Each person at least once dealt with paint or glue and at the same time paid attention to a number of properties, characteristic of these substances, among which the basic is viscosity. However very few people know in what cases the viscosity of substance increases and in what - decreases. On production and in life it is necessary to come up against situations in which the viscosity needs to be reduced. It is possible to carry out it in various ways.
1. The concept of viscosity extends both to liquids, and to gases. And, the viscosity of liquids strongly differs from similar characteristic of gases. It depends on a number of parameters: type of liquid or gas, temperature, pressure, speed of the movement of layers, etc. Viscosity is called the property of substance of gas to show resistance to the some layers concerning others. Thus, it represents proportionality coefficient which depends on a substance sort. If this coefficient is big, also forces of internal friction which arise when moving layers of substance are considerable. They also depend on the speed of the movement of layers and surface area of a layer. Forces of internal friction pay off as follows: F=η*S*Δv/Δx, where η - dynamic viscosity.
2. The concept of kinematic viscosity is most often applied to the closed current sources (pipes, tanks). It is connected with dynamic viscosity by a formula: ν=η/ρ, where ρ - liquid density. There are two modes of a current of substance: laminar and turbulent. At the laminar movement the layers slip among themselves, and at turbulent - mix up. If substance differs in high viscosity, then the second situation most often takes place. The nature of the movement of substance can be recognized by Reynolds number: Re=ρ*v*d/η=v*d/ν At Re <a current is considered 1000 laminar, at Re> 2300 - turbulent.
3. The viscosity of substance changes under the influence of a number of external factors. Long ago the dependence of this characteristic on temperature is known. It influences gases and liquids differently. If temperature of liquid increases, then its viscosity decreases. At gases, on the contrary, the viscosity at increase in temperature grows. Gas molecules at temperature increase begin to move quicker, and at liquids the return phenomenon is observed - they lose energy of intermolecular interaction, and, according to a molecule move more slowly. It also causes distinction of viscosity at liquids and gases at the same temperature. Besides, the important factor affecting viscosity is also pressure. The viscosity both liquid, and gas at increase in pressure increases. In addition, the viscosity promptly grows also at increase in molar mass of substance. Especially it is noticeable at low-molecular liquids. At suspensions the viscosity increases at increase in volume of a disperse phase.
4. As it is already told above, the nature of change of viscosity under the influence of external factors depends on a substance sort. So for example, when heating oils perhaps considerable reduction of viscosity for two reasons: first, oils have complex molecular structure, and secondly, already noted dependence of viscosity on temperature affects. Therefore to lower viscosity of liquid, the first that needs to be made is to increase its temperature. If it is about gas, then temperature for decrease in its viscosity should be lowered. The second way of decrease in viscosity of substance - decrease in its pressure. It is suitable both for liquids, and for gases. At last, the third way decrease in viscosity - dilution of viscous substance by less viscous. Water can be applied to many liquid substances as thinner. All listed ways of decrease in viscosity can be applied to substance both separately, and in common.