The property of any liquid, which is manifested in its ability to prevent free displacement or shear of its own particles, characterizes the concept of fluid viscosity. The physicochemical meaning of this property is that certain forces of internal friction between molecules arise in a moving fluid, which, in turn, owe their appearance to the presence of molecular attraction forces.
The fact is that in liquids the distance between the molecules is very small, and therefore they are less mobile than, say, gas molecules. Penetration into another layer is possible only if a certain free space is formed in it, sufficient for the liquid molecule to penetrate there. A certain energy is spent on the formation of such a cell, which accordingly decreases with increasing temperature and decreasing pressure and vice versa.
To date, it should be recognized that a rigorous scientific theory of this phenomenon has not yet been created.
The main indicators characterizing the viscosity of the liquid are the dynamic coefficient, denoted by μ, and, in addition to it, the kinematic coefficient denoted by ν.
To measure the value of the dynamic coefficient in metric GHS, the Poise unit (P) is used, which is equal to: 1 dyne • x s / cm2 = 1 g / cm • x s). In the ICSS system, this coefficient is measured in kgf • s / m2; and in the most common SI system - in Pa • x s. It is possible to establish the mathematical dependence of these quantities among themselves. It is as follows: 1 P is approximately 0.0101 kgf • s / m2, which in turn is 0.1 Pa • x s. In addition, 1 kgf • x s / m2 = 98.1 P, which is equal to 9.81 Pa • x s.
By the formula: ν = μ / ρ, the kinematic coefficient of viscosity of the liquid can be calculated, and the unit of measurement is stokes (St, in the GHS system), which is 1 cm2 / s. In other systems - MKGSS and SI, a unit equal to 1 m2 / s, which is 10,000 Art.
The physical regularity is that with increasing temperature, the viscosity of the liquid decreases. This dependence for the dynamic coefficient is determined by the equation μ = μ0 • e x a (t-t0), in which are indicated: μ and μ0 are the corresponding values of the coefficient at given temperatures t and t0, and is the exponent, the value of which is determined by the characteristics of the liquid itself, and which is variable, for example, for oils, its values range from 0.025 to 0.035.
There is also a dependence of this indicator on the temperature regime. This is especially important when it comes to oils or other lubricants that are used in various mechanical devices, assemblies, machines. The formula for this dependence has the form: νt = ν x 50 • x (50 / t0) n. It indicates: νt is the value of the kinematic coefficient at a certain considered temperature, ν x 50 is the coefficient at a temperature of 50 C, t is the temperature at which we need to determine the coefficient, n is the characteristic of the properties of the liquid, which varies depending on temperature, and from the value of ν x 50.
Correctly enough, you can calculate the value of n, if you use the formula n = log ν x 50 + 2.7. In order not to make calculations in each case, the values of n, characterizing the initial viscosity of the liquid at 50 C, are reduced to a special table. This greatly speeds up the calculation process.
In addition, there is also a summary table of the viscosity of liquids, which immediately reflects all the data and the corresponding indicators of both dynamic and kinematic coefficients for certain types of liquids.
The correct determination of the values that characterize the viscosity of a liquid is of great practical importance; it is the very efficiency and longevity of the work of many mechanisms that we use both in production and in life that depends on it.