Viscosity characterizes the ability of gases or liquids to create resistance between layers of fluid (not solid) bodies moving with respect to each other. That is, this value corresponds to the force of internal friction (English term: viscosity) that occurs when a gas or liquid moves. For different bodies, it will be different, since 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 means mistletoe. This is due to bird glue, which was made from mistletoe berries and used to catch birds. Tree branches were smeared with adhesive, and birds, sitting on them, became easy prey for humans.
What is viscosity? The units of measurement for this characteristic will be given, as is customary, in the SI system, as well as in other off-system 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) that causes the shear layers to move body. The ratio (v2 - v1) / (z2 - z1) shows the rate of change of the velocity of the fluid or gas during the transition from one moving layer to another. 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 the moving body. The proportionality coefficient ƞ is the dynamic viscosity coefficient of a given body. The inverse of j = 1 / ƞ is the 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. Units of measurement in the SI system are expressed as pascal per second.
Viscosity is the most important physical and chemical characteristic of many substances. Its value is taken into account when designing and operating pipelines and apparatuses in which movement (for example, if they serve for pumping) of a liquid or gaseous medium occurs. This may be oil, gas or products of their processing, molten slag or glass and so on. Viscosity in many cases is a qualitative characteristic of intermediates and finished products of various industries, since it directly depends on the structure of the substance and shows the physicochemical state of the material and the changes that occur in the technology. Often, kinematic viscosity is used, not dynamic, 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 dynamic viscosity (µ) to medium density (ρ): v = µ / ρ.
Kinematic viscosity is a physicochemical characteristic of a material, showing its ability to resist flow under the influence of gravitational forces.
In the SI system, the units of kinematic viscosity are recorded as m 2 / s.
In the GHS system, viscosity is measured in stokes (St) or centistokes (cSt).
The following relationship exists between these units: 1 St = 10 -4 m 2 / s, then 1 sSt = 10 -2 St = 10 -6 m 2 / s = 1 mm 2 / s. Often, a different off-system unit of measurement is used for kinematic viscosity - 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.
To recalculate system units of measurement of dynamic viscosity into off-system units, you can use the equality: 1 Pa • s = 10 poise. Short designation is written: P.
Typically, the units for measuring the viscosity of a liquid are regulated by the normative documentation for the finished (commercial) product or the technological regulations for the intermediate product along with the permissible range of variation of this qualitative characteristic, as well as with the error of its measurement.
To determine the viscosity in laboratory or industrial conditions, viscometers of various designs are used. They can be rotational, with a ball, capillary, ultrasonic. The principle of measuring viscosity in a glass capillary viscometer is based on determining the time of fluid flow 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 temperature (it will decrease with increasing it, which is explained by the molecular kinetic theory as a result of the acceleration of chaotic motion and interaction of molecules), therefore, the test sample must be kept for a while at a certain temperature to average the latter over the entire volume of the sample. There are several standardized methods for testing viscosity, but the most common is the interstate standard GOST 33-2000, based on which the kinematic viscosity is determined, the unit of measurement in this case is mm 2 / s (cSt), and the dynamic viscosity is recalculated as the product of the kinematic viscosity and density .