Kinematic viscosity is the fundamental physical characteristic of all gas and liquid media. This indicator is of key importance in determining the drag of moving solids and the load they experience. As you know, in our world, any movement occurs in air or water. 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 higher 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?
The kinematic viscosity, defined as internal friction, is due to the transfer of momentum of the molecules of the substance perpendicular to the direction of motion of its layers having different speeds. For example, in liquids, each of the structural units (a molecule) is surrounded on all sides by its nearest neighbors, located at a distance approximately equal to their diameter. Each molecule oscillates around the so-called equilibrium position, but, taking an impulse from its neighbors, it makes a sharp jump in the direction of a new center of vibration. In a second, each such structural unit of matter manages to change its place of settledness about a hundred million times, making between jumps from one to hundreds of thousands of oscillations. Of course, the stronger this molecular interaction, the less will be the mobility of each structural unit and, accordingly, the more kinematic viscosity of the substance.
If any molecule is affected by constant external forces from neighboring layers, then in this direction the particle performs more displacements in a certain unit of time than in the opposite. Therefore, its chaotic wandering is converted into an ordered movement with a certain speed, depending on the forces acting on it. This viscosity is characteristic, for example, of motor oils. It also matters that the external forces applied to the particle in question perform the work of expanding the layers through which this molecule squeezes. Such an effect ultimately increases the rate of 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 the substance, which precisely determines the kinematic viscosity coefficient.
With increasing temperature, the mobility of the molecules begins to increase, which leads to a slight decrease in the resistance of the layers of the substance, 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 motionless one. A significant indicator of the kinematic viscosity, measured in Stokes or Pascal seconds, have polymer solutions. This is due to the presence in their structure of long tightly bound molecular chains. But with increasing temperature, their viscosity decreases rapidly. When pressing plastic products, its filamentary, bizarrely intertwined molecules forcibly take on 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 -5 Pa * s. For example, the kinematic viscosity of air, helium, oxygen and hydrogen under such conditions will be equal to 1.82 * 10 -5 , respectively; 1.96 * 10 -5 ; 2.02 * 10 -5 ; 0.88 * 10 -5 Pa * s. And liquid helium generally has an amazing property of superfluidity. This phenomenon, discovered by academician P.L. Kapitsa, lies in the fact that this metal in such an aggregate state has almost no viscosity. For him, this figure is almost zero.