Kinematic viscosity is a fundamental physical characteristic of all gas and liquid media. This indicator is of key importance in determining the drag of moving solid bodies and the load that they experience. As you know, in our world, any movement occurs in the air or water environment. In this case, moving bodies are always affected by forces whose vector is opposite to the direction of movement of the objects themselves. Accordingly, the greater the kinematic viscosity of the medium, the stronger the load experienced by the solid. What is the nature of this property of liquids and gases?
Kinematic viscosity, defined as internal friction, is due to the momentum transfer of substance molecules perpendicular to the direction of movement of its layers with different speeds. For example, in liquids, each of the structural units (molecule) is surrounded on all sides by its closest neighbors, located approximately at a distance equal to their diameter. Each molecule oscillates around a so-called equilibrium position, but, taking momentum from its neighbors, it makes a sharp jump towards a new center of oscillation. In a second, each such structural unit of matter has time to change its place of residence about a hundred million times, making between jumps from one to hundreds of thousands of oscillations. Of course, the stronger such molecular interaction, the lower will be the mobility of each structural unit and, accordingly, the greater the kinematic viscosity of the substance.
If any molecule is acted upon by constant external forces from neighboring layers, then in this direction the particle makes more displacements per unit of time than in the opposite direction. Therefore, its chaotic wandering is transformed into an ordered movement with a certain speed, depending on the forces acting on it. This viscosity is typical, for example, of motor oils. Here, the fact that the external forces applied to the particle under consideration perform work on a kind of pushing apart the layers through which the given molecule squeezes is also important. Such an impact ultimately increases the speed of the thermal random motion of particles, which does not change with time. In other words, liquids are characterized by a uniform flow, despite the constant influence of multidirectional external forces, since they are balanced by the internal resistance of the layers of matter, which just determines the coefficient of kinematic viscosity.
With increasing temperature, the mobility of molecules begins to increase, which leads to some decrease in the resistance of the layers of matter, since in any heated substance more favorable conditions are created for the free movement of particles in the direction of the applied force. This can be compared to how it is much easier for a person to squeeze through a randomly moving crowd than through a stationary one. Polymer solutions have a significant indicator of kinematic viscosity, measured in Stokes or Pascal seconds. This is due to the presence in their structure of long rigidly bound molecular chains. But as the temperature rises, their viscosity decreases rapidly. When plastic products are pressed, its filamentous, intricately intertwined molecules are forced into a new position.
The viscosity of gases at a temperature of 20°C and atmospheric pressure of 101.3 Pa is of the order of 10-5Pas. For example, the kinematic viscosity of air, helium, oxygen and hydrogen under such conditions will be equal to 1.8210-5, respectively; 1, 9610-5; 2, 0210-5; 0.8810-5 Pas. And liquid helium generally has the amazing property of superfluidity. This phenomenon, discovered by Academician P. L. Kapitsa, lies in the fact that this metal in such a state of aggregation has almost no viscosity. For him, this figure is almost zero.
