Change in surface tension. Lesson on "surface tension". Manifestations of surface tension forces

Concept of surface tension

Surface tension is called the thermodynamic characteristic of the interface, defined as the work of reversible isothermal formation of a unit area of ​​this surface. For a liquid, surface tension is considered as a force acting per unit length of the surface contour and tending to reduce the surface to a minimum for given phase volumes.

Oil is an oil dispersed system consisting of a dispersed phase and a dispersion medium.

The surface of a dispersed phase particle (for example, an asphaltene associate, a water globule, etc.) has some excess free surface energy F s, proportional to the interface area S:

Magnitude σ can be considered not only as specific surface energy, but also as a force applied per unit length of the contour limiting the surface, directed along this surface perpendicular to the contour and tending to tighten or reduce this surface. This force is called surface tension.

The action of surface tension can be visually represented as a set of forces that pull the edges of the surface towards the center.

The length of each vector arrow reflects the magnitude of surface tension, and the distance between them corresponds to the accepted unit of surface contour length. As a dimension of the quantity σ both [J/m 2 ] = 10 3 [erg/cm 2 ] and [N/m] = 10 3 [dyne/cm] are used equally.

As a result of the action of surface tension forces, the liquid tends to reduce its surface, and if the influence of the force of gravity is insignificant, the liquid takes the shape of a sphere with a minimum surface area per unit volume.

Surface tension varies for different groups of hydrocarbons - maximum for aromatics and minimum for paraffinics. As the molecular weight of hydrocarbons increases, it increases.

Most heteroatomic compounds, having polar properties, have a surface tension lower than hydrocarbons. This is very important, since their presence plays a significant role in the formation of water-oil and gas-oil emulsions and in the subsequent processes of destruction of these emulsions.

Parameters affecting surface tension

Surface tension significantly depends on temperature and pressure, as well as on the chemical composition of the liquid and the phase in contact with it (gas or water).

With increasing temperature, surface tension decreases and at the critical temperature is zero. As pressure increases, surface tension in the gas-liquid system also decreases.

The surface tension of petroleum products can be found by calculation using the equation:

Recalculation σ from one temperature T0 to another T can be carried out according to the relationship:

Surface tension values ​​for some substances.

Substances whose addition to a liquid reduces its surface tension are called surfactants(surfactant).

The surface tension of oil and petroleum products depends on the amount of surface-active components present in them (resinous substances, naphthenic and other organic acids, etc.).

Petroleum products with a low content of surface-active components have the highest surface tension at the interface with water, while those with a high content have the lowest.

Well-refined petroleum products have high surface tension at the interface with water.

The decrease in surface tension is explained by the adsorption of surfactants at the interface. With increasing concentration of the added surfactant, the surface tension of the liquid first intensively decreases and then stabilizes, which indicates complete saturation of the surface layer with surfactant molecules. Natural surfactants that sharply change the surface tension of oils and petroleum products are alcohols, phenols, resins, asphaltenes, and various organic acids.

Surface forces at the interface between solid and liquid phases are associated with wetting and capillary phenomena, on which the processes of oil migration in formations, the rise of kerosene and oil along the wicks of lamps and oil cans, etc. are based.

Experimental determination of surface tension

Various methods are used to experimentally determine the surface tension of oils and petroleum products.

The first method (a) is based on measuring the force required to separate the ring from the interface between the two phases. This force is proportional to twice the circumference force of the ring. With the capillary method (b), the height of the rise of liquid in the capillary tube is measured. Its disadvantage is the dependence of the height of liquid rise not only on the value of surface tension, but also on the nature of wetting of the capillary walls with the liquid under study. A more accurate version of the capillary method is the hanging drop method (c), based on measuring the mass of a drop of liquid coming off a capillary. The measurement results are affected by the density of the liquid and the size of the drop and are not affected by the contact angle of the liquid on the solid surface. This method allows the determination of surface tension in pressure vessels.

The most common and convenient way to measure surface tension is the method of the highest pressure of bubbles or drops (g), which is explained by the simplicity of the design, high accuracy and independence of the determination from wetting.

This method is based on the fact that when squeezing an air bubble or a drop of liquid from a narrow capillary into another liquid, surface tension σ at the boundary with the liquid into which the drop is released, in proportion to the highest pressure required to squeeze out the drop.

Main part.

To understand the basic properties and patterns of the liquid state of a substance, it is necessary to consider the following aspects:

Structure of liquid. Movement of liquid molecules.

A liquid is something that can flow.

The so-called short-range order is observed in the arrangement of liquid particles. This means that with respect to any particle, the location of its nearest neighbors is ordered.

However, as you move away from a given particle, the arrangement of other particles in relation to it becomes less and less ordered, and quite quickly the order in the arrangement of particles completely disappears.

Liquid molecules move much more freely than solid molecules, although not as freely as gas molecules.

Each molecule of liquid moves here and there for some time, without moving away, however, from its neighbors. But from time to time, a liquid molecule breaks out of its environment and moves to another place, ending up in a new environment, where again for some time it performs movements similar to vibration. Significant achievements in the development of a number of problems in the theory of the liquid state belong to the Soviet scientist Ya. I. Frenkel.

According to Frenkel, thermal motion in liquids has the following character. Each molecule oscillates around a certain equilibrium position for some time. From time to time, a molecule changes its place of equilibrium, moving abruptly to a new position, separated from the previous one by a distance of the order of the size of the molecules themselves. That is, the molecules only move slowly inside the liquid, staying part of the time near certain places. Thus, the movement of liquid molecules is something like a mixture of movements in a solid and in a gas: oscillatory movement in one place is replaced by a free transition from one place to another.

Fluid pressure

Everyday experience teaches us that liquids act with known forces on the surface of solid bodies in contact with them. These forces are called fluid pressure forces.

When we cover the opening of an open water tap with our finger, we feel the pressure of the liquid on our finger. The ear pain experienced by a swimmer who has dived to great depths is caused by the forces of water pressure on the eardrum. Thermometers for measuring temperature in the deep sea must be very durable so that water pressure cannot crush them.

Pressure in a liquid is caused by a change in its volume - compression. Liquids are elastic in relation to changes in volume. Elastic forces in a liquid are pressure forces. Thus, if a liquid acts with pressure forces on bodies in contact with it, this means that it is compressed. Since the density of a substance increases during compression, we can say that liquids have elasticity with respect to changes in density.

The pressure in a liquid is perpendicular to any surface placed in the liquid. The pressure in the liquid at depth h is equal to the sum of the pressure on the surface and a value proportional to the depth:

Due to the fact that liquids can transmit static pressure, almost no less than their density, they can be used in devices that provide an advantage in strength: a hydraulic press.

Archimedes' Law

Pressure forces act on the surface of a solid body immersed in a liquid. Since pressure increases with depth of immersion, the pressure forces acting on the lower part of the liquid and directed upward are greater than the forces acting on the upper part and directed downward, and we can expect that the resultant of the pressure forces will be directed upward. The resultant of the pressure forces on a body immersed in a liquid is called the supporting force of the liquid.

If a body immersed in a liquid is left to its own devices, it will sink, remain in equilibrium, or float to the surface of the liquid, depending on whether the supporting force is less than, equal to, or greater than the force of gravity acting on the body.

Archimedes' law states that a body in a liquid is subjected to an upward buoyancy force equal to the weight of the displaced liquid. A body immersed in a liquid is subject to a buoyant force (called the Archimedes force)

where ρ is the density of the liquid (gas), is the acceleration of free fall, and V- the volume of the submerged body (or the part of the volume of the body located below the surface).

If a body immersed in a liquid is suspended from a scale, then the scale shows the difference between the weight of the body in the air and the weight of the displaced liquid. Therefore, Archimedes' law is sometimes given the following formulation: a body immersed in a liquid loses as much in its weight as the weight of the liquid displaced by it.

It is interesting to note such an experimental fact that, being inside another liquid of greater specific gravity, the liquid, according to Archimedes’ law, “loses” its weight and takes on its natural, spherical shape.

Evaporation

In the surface layer and near the surface of the liquid, forces act that ensure the existence of the surface and do not allow molecules to leave the volume of the liquid. Due to thermal motion, some of the molecules have high enough speeds to overcome the forces holding the molecules in the liquid and leave the liquid. This phenomenon is called evaporation. It is observed at any temperature, but its intensity increases with increasing temperature.

If the molecules that have left the liquid are removed from the space near the surface of the liquid, then eventually all the liquid will evaporate. If the molecules that have left the liquid are not removed, they form steam. Vapor molecules that enter the area near the surface of the liquid are drawn into the liquid by attractive forces. This process is called condensation.

Thus, if molecules are not removed, the evaporation rate decreases with time. With a further increase in vapor density, a situation is reached where the number of molecules leaving the liquid in a certain time will be equal to the number of molecules returning to the liquid in the same time. A state of dynamic equilibrium occurs. Vapor in a state of dynamic equilibrium with liquid is called saturated.

With increasing temperature, the density and pressure of saturated vapor increase. The higher the temperature, the more liquid molecules have enough energy to evaporate, and the greater the vapor density must be for condensation to equal evaporation.

Boiling

When, when heating a liquid, a temperature is reached at which the saturated vapor pressure is equal to the external pressure, equilibrium is established between the liquid and its saturated vapor. When an additional amount of heat is imparted to the liquid, the corresponding mass of liquid immediately transforms into steam. This process is called boiling.

Boiling is the intense evaporation of a liquid, occurring not only from the surface, but throughout its entire volume, inside the resulting vapor bubbles. To change from liquid to vapor, molecules must acquire the energy necessary to overcome the attractive forces holding them in the liquid. For example, to evaporate 1 g of water at a temperature of 100 ° C and a pressure corresponding to atmospheric pressure at sea level, it is necessary to spend 2258 J, of which 1880 are used to separate molecules from the liquid, and the rest are used to increase the volume occupied by the system, against forces of atmospheric pressure (1 g of water vapor at 100 ° C and normal pressure occupies a volume of 1.673 cm 3, while 1 g of water under the same conditions - only 1.04 cm 3).

The boiling point is the temperature at which the saturated vapor pressure becomes equal to the external pressure. As pressure increases, the boiling point increases, and as pressure decreases, it decreases.

Due to the change in pressure in the liquid with the height of its column, boiling at different levels in the liquid occurs, strictly speaking, at different temperatures. Only saturated steam above the surface of a boiling liquid has a certain temperature. Its temperature is determined only by external pressure. This is the temperature that is meant when we talk about the boiling point.

The boiling points of various liquids differ greatly from each other, and this is widely used in technology, for example, in the distillation of petroleum products.

The amount of heat that must be supplied in order to isothermally convert a certain amount of liquid into vapor, at an external pressure equal to the pressure of its saturated vapor, is called the latent heat of vaporization. This value is usually referred to as one gram, or one mole. The amount of heat required for isothermal evaporation of a mole of liquid is called the molar latent heat of vaporization. If this value is divided by the molecular weight, the specific latent heat of vaporization is obtained.

Surface tension of a liquid

The property of a liquid to reduce its surface to a minimum is called surface tension. Surface tension is a phenomenon of molecular pressure on a liquid caused by the attraction of molecules in the surface layer to molecules inside the liquid. On the surface of a liquid, molecules experience forces that are not symmetrical. On average, a molecule located inside a liquid is subject to a force of attraction and adhesion from its neighbors evenly on all sides. If the surface of the liquid is increased, the molecules will move against the holding forces. Thus, the force tending to contract the surface of the liquid acts in the opposite direction to the external force stretching the surface. This force is called surface tension and is calculated by the formula:

Surface tension coefficient()

Liquid surface boundary length

Please note that easily evaporating liquids (ether, alcohol) have less surface tension than non-volatile liquids (mercury). The surface tension of liquid hydrogen and, especially, liquid helium is very low. In liquid metals, surface tension, on the contrary, is very high. The difference in surface tension of liquids is explained by the difference in the adhesive forces of different molecules.

Measurements of the surface tension of a liquid show that surface tension depends not only on the nature of the liquid, but also on its temperature: with increasing temperature, the difference in liquid densities decreases, and therefore the surface tension coefficient - decreases.

Due to surface tension, any volume of liquid tends to reduce its surface area, thus reducing its potential energy. Surface tension is one of the elastic forces responsible for the movement of ripples in water. In bulges, surface gravity and surface tension pull water particles down, trying to make the surface smooth again.

Liquid films

Everyone knows how easy it is to get foam from soapy water. Foam is a set of air bubbles bounded by a thin film of liquid. A separate film can easily be obtained from a foam-forming liquid.

These films are very interesting. They can be extremely thin: in the thinnest parts their thickness does not exceed a hundred thousandth of a millimeter. Despite their thinness, they are sometimes very resistant. The soap film can be stretched and deformed, and a stream of water can flow through the soap film without destroying it.

How can we explain the stability of films? An indispensable condition for the formation of a film is the addition of substances dissolving in it to a clean liquid, moreover, those that greatly reduce the surface tension

In nature and technology, we usually encounter not individual films, but a collection of films - foam. You can often see in streams, where small streams fall into calm water, abundant formation of foam. In this case, the ability of water to foam is associated with the presence of a special organic substance in the water, released from the roots of plants. Construction equipment uses materials that have a cellular structure, such as foam. Such materials are cheap, lightweight, do not conduct heat and sound well, and are quite durable. To make them, substances that promote foaming are added to the solutions from which building materials are formed.

Wetting

Small drops of mercury placed on a glass plate take on a spherical shape. This is the result of molecular forces tending to reduce the surface of the liquid. Mercury placed on the surface of a solid does not always form round droplets. It spreads over the zinc plate, and the total surface of the droplet will undoubtedly increase.

A drop of aniline also has a spherical shape only when it does not touch the wall of the glass vessel. As soon as it touches the wall, it immediately sticks to the glass, stretching across it and acquiring a large total surface.

This is explained by the fact that in the case of contact with a solid body, the adhesion forces between liquid molecules and solid molecules begin to play a significant role. The behavior of a liquid will depend on which is greater: the cohesion between liquid molecules or the cohesion of a liquid molecule with a solid molecule. In the case of mercury and glass, the adhesive forces between the mercury and glass molecules are small compared to the adhesive forces between the mercury molecules, and the mercury collects into a drop.

This liquid is called non-wetting solid. In the case of mercury and zinc, the cohesive forces between the molecules of the liquid and the solid exceed the cohesive forces acting between the molecules of the liquid, and the liquid spreads over the solid. In this case the liquid is called wetting solid.

It follows that when speaking about the surface of a liquid, we must mean not only the surface where the liquid borders air, but also the surface bordering other liquids or a solid body.

Depending on whether the liquid wets the walls of the vessel or does not, the shape of the surface of the liquid at the point of contact with the solid wall and gas has one form or another. In the case of non-wetting, the shape of the liquid surface at the edge is round and convex. When wetted, the liquid at the edge takes on a concave shape.

Capillary phenomena

In life, we often deal with bodies penetrated by many small channels (paper, yarn, leather, various building materials, soil, wood). When such bodies come into contact with water or other liquids, they often absorb them. This is the basis for the action of a towel when drying hands, the action of a wick in a kerosene lamp, etc. Similar phenomena can also be observed in narrow glass tubes. Narrow tubes are called capillary or hair tubes.

When such a tube is immersed at one end into a wide vessel in a wide vessel, the following happens: if the liquid wets the walls of the tube, then it will rise above the level of the liquid in the vessel and, moreover, the higher the narrower the tube; if the liquid does not wet the walls, then, on the contrary, the liquid level in the tube is set lower than in a wide vessel. The change in the height of the liquid level in narrow tubes or gaps is called capillarity. In a broad sense, capillary phenomena mean all phenomena caused by the existence of surface tension.

The height of liquid rise in capillary tubes depends on the radius of the channel in the tube, surface tension and density of the liquid. Between the liquid in the capillary and in the wide vessel, such a level difference h is established so that the hydrostatic pressure rgh balances the capillary pressure:

where s is the surface tension of the liquid

R is the radius of the capillary.

The height of the liquid rising in a capillary is proportional to its surface tension and inversely proportional to the radius of the capillary channel and the density of the liquid (Jurin’s law)

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 – dull, i.e. the liquid does not wet the solid surface.

The contact angle satisfies condition (10.3) if

(σ 13 - σ 12)/σ 23 ≤1.

If the condition is not met, then a drop of liquid at any value θ cannot be in balance. If σ 13 >σ 12 +σ 23, then the liquid spreads over the surface of the solid, covering it with a thin film (for example, kerosene on the surface of glass), – this occurs complete wetting(in this case θ =0).

If σ 12 >σ 13 +σ 23, then the liquid contracts into a spherical drop, in the limit having only one point of contact with it (for example, a drop of water on the surface of paraffin), - complete non-wetting(in this case θ =π).

Wetting and non-wetting are relative concepts, i.e. a liquid that wets one solid surface does not wet another. For example, water wets glass, but does not wet paraffin; Mercury does not wet glass, but it does wet clean metal surfaces.

The phenomena of wetting and non-wetting are of great importance in technology. For example, in the method of flotation beneficiation of ore (separation of ore from waste rock), it, finely crushed, is shaken in a liquid that wets the waste rock and does not wet the ore. Air is blown through this mixture and then it settles. In this case, rock particles moistened with liquid sink to the bottom, and grains of minerals “stick” to air bubbles and float to the surface of the liquid. When machining metals, they are moistened with special liquids, which facilitates and speeds up surface treatment.

In construction, the phenomenon of wetting is important for the preparation of liquid mixtures (putty, putty, mortars for bricklaying and concrete preparation). It is necessary that these liquid mixtures well wet the surfaces of the building structures to which they are applied. When selecting mixture components, not only the contact angles for mixture-surface pairs are taken into account, but also the surface-active properties of the liquid components.

This lesson will discuss 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, to which we usually pay little or no attention. 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 bringing into play the forces that we will discuss. These are surface tension forces.

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

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

The force of surface tension 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 in more detail on the concept of “surface tension coefficient”.

The surface tension coefficient is numerically equal to the force acting per unit length of the wetting perimeter and directed perpendicular to this perimeter.

Also, the 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 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– excess potential energy possessed by molecules on the surface of a liquid.

The coefficient of surface tension is measured in newtons divided by meter.

Let's discuss what the coefficient of surface tension of a liquid depends on. To begin with, let us remember that the surface tension coefficient characterizes the specific interaction energy of molecules, which means that 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 ("volatile" liquids, such as ether, alcohol and gasoline, have less surface tension than "non-volatile" liquids - water, mercury and liquid metals).

2. Temperatures (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 gas bordering liquid.

Surface tension forces determine the shape and properties of liquid droplets and soap bubbles. These forces hold the steel needle and the water strider insect on the surface of the water and retain 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 a wire ring in two places, so that the length of the thread is slightly greater than the length of the chord connecting the points of attachment of the thread, and the wire ring is dipped in a soap solution, the soap film will cover the entire surface of the ring and the thread will lie on the soap film. If you now tear the film on one side of the thread, the soap film remaining on the other side of the thread will contract and tighten 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 upward.

Let us 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 states that if a drop is in conditions where the main influence on its shape is exerted by surface tension forces, it takes the shape with the smallest surface area, that is, spherical.

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

Steps

Using 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- length of the needle used in the experiment. Let us express 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. Construct a small rocker arm. In this experiment, a rocker and a small needle that floats on the surface of the water are used to determine surface tension. It is necessary to carefully consider the construction of the rocker, since the accuracy of the result depends on this. You can use various materials, the main thing is to make a horizontal crossbar from something hard: wood, plastic or thick cardboard.

   • Locate the center of the rod (such as a straw or plastic ruler) that you intend to use as the crossbar and drill or poke a hole at that location; this will be the fulcrum of the crossbar on which it will rotate freely. If you are using a plastic straw, simply poke it with a pin or nail.
   • Drill or poke holes at the ends of the crossbar so that they are the same distance from the center. Thread threads through the holes to hang the weight cup and needle.
   • If necessary, support the rocker arm with books or other sufficiently hard objects to keep the crossbar horizontal. It is necessary that the crossbar rotates freely around a nail or rod inserted into its middle.

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

   • Hang a 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 bar so that it is horizontal. Tie a needle or paper clip horizontally to the thread that hangs from the other end of the crossbar. For the experiment to be successful, it is necessary to position the needle or paper clip exactly horizontally.

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

   • This is called balancing.

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

   • 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 drop of water to the aluminum saucer on the rocker arm. In this case, it is necessary to know the exact weight at which the needle will come off the surface of the water.

   • 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.
   • Let's say 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, to the aluminum foil saucer until the pin lifts off the surface of the water. Gradually add one pin or drop of water at a time. Watch the needle carefully so as not to miss the moment when, after the next increase in the load, it comes off the water. Once the needle leaves the surface of the liquid, stop adding pins or drops of water.

   • Count the number of pins or drops of water before the needle at the opposite end of the bar breaks away from the surface of the water.
   • Write down the result.
   • Repeat the experiment several (5 or 6) times to get more accurate results.
   • Calculate the average 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 that was required to lift the needle from the surface of the water. Since you calculated the weight of the pins in the previous step, to determine the force, simply multiply that weight by 0.00981 N/g.

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

9. Substitute the resulting values ​​into the equation and find the desired value. Using the results obtained during the experiment, surface tension can be determined. Simply plug in the values ​​found and calculate the result.

   • Let's say that in the example above, the length of the needle is 0.025 meters. We substitute the values ​​into the equation and 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.