liquid is defined as the physical body, the ability to change its shape at an arbitrarily small impact on him.Usually there are two main types of liquids and gases drip.Liquid drops - are liquids in the conventional sense: water, kerosene, petroleum, oil and so on.Gaseous fluids - gases which under normal conditions are, for example, gaseous substances such as air, nitrogen, propane, oxygen.
These materials differ in molecular structure and type of interaction of the molecules with each other.However, from the mechanical point of view, they are solid media.And because of this, they provide some basic mechanical properties: the density and specific gravity;as well as the basic physical properties: compressibility, thermal expansion, tensile strength, surface tension and viscosity.
Under property understand viscosity liquid material to resist sliding or shift its layers to each other.The essence of this concept is the emergence of friction between the different layers within the liquid during their relative motion.There are concepts of "dynamic viscosity" and its "kinetic viscosity".Next, take a closer look, what is the difference between these concepts.
Concepts and dimension
strength of the internal friction F, arising from moving relative to each other adjacent layers generalized fluid is directly proportional to the velocity of the layers and the area of contact S. This force acts in a direction perpendicular to the motion, and analytically expressed by the equationNewton
F = μS (ΔV) / (Δn),
where (ΔV) / (Δn) = GV - velocity gradient in the direction normal to the moving layers.
proportionality coefficient μ - has a dynamic viscosity or viscosity of a generalized liquid.From Newton's equations it is
μ = F / (S ∙ GV).
The physical unit of viscosity measurement system is defined as the viscosity of the medium in which the velocity gradient at unit GV = 1 cm / sec per square centimeter of layer frictional force acts in 1 dyne.Accordingly, the dimension of the units in the system is expressed in dyne sec ∙ ∙ cm ^ (- 2) = g ∙ cm ^ (- 1) ∙ s ^ (- 1).
This unit of dynamic viscosity is called the poise (P).
1 P = 0.1 Pa ∙ s = 0.0102 kgf ∙ a ∙ m ^ (- 2).
apply and smaller units, namely: 1 P = 100 cP (cps) = 1000 Mn (millipuaz) = 1,000,000 INC (mikropuaz).The technical system of the unit value of viscosity take kgf ∙ a ∙ m ^ (- 2).
In the international system of units, the viscosity is defined as the viscosity of the medium in which the velocity gradient at a single GV = 1 m / s at 1 meter per square meter of the liquid layer in the force of friction 1 N (Newton).The dimension of the value of μ in the SI expressed in kg ∙ m ^ (- 1) ∙ c ^ (- 1).
Besides such features as dynamic viscosity, liquid introduces the concept of kinematic viscosity coefficient μ as a ratio to the density of the liquid.The coefficient of kinematic viscosity is measured in Stokes (1st Class = 1 cm ^ (2) / s).
Viscosity is numerically equal to the amount of traffic carried in the moving gas per unit time in the direction perpendicular to the motion, per unit area, when the velocity is different per unit of gas velocity layers, separated by one length.Viscosity is dependent on the nature and state of the material (temperature and pressure).
Dynamic viscosity and kinematic viscosity of liquids and gases are highly dependent on temperature.It was noted that both of these factors decrease with increasing temperature of dropping liquid and, conversely, increases with increasing temperature - for gases.This difference can be explained according to the physical nature of the interaction of molecules in the droplet liquids and gases.
physical meaning
In terms of the molecular-kinetic theory of gases viscosity phenomenon is that in a moving medium due to the random motion of the molecules is an alignment of speeds of different layers.Thus, if the first layer in a direction moving faster than its adjacent second layer, the first layer of the second moving faster molecule, and vice versa.
Therefore, the first layer tends to accelerate the movement of the second layer, and the second - to slow down first.Thus, the total amount of movement of the first layer will be reduced, and the second - to increase.The resulting change in the amount of this movement is characterized by the viscosity of gases.
The droplet, as opposed to gas, the internal friction is increasingly determined by the action of intermolecular forces.And, since the distance between the molecules of the liquid drops are small compared with the gaseous medium, the force of interaction of the molecules at the same time - are significant.The molecules of the liquid as solids and molecules oscillate near the equilibrium positions.However, these fluids are not stationary position.After a certain period of time a molecule of the liquid rapidly moves to a new position.At this time, during which the position of the molecules in the liquid does not change its time called "sedentary life."
intermolecular forces depend strongly on the type of liquid.If the viscosity becomes small, it is called a "fluid", as the flowability and the dynamic viscosity of the fluid - is inversely proportional.Conversely, a material with a high viscosity may have a mechanical hardness, such as a resin.The viscosity of the substance while significantly depends on the composition of the impurities and their amounts and the temperature.With increasing temperature, the time of "sedentary life" is reduced, thereby increasing mobility and reducing the viscosity of the liquid substance.
phenomenon of viscosity, as well as other molecular transport phenomena (diffusion and thermal conductivity), is an irreversible process leading to the attainment of the equilibrium state corresponding to maximum entropy and the free energy minimum.
