A liquid is defined as a physical body capable of changing its shape with an arbitrarily small effect on it. Usually there are two main types of liquids: drip and gaseous. Drop liquids are liquids in the usual sense: water, kerosene, oil, oil, and so on. Gaseous liquids are gases that, under ordinary conditions, are, for example, gaseous substances such as air, nitrogen, propane, oxygen.
These substances differ in molecular structure and type of interaction of molecules with each other. However, from the point of view of mechanics, they are continuous media. And because of this, some common mechanical characteristics are determined for them: density and specific gravity; as well as basic physical properties: compressibility, thermal expansion, tensile strength, surface tension and viscosity.
By viscosity is meant the property of a liquid substance to resist sliding or shear of its layers relative to each other. The essence of this concept is the appearance of the friction force between different layers inside the liquid during their relative motion. Distinguish the concepts of "dynamic viscosity of a liquid" and its "kinetic viscosity". Next, we consider in more detail what the difference between these concepts consists of.
Basic concepts and dimension
The force of internal friction F arising between neighboring relative layers of a generalized fluid moving relative to each other is directly proportional to the velocity of the layers and the area of their contact S. This force acts in the direction perpendicular to the movement and is analytically expressed by Newton’s equation
F = μS (∆V) / (∆n),
where (∆V) / (∆n) = GV is the velocity gradient in the direction of the normal to the moving layers.
The proportionality coefficient μ is the dynamic viscosity or simply the viscosity of the generalized fluid. From Newton's equation it is equal to
μ = F / (S ∙ GV).
In the physical measurement system, the unit of viscosity is defined as the viscosity of the medium in which, at a unit velocity gradient GV = 1 cm / sec, a friction force of 1 dyne acts on each square centimeter of the layer. Accordingly, the unit dimension in this system is expressed in dyn ∙ sec ∙ cm ^ (- 2) = g ∙ cm ^ (- 1) ∙ sec ^ (- 1).
This unit of dynamic viscosity is called Poise (P).
1 P = 0.1 Pa ∙ s = 0.0102 kgf ∙ s ∙ m ^ (- 2).
Smaller units are also used, namely: 1 P = 100 cP (centipoise) = 1000 mP (millipoise) = 1,000,000 μP (micropoise). In the technical system, the unit of viscosity is kgs ∙ s ∙ m ^ (- 2).
In the international system, the unit of viscosity is defined as the viscosity of the medium in which, with a unit velocity gradient GV = 1 m / s per 1 m, a friction force of 1 N (newton) acts on each square meter of the liquid layer. The dimension of μ in the SI system is expressed in kg ∙ m ^ (- 1) ∙ s ^ (- 1).
In addition to such characteristics as dynamic viscosity, the concept of kinematic viscosity is introduced for liquids as the ratio of the coefficient μ to the density of the liquid. The value of the kinematic viscosity coefficient is measured in stokes (1 st = 1 cm ^ (2) / s).
The viscosity coefficient is numerically equal to the amount of motion carried in a moving gas per unit of time in a direction perpendicular to the movement through the unit area, when the speed of movement differs by unit of speed in the gas layers spaced by unit of length. The viscosity coefficient depends on the type and state of the substance (temperature and pressure).
The dynamic viscosity and kinematic viscosity of liquids and gases are highly dependent on temperature. It was noted that both of these coefficients decrease with increasing temperature for dropping liquids and, conversely, increase with increasing temperature for gases. The difference in this dependence can be explained by the physical nature of the interaction of molecules in droplet liquids and gases.
Physical meaning
From the point of view of molecular kinetic theory, the phenomenon of viscosity for gases consists in the fact that in a moving medium, due to the random motion of molecules, the velocities of different layers are equalized. So, if the first layer moves in a certain direction faster than the second layer adjacent to it, then faster molecules pass from the first layer to the second, and vice versa.
Therefore, the first layer seeks to accelerate the movement of the second layer, and the second - to slow down the movement of the first. Thus, the total momentum of the first layer will decrease, and the second - increase. The resulting change in momentum is characterized by a viscosity coefficient for gases.
In droplet fluids, unlike gases, internal friction is largely determined by the action of intermolecular forces. And, since the distances between the molecules of the droplet liquid are small compared to gaseous media, the forces of interaction of the molecules in this case are significant. Molecules of a liquid, like molecules of solids, oscillate near equilibrium positions. However, in liquids these positions are not stationary. After a lapse of time, the liquid molecule abruptly moves to a new position. Moreover, the time during which the position of the molecule in the liquid does not change is called the time of its "settled life".
The forces of intermolecular interaction depend significantly on the type of liquid. If the viscosity of the substance is small, then it is called "fluid", since the yield coefficient and dynamic viscosity of the liquid are inversely proportional. Conversely, substances with a high viscosity coefficient may have mechanical hardness, such as resin. In this case, the viscosity of a substance substantially depends both on the composition of impurities and their amount, and on temperature. With increasing temperature, the “settled life” time decreases, as a result of which the mobility of the liquid increases and the viscosity of the substance decreases.
The phenomenon of viscosity, like other phenomena of molecular transfer (diffusion and thermal conductivity), is an irreversible process, leading to an equilibrium state corresponding to a maximum of entropy and a minimum of free energy.