EntranceThe concept of surface energy and surface tension. Fundamental research The surface tension energy of a liquid layer is directly proportional to
1The issue of predicting and forming the friction coefficient in movable detachable joints is considered. As a tool for controlling the value of the required friction coefficient, energy “pumped” into the surface layer of the mating surfaces is proposed. Saturation of the surface layer with energy occurs during the implementation of the technological process of manufacturing a specific part at all stages of production, from the creation of a workpiece to the finishing operation. It is assumed that to obtain lowest value To determine the friction coefficient of mating surfaces, a minimum difference in their surface layer energies is required. Friction control using technological influence methods will allow us to get closer to solving the problem of smooth movement of contacting surfaces and positioning accuracy. The proposed approach will allow us to improve modern product designs and avoid significant economic costs.
surface layer energy
friction coefficient
technological impact
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4. Musokhranov M.V. Technological assurance of the quality of the surface layer of mechanical engineering guide elements: diss. Ph.D. tech. Sciences – M. 2006. – P. 65.
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The presence in the reference literature of standardized data for various cases of arrangement of mating parts, it would seem, completely solves the problem of their operation. At the same time, it becomes obvious that in this case, when designing the surfaces of machine parts, both rubbing and constituting stationary pairs of mating parts, are characterized very approximately. The effects of technological impact, as well as many natural properties of kinematic pairs, are not taken into account at all. The energy of surface layers is, at best, only stated, but not used in practice. As a result, there may be cases of design errors and, as a result, certain economic costs. Determining such costs requires special calculations in relation to individual pairs of parts and machines as a whole.
Influence of surface layer energy on friction coefficient
In the overwhelming majority of cases, for the functioning of friction pairs, they try to create structures with a relatively low friction coefficient. This usually leads to some improvement in designs. In other cases, a relatively high coefficient of friction is required, which can be called a coefficient of adhesion with a value greater than one, and then the guide element plays a special role in determining the position of the part in space. Thus, the design becomes more perfect.
The situation with the use of the friction coefficient in the general view conditionally presented in Fig. 1. The coefficient f1, taken from reference literature, is most often used. At the same time, the actual friction coefficient may turn out to be equal to f1′, since the influence of microgeometry and energy of the surface layer will certainly manifest itself. In another case, for the same reasons, the friction coefficient may be equal to f1″. But the designer is confident that in his case the coefficient f1 applies.
Determining the actual values of friction coefficients, as well as ensuring their values by technological methods to the required ones, is a priority task modern technology mechanical and instrument engineering.
Rice. 1. Conventional representation of the use of friction coefficients. A typical case is when the coefficients fluctuate in the interval Δ: Δ= f1″ - f1′
For kinematic pairs of machines, it is important to constantly reduce friction coefficients in practice. The need for this is determined by the following circumstances:
Reducing the coefficients invariably leads to economic benefits, which in turn has a beneficial effect on the values of the efficiency coefficients of products;
Numerical determination of economic benefits does not cause difficulties. This was especially clearly noted during the creation of the bearing industry, when sliding friction was replaced by rolling friction;
The problem of smooth movement of contacting parts. The very nature of the profile of the rubbing surfaces already predetermines the presence of so-called intermittent oscillations of the elements in their relative movement. Any movement is always uneven, spasmodic. Reducing the friction coefficient values will invariably increase the uniformity of movement;
Requirements for positioning accuracy. In particular, this applies to the whole direction in electrical vacuum engineering. At the same time, studies devoted to friction coefficients both in ordinary atmosphere and in vacuum are of great interest. Positioning accuracy is one of the most important indicators of the quality of computer-controlled machines used not only in mechanical engineering.
The synergistic approach to the collision of two microprotrusions of contacting surfaces requires special consideration. However, even here, considering one’s own collision as a bifurcation, one can use the friction coefficient as a kind of tool for the formation of a post-bifurcation self-organizing space. Such a process can be extended to the entire surface of contacting parts.
Traditionally, contact is considered as a result of the engagement and deformation of mutually embedded roughnesses (microroughnesses) of two mating surfaces. According to this hypothesis, the lower the roughness, i.e., the more thoroughly the rubbing surfaces are processed, the lower the friction coefficient will be.
This point of view fits very well in the minds of designers and technologists. However, in the light of considering the issue of contacting parts, it is necessary to focus on the diagram in Fig. 2. With relative movement of parts Dg and the presence of force Q, elastoplastic states arise at the places of microcontacts of roughness. Deformation of microsurfaces almost always occurs, despite the fact that angles β (according to the diagram in Fig. 2) are small and do not exceed almost 35...40°, depending on the processing method. One pair of conjugate microprotrusions is very conventionally shown in a deformed form.
Rice. 2. Scheme of interaction of microroughnesses of contacting parts
The manifestation of the energy field is shown conventionally with arrows so that each mating part transfers the corresponding portion of energy to the other part. In turn, another part, which also has energy potential, transfers the energy of the first part. The mutual transfer of energies is shown schematically by solid and dotted arrows. It is obvious that both the deformation of microprotrusions and the transfer of energy always occur, even when there is a small gap between the contacting surfaces. Experience shows that with very smooth, “cleanly” polished surfaces, the friction forces not only do not decrease, but increase significantly. An example is the sticking effect when connecting gauge blocks.
The assumed dependence of the energy state and the friction coefficient can be shown by an example where the moment of jamming of two rubbing surfaces of the same roughness Ra = 0.08 μm, the value of which was obtained by different technological operations, was studied. In Fig. 3. The first column of the diagram illustrates that when grinding two contacting surfaces with the periphery of a wheel along the intended movement, the force Q at which the jamming moment occurs is 1.8 MPa. The second column of the diagram illustrates the moment of sticking when the ground surface comes into contact with the scraped surface. The third - two scraped surfaces. The fourth is the polished surface and the surface processed by lapping. Fifth - two ground surfaces. The sixth is ground in and scraped. V - direction of movement speed. The diagram shows that Q changes more than 3 times. Its change is due different meanings friction coefficient. Since the roughness of all surfaces is identical, they apparently have different levels of energy that they received from the technological impact.
Rice. 3. Effect of processing method on the moment of jamming
Thus, based on all of the above, contact must be considered not only as a result of the engagement of microprotrusions, but also taking into account the forces of energetic interaction that manifest themselves during the interaction of two surfaces. For very small gaps and distances between parts, several surface molecules of mating parts exchange accumulated energy more intensively, thereby changing the nature of the interaction - the friction coefficient. This hypothesis can probably more fully explain the nature and cause of friction that occurs as a result of the interaction of carefully processed surface layers of mechanical engineering parts.
The appearance of various mathematical dependencies has the positive significance that it makes it possible to link together the parameters of solid state physics, contrary to both design and technology. At the same time, it is obvious that the use of the presented scientific data directly in practice is difficult due to the lack numerical values at the disposal of enterprises. It is necessary to continue the work begun, as well as the creation of a methodology for determining the energy components of friction coefficients, primarily for precision engineering.
Of great scientific interest is the process of energy transfer, which depends on the gap between microprotrusions and plastic deformation, leading to intense energy exchange in the contact zones of mating parts. This usually occurs to the greatest extent on surfaces whose roughness parameters are 0.1< Ra < 2,5 мкм, а радиусы кривизны микронеровностей 30-670 мкм, толщина деформированного слоя 17-58 мкм. И вероятно, обмен энергией идет по принципу перетекания ее из «объемов» с большим количеством - в меньшие.
Conclusion
Consequently, to create the smallest coefficient of friction, it is necessary, as follows from the above, that the difference in the energy values of the rubbing pair be minimal. The best option is when the energies of the parts are the same and the difference is zero.
Reviewers:Astakhov M.V., Doctor of Technical Sciences, Professor, Head of the Department of Applied Mechanics, Kaluga branch of the Federal State Budgetary Educational Institution VPO "Moscow State Technical University named after. N.E. Bauman", Kaluga;
Shatalov V.K., Doctor of Technical Sciences, Professor, Head of the Department of Materials Processing Technologies, Kaluga Branch of the Moscow State Technical University. N.E. Bauman", Kaluga.
The work was received by the editor on February 12, 2015.
Bibliographic link
Musokhranov M.V., Kalmykov V.V., Malyshev E.N., Zenkin N.V. ENERGY OF THE SURFACE LAYER OF METALS AS A TOOL OF INFLUENCE ON THE AMOUNT OF FRICTION COEFFICIENT // Fundamental Research. – 2015. – No. 2-2. – P. 251-254;URL: http://fundamental-research.ru/ru/article/view?id=36797 (access date: 07/14/2019). We bring to your attention magazines published by the publishing house "Academy of Natural Sciences"
When water from an overturned glass spills on the floor or when we blow a soap bubble, the surface of the liquid increases. In this case, new areas of the rarefied surface layer appear. The average distance between molecules when they move from the depth of a liquid to its surface increases. The forces of attraction between the molecules of the liquid do negative work. In accordance with the laws of mechanics, this means an increase in the potential energy of molecules moving from the depths of the liquid to the surface.
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.
The excess potential energy possessed by molecules on the surface of a liquid is called surface energy.
From a macroscopic (thermodynamic) point of view, surface energy is one of the types internal energy, absent in gases, but present in liquids*.
*Solid bodies also have surface energy. After all, the special conditions in which molecules are found on the surface of a liquid are also characteristic of the surface of solids.
When water spreads from an overturned glass on the floor, the energy of the surface layer molecules increases due to the work of gravity. And when blowing a soap bubble, the potential energy of the surface layer molecules increases due to the work of air pressure forces in the bubble. After all, in order for a bubble to inflate, the air pressure in it must be greater than atmospheric pressure.
Surface tension
Molecules in all areas of the surface layer of a liquid are in the same conditions, and two areas of the same area have the same surface energy. It means that surface energy is directly proportional to the surface area of the liquid. Therefore, the surface energy ratio U n section of the surface of the liquid to the area S of this section is a constant value that does not depend on the area S. This quantity is called the surface tension coefficient or simply surface tension and is denoted by the letter σ:
Surface tension is specific surface energy, i.e., the energy per unit surface area.
In SI, surface tension is expressed in joules per square meter (J/m2). Since 1 J = 1 N m, surface tension can be expressed in newtons per meter (N/m).
Surface tension a depends on the nature of the surrounding media and on temperature. As the temperature rises, the difference between the liquid and its saturated vapor gradually disappears and at a critical temperature disappears completely. Accordingly, the surface tension for the liquid-saturated vapor interface decreases with increasing temperature and becomes equal to zero at the critical temperature.
From formula (7.3.1) it follows that
(7.3.2)
Therefore, as the surface area decreases, the surface energy decreases. In this case, molecular forces perform positive work, since the distances between molecules decrease when they move from the surface layer into the depths of the liquid. In a state of fluid equilibrium, the surface energy has a minimum value. This corresponds to the minimum surface area for a given volume. Therefore, as discussed in § 7.1, a liquid takes the shape of a sphere if there are no other forces distorting its natural spherical shape.
The surface layer of a liquid stores energy directly proportional to the surface area. Surface energy- one of the forms of internal energy.
Surface tension of a liquid.
Surface layer of liquid.
All liquids and solids are limited by an outer surface at which they come into contact with phases of a different composition and structure, for example, vapor, another liquid or a solid.
Properties of matter in this interfacial surface, several diameters of atoms or molecules thick, differ from the properties inside the phase volume.
Inside the volume of a pure substance in a solid, liquid or gaseous state, any molecule is surrounded by similar molecules.
In the boundary layer, molecules are in interaction with another number of molecules (different in comparison with the interaction inside the volume of the substance). This happens, for example, at the interface of a liquid with its vapor.
The average value of the resultant molecular forces of attraction applied to a molecule located inside a liquid is close to zero. In the figure below this molecule is labeled M1.
Random fluctuations of this resultant cause the molecule M1 perform only chaotic movement within the liquid.
The situation is different with molecules located in the surface layer of liquid.
Consider a molecule located directly at the interface. In the figure we denote it M2.
If there are molecules around M2 describe the sphere of molecular action, then inside this sphere there will be centers of many other molecules that will interact with our molecule. The radius of such a sphere is approximately 10 -9 m.
For a molecule M2 in the lower hemisphere there will be many molecules, and in the upper hemisphere there will be much less, since there is liquid below and steam or air above.
Therefore for a molecule M2 resultant of molecular forces of attraction in the lower hemisphere much more resultant of molecular forces in the upper hemisphere. The forces acting in the upper hemisphere are so small that they can be neglected.
If we consider another molecule, which, in comparison with M2 will be a little more “recessed” into the liquid, but also located in the surface layer. Let's denote it M3.
Since in the upper hemisphere M3 there will be other liquid molecules, they will attract M3 towards itself and partially balance the forces of attraction of molecules located in the lower hemisphere M3.
As a result, the total resultant of the forces acting on M3 will be less than the total resultant M2.
Both resultants will be directed into the liquid perpendicular to its surface.
Thus, all molecules of the liquid located in the surface layer with a thickness equal to the radius of molecular action are drawn into the liquid.
But the space inside the liquid is occupied by other molecules, so the surface layer creates pressure on the liquid, which is called molecular pressure.
Energy of the surface layer of a liquid.
Since the molecules of a liquid located in its surface layer are drawn into the liquid, their potential energy is greater than that of the molecules inside the liquid.
This additional potential energy of the molecules of the surface layer of the liquid is called free energy. Due to it, work can be done associated with a decrease in the free surface of the liquid.
And, conversely, in order to bring the molecules located inside the liquid to its surface, it is necessary to overcome the opposition of molecular forces, i.e. produce the work needed to increase the free energy of the surface layer of the liquid.
In this case, the change in free energy is directly proportional to the change in the surface area of the liquid.
Since any system spontaneously transitions into a state in which its potential energy is minimal, the liquid must spontaneously transition into a state in which its free surface area has the smallest value.
For example, a drop of rain or fog in the air takes on the shape of a ball, a shape corresponding to the lowest level of free energy.
Surface tension coefficient
Surface tension coefficient- this is a quantity that characterizes the dependence of the work of molecular forces going on to change the area of the free surface of a liquid and the area of change of this surface itself.
σ = А/ΔS
σ - surface tension coefficient
A– the work of molecular forces to change the surface area of a liquid
ΔS- change in liquid surface area
σ measured by the work done by molecular forces when the free surface area of a liquid decreases by one.
The surface tension coefficient depends on the type of liquid and external conditions, for example, temperature.
Molecule M1, which is located on the surface of the liquid, interacts not only with the molecules located inside the liquid, but also with molecules on the surface of the liquid located within the sphere of molecular action.
For a molecule M1 resultant R molecular forces directed along the surface of the liquid is zero, and for a molecule M2, located at the edge of the surface, R is different from zero.
From the figure it can be seen that the force R directed perpendicular to the boundary of the free surface and tangential to the surface itself.
Molecular forces directed along the surface of a liquid act on any closed line on the free surface of the liquid normal to this line in such a way that they tend to reduce the surface area of the liquid limited by the closed line.
This can be shown in the following experiment.
A thread of length is attached to a wire ring L.
If you tighten the ring with soap film, then the thread will rest freely on this film(Figure A). The surface area of the soap film will be determined by the contour of the frame.
If you break through the soap film from the underside of the thread, then molecular forces will reduce the surface, now limited by the upper part of the contour and the thread. Wherein the thread will stretch(Fig. B).
The force caused by the interaction of liquid molecules, causing a reduction in the area of its free surface and directed tangentially to this surface, is called surface tension force.
Molecular Pressure Forces draw molecules from the surface into the liquid and reduce the free surface area, i.e. close the formed “windows” on this surface.
So, the surface layer of a liquid is always in a state of tension. However, this state cannot be compared with the tension of an elastic stretched film. Elastic forces increase as the area of the stretched film increases, and surface tension forces do not depend on the surface area of the liquid.
Experience shows that surface tension coefficient influenced by the environment and temperature of the liquid. As the temperature of the liquid increases, it surface tension decreases and becomes zero at the critical temperature.
Lecture 11. Characteristics of the liquid state of matter. Surface layer of liquid. Energy of the surface layer. Phenomena at the interface between a liquid and a solid. Capillary phenomena.
CHARACTERISTICS OF THE LIQUID STATE OF MATTER
Liquid is a state of aggregation of a substance, intermediate between gaseous and solid.
A substance in a liquid state retains its volume, but takes the shape of the vessel in which it is located. Conservation of volume in a liquid proves that attractive forces act between its molecules.
If we describe a sphere of molecular action around a liquid molecule, then inside this sphere there will be centers of many other molecules that will interact with our molecule. These interaction forces hold the liquid molecule near its temporary equilibrium position for approximately 10 -12 -10 -10 s, after which it jumps to a new temporary equilibrium position approximately the distance of its diameter. Between jumps, the liquid molecules undergo oscillatory motion around a temporary equilibrium position.
The time between two jumps of a molecule from one position to another is called the settling time.
This time depends on the type of liquid and temperature. When a liquid is heated, the average residence time of molecules decreases.
So, in a small volume of liquid there is an ordered arrangement of its molecules, but in a large volume it turns out to be chaotic. In this sense, they say that in a liquid there is short-range order in the arrangement of molecules and no long-range order. This structure of the liquid is called quasicrystalline (crystal-like).
LIQUID PROPERTIES
1. If the time of action of the force on the liquid is short, then the liquid exhibits elastic properties. For example, when a stick hits the surface of the water sharply, the stick may fly out of the hand or break; A stone can be thrown so that when it hits the surface of the water it bounces off it, and only after making a few jumps does it sink in the water.
2. If the time of exposure to the liquid is long, then instead of elasticity, the fluidity of the liquid appears. For example, the hand easily penetrates into the water.
3. When a short-term force is applied to a liquid stream, the latter exhibits fragility. The tensile strength of liquids, although less than that of solids, is not much inferior to them in magnitude. For water it is 2.5-10 7 N/m 2.
4. The compressibility of a liquid is also very small, although it is greater than that of the same substances in the solid state. For example, when pressure increases by 1 atm, the volume of water decreases by 50 ppm.
Breaks inside a liquid that does not contain foreign substances, such as air, can only occur under intense influence on the liquid, for example, when propellers rotate in water, or when ultrasonic waves propagate through the liquid. This kind of void inside a liquid cannot exist for a long time and suddenly collapses, i.e., disappears. This phenomenon is called cavitation (from the Greek “cavitas” - cavity). This causes rapid wear of the propellers.
SURFACE LAYER OF LIQUID
The average value of the resultant molecular forces of attraction applied to a molecule located inside the liquid (Fig. 2) is close to zero. Random fluctuations of this resultant force the molecule to perform only chaotic motion within the liquid. The situation is somewhat different with molecules located in the surface layer of a liquid.
Let us describe spheres of molecular action with a radius R (about 10 -8 m) around the molecules. Then for the top molecule there will be many molecules in the lower hemisphere, and much less in the top, since there is liquid below, and vapor and air above. Therefore, for the upper molecule, the resultant of the molecular forces of attraction in the lower hemisphere is much greater than the resultant of the molecular forces in the upper hemisphere.
Thus, all liquid molecules located in the surface layer with a thickness equal to the radius of molecular action are drawn into the liquid. But the space inside the liquid is occupied by other molecules, so the surface layer creates pressure on the liquid, which is called molecular pressure.
Forces acting in the horizontal plane pull the surface of the liquid together. They're called surface tension forces
Surface tension - physical quantity, equal to the ratio of the surface tension force F applied to the boundary of the surface layer of the liquid and directed tangentially to the surface, to the length l of this boundary:
The unit of surface tension is newton per meter (N/m).
Surface tension varies for different liquids and depends on temperature.
Generally, surface tension decreases with increasing temperature and at the critical temperature, when the densities of the liquid and vapor are the same, the surface tension of the liquid is zero.
Substances that reduce surface tension are called surface active (alcohol, soap, washing powder)
To increase the surface area of a liquid, work must be done against surface tension.
There is another definition of the surface tension coefficient - energy. It proceeds from the fact that if the surface area of a liquid increases, then a certain number of molecules from its volume rise to the surface layer. For this purpose, external forces do work against the molecular forces of adhesion of molecules. The amount of this work will be proportional to the change in surface area of the liquid:
The proportionality coefficient σ is called the surface tension of the liquid.
Let us derive the unit of surface tension a in SI: o = 1 J/1 m 2 = 1 J/m 2.
Molecules in a liquid have kinetic energy of thermal motion and potential energy of intermolecular interaction. To move a molecule from the depth of a liquid to the surface, work must be done to overcome the force of molecular pressure. This work is done by the molecule due to the reserve kinetic energy and goes to increase its potential energy. Therefore, the molecules of the surface layer have additional potential energy compared to the molecules inside the liquid. This additional potential energy possessed by the surface layer molecules is called surface energy.
If the surface of a liquid is stretched, more and more molecules will come to the surface, and the potential energy of the surface layer will increase. Consequently, the surface energy is proportional to the area of the liquid surface itself (Fig. 4).
Where A– work of surface tension force; F– surface tension force; D x– film stretching; D S– change in the surface area of the film.
From this expression we can give another definition of the surface tension coefficient.
The surface tension coefficient is equal to the free surface energy per unit surface area. In this case, the unit of measurement is [a] = [J/m 2 ].
The impurities in the liquid have a great influence on the surface tension. For example, soap dissolved in water reduces the surface tension coefficient to 0.045 N/m, while sugar or salt increases it. Substances that change surface tension are called superficially active. These include oil, soap, alcohol. This phenomenon is explained by intermolecular interaction between molecules. If the interaction between the molecules of the liquid itself is greater than between the molecules of the liquid and the impurity, then the impurity molecules are pushed to the surface and the concentration of the impurity on the surface is greater; than in volume, which leads to a decrease in surface tension.
Surfactants are widely used when cutting metals, drilling rocks, etc., since the destruction of rocks in their presence occurs more easily; adsorbed on the surface of a solid, they penetrate into microcracks and contribute to the further development of these cracks deeper.