60 drops of liquid were poured from the burette into a glass, counting exactly the number of drops poured.

Weighed a glass of liquid. (M)

Substituted the obtained values ​​into the formula ơ = ((M-M0)g)/πkdn

Calculate the coefficient of surface tension.

Tried three times

Calculate the average value of the coefficient of surface tension.

The surface tension coefficient in the SI system is measured in N/m.

Table #1

The results of determining the coefficient of surface tension (N/m)

Liquid	Surface tension coefficient
	measured	Tabular
Ethanol
Milk (2.5)
Milk (cow's homemade)
Powder solution "Myth"
Powder solution "Persil"
Detergent "Fairy"
Detergent "Aos"

Output: Of the studied kitchen detergents, with all other identical parameters affecting the quality of "laundering", it is better to use the " Fairy". From the studied washing powders " Myth”, because it is their solutions that have the lowest surface tension. Therefore, the first means (" Fairy”) better helps to wash off water-insoluble fats from dishes, being an emulsifier - a tool that facilitates the production of emulsions (suspensions of tiny particles of a liquid substance in water). The second (" Myth”) better washes clothes, penetrating into the pores between the fibers of 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 breaks down into tiny particles. During this time, it is recommended to wash off the applied detergent with a stream of clean water, and not to rinse the dishes after a while in the container. In addition, the surface tension of shampoos and shower gels was studied. Due to the rather high viscosity of these liquids, it is difficult to accurately determine their surface tension coefficient, but it can be compared. Shampoos have been tested (by tearing off drops) Pantene», "Schauma" And " frutis» as well as shower gels Sensen», "Montpensier" And " Discover».

Output:

Surface tension decreases in shampoos in a row Frutis - "Schauma" - "Pantene" in gels - in a row "Montpensier" - 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 that affect the quality of washing, it is better to use from the studied shampoos "Pantene" (Fig. 9), of the studied shower gels - "Senses" (Fig.10).

The tear-off method, although not very precise, is used in medical practice. This method determines for diagnostic purposes the surface tension of the cerebrospinal fluid, bile, etc.

Conclusion

1. Experimental confirmation of theoretical conclusions has been obtained , proving that a homogeneous liquid takes a form 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 surface tension force.

3. To determine the coefficient of surface tension of liquids

a) studied brief theory droplet separation method;

b) an experimental setup has been designed and assembled;

c) the average values ​​of the surface tension coefficient of various liquids are calculated, conclusions are drawn.

4. The results of experiments and research are presented in the form of a table and photographs.

Working on the project allowed me to acquire a broader knowledge of the physics section "Surface Tension".

I would like to end my project with the words of the great scientist 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 everything that exists and actively fight to grab even the smallest grain of reason that manifests itself in Nature”

List of used sources and literature

http://www.physics.ru/

http://greenfuture.ru/

http://www.agym.spbu.ru/

Bukhovtsev B.B., Klimontovich Yu.L., Myakishev G.Ya., Physics, textbook for grade 9 of secondary school - 4th edition - M .: Education, 1988 - 271 p.

Kasyanov V.A., Physics, grade 10, textbook for general education educational institutions, M.: Bustard, 2001 - 410 p.

Pinsky A.A. Physics: textbook. Allowance for 10 classes with in-depth study physics. M.: Enlightenment, 1993. - 416 p.

Yufanova I.L. Entertaining evenings in physics in high school: a book for the teacher. - M.: Enlightenment, 1990. -215s

Chuyanov V.Ya., Encyclopedic Dictionary of a Young Physicist, M .: Pedagogy, 1984. - 350 s.

1 1 http://www.physics.ru/

2 http://greenfuture.ru

Liquid is state of aggregation substance intermediate between gaseous and solid, therefore it has the properties of both gaseous and solids. 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, therefore, 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 IN, 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 through kinetic energy 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.

Since the equilibrium state is characterized by a minimum of 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, 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 solid body more 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 forces of attraction between the molecules of the liquid and the solid are 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 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 . Injection 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 σ 13 >σ 12 , then cos θ >0 and angle θ sharp, 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 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 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 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 the components of mixtures, not only the contact angles for mixture-surface pairs are taken into account, but also the surface-active properties of liquid components.