Viscosity characterizes the ability of gases orliquids create resistance between moving relative to each other layers of fluid (not solid) bodies. That is, this value corresponds to the strength of internal friction (English term: viscosity), arising from the movement of gas or liquid. For different bodies, it will be different, as it depends on their nature. For example, water has a low viscosity compared to honey, whose viscosity is much higher. Internal friction or fluidity of solid (bulk) substances is characterized by rheological characteristics.
Слово вязкость происходит от латинского слова Viscum, which means mistletoe. This is due to bird glue, which was made from mistletoe berries and used for catching birds. The branches of trees were spread with adhesive material, and birds, sitting on them, became easy prey for humans.
What is viscosity? The units of measurement of this characteristic will be given, as is customary, in the SI system, as well as in other off-system units.
Isaac Newton in 1687 established the basic lawflow of liquid and gaseous bodies: F = ƞ • {(v2 - v1) / (z2 - z1)} • S. In this case, F is the force (tangential) that causes the layers of the moving body to shift. The ratio (v2 - v1) / (z2 - z1) indicates the rapidity of the change in the flow rate of a liquid or gas when moving from one moving layer to another. This is otherwise called the gradient of the flow velocity or shear rate. The value of S is the area (in cross section) of the flow of a moving body. The coefficient of proportionality, ƞ, is the coefficient of viscosity of the dynamic body. The value, its inverse j = 1 /, is fluidity. The force acting per unit area (in cross section) of the flow can be calculated by the formula: µ = F / S. This is the absolute or dynamic viscosity. Its units in the SI system are expressed as pascal for a second.
Viscosity is the most important physico-chemicalcharacteristic of many substances. Its value is taken into account in the design and operation of pipelines and devices in which movement occurs (for example, if they are used for pumping) a liquid or gaseous medium. It can be oil, gas or their products, molten slag or glass and so on. Viscosity in many cases is a qualitative characteristic of intermediates and finished products of various industries, as it directly depends on the structure of the substance and shows the physicochemical state of the material and changes occurring in technology. Often, to estimate the value of resistance to deformation or outflow, they use not kinetic viscosity, but kinematic viscosity, the units of measurement of which in the SI system are expressed in square meters per second. Kinematic viscosity (denoted by ν) is the ratio of the dynamic viscosity (µ) to the density of the medium (ρ): v = µ / ρ.
Kinematic viscosity is a physico-chemical characteristic of a material, showing its ability to resist flow under the action of gravity.
In the SI system, the units of measurement of kinematic viscosity are recorded as m2 / s.
In the GHS system, viscosity is measured in stocks (St) or centistokes (cSt).
Между этими единицами измерения существует the following connection: 1 St = 10-4 m2 / s, then 1 cSt = 10-2 St = 10-6 m2 / s = 1 mm2 / s. Often, for kinematic viscosity, another non-sys- tematic unit of measurement is used - these are Engler degrees, which can be converted to Stokes by the empirical formula: v = 0.073oE - 0.063 / oE or according to the table.
For converting the system units of measurement of dynamic viscosity into non-systemic, you can use the equation: 1 Pa • s = 10 poise. Short designation is written: P.
Normally, viscosity unitsregulated by the regulatory documentation on the finished (commodity) product or process regulations on intermediate with the permissible range of variation of this quality characteristic, as well as the error of its measurement.
Для определения вязкости в лабораторных или working conditions use viscometers of various designs. They can be rotary, with a ball, capillary, ultrasonic. The principle of measuring viscosity in a glass capillary viscometer is based on determining the flow time of a fluid through a calibrated capillary of a certain diameter and length, and the constant viscometer should be taken into account. Since the viscosity of a material depends on temperature (as it increases, it will decrease, which is explained by molecular kinetic theory as a result of acceleration of chaotic movement and interaction of molecules), therefore, the test sample must be kept at a certain temperature for averaging the latter over the entire sample volume. There are several standardized methods for testing viscosity, but the most common is the interstate standard GOST 33-2000, on the basis of which the kinematic viscosity is determined, the units of measure in this case mm2 / s (cSt), and the dynamic viscosity is recalculated as the product of kinematic viscosity and density.
