What is viscosity? Units of measurement of viscosity

Viscosity characterizes the ability of gases or liquids to create resistance between layers of fluid (not solid) moving in relation to each other. That is, this value corresponds to the force of internal friction (English term: viscosity), which occurs when a gas or liquid moves. For different bodies it will be different, because 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.

The word viscosity comes from the Latin word Viscum, which in translation means mistletoe. This is due to the bird's glue, which was made from mistletoe berries and used for catching birds. Adhesive material spread tree branches, and birds, sitting down on them, became easy prey for a person.

What is viscosity? Units of measurement of this characteristic will be given, as is customary, in the SI system, as well as in other non-systemic units.

Isak Newton in 1687 established the basic law of the flow of liquid and gaseous bodies: F = ƞ • {(v2 - v1) / (z2 - z1)} • S. In this case, F is the force (tangential), which causes a displacement of the layers of the mobile Body. The ratio (v2 - v1) / (z2 - z1) shows the rapidity of the change in the velocity of the fluid or gas flow during the transition from one moving layer to another. Otherwise it is called a gradient of flow velocity or shear rate. The value of S is the area (in cross section) of the flow of the moving body. The coefficient of proportionality ƞ is the coefficient of viscosity of the dynamic given body. The quantity inverse to it is j = 1 / ƞ, is fluidity. The force acting per unit area (in cross section) of the flow can be calculated from the formula: μ = F / S. This is the absolute or dynamic viscosity. The units of measurement in the SI system are expressed as pascal per second.

Viscosity is the most important physico-chemical characteristic of many substances. Its importance is taken into account in the design and operation of pipelines and apparatus in which motion occurs (for example, if they serve for pumping) of a liquid or gaseous medium. It can be oil, gas or products of their processing, molten slag or glass and so on. Viscosity in many cases is a qualitative characteristic of semi-finished products and finished products of various industries, since it directly depends on the structure of the substance and shows the physico-chemical state of the material and the changes taking place in the technology. Often, to estimate the value of the resistance to deformation or flow, the dynamic viscosity is not dynamic but kinematic, whose units of measurement are expressed in square meters per second in the SI system. The kinematic viscosity (denoted by ν) is the ratio of the viscosity of the dynamic (μ) to the density of the medium (ρ): v = μ / ρ.

Kinematic viscosity is the physico-chemical characteristic of the material, showing its ability to resist the flow under the influence of gravitational forces.

In the SI system, units of kinematic viscosity measurement are recorded as m2 / s.

In the GHS system, the viscosity is measured in Stokes (St) or centistokes (cSt).

Between these units of measurement there is the following relationship: 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-system unit of measurement is used - these are the Engler degrees, whose translation into Stokes can be carried out according to the empirical formula: v = 0,073oE - 0,063 / oE or according to the table.

To recalculate the system units of dynamic viscosity into non-systemic units, we can use the equality: 1 Pa • s = 10 poise. A short notation is written: P.

Usually units of fluid viscosity are regulated by normative documentation for the finished (commodity) product or technological regulations for the semi-product together with the permissible range of variation of this quality characteristic, as well as with the error of its measurement.

To determine the viscosity in laboratory or production conditions use viscosimeters of various designs. They can be rotational, with a ball, capillary, ultrasonic. The principle of viscosity measurement in a glass capillary viscometer is based on determining the time for the flow of liquid through a calibrated capillary of a certain diameter and length, while the constant of the viscometer must be taken into account. Since the viscosity of the material depends on the temperature (as it increases, it will decrease, which is explained by molecular-kinetic theory as a result of acceleration of chaotic motion and interaction of molecules), so the test sample must be conditioned for a certain time at a certain temperature to averaged over the whole volume of the sample. 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 measurement in this case are mm2 / s (cSt), and the dynamic viscosity is recalculated as the product of the kinematic viscosity by density.