How Is the Viscosity of Liquids Found?

Liquid viscosity regards the flowing liquid as a number of liquid layers moving in parallel with each other, and the speed of each layer is different, forming a speed gradient (dv / dx), which is the basic feature of flow.

Liquid viscosity

Right!

Liquid viscosity regards the flowing liquid as a number of liquid layers moving in parallel with each other, and the speed of each layer is different, forming a speed gradient (dv / dx), which is the basic feature of flow.

Chinese name

Liquid viscosity

Features

Different layers have different velocities, forming different speed gradients

Related formulas

(D) D = dv / dx (S-1)

Common areas

Europe and America

Liquid viscosity regards the flowing liquid as many liquid layers moving in parallel with each other.The speed of each layer is different, forming a speed gradient (dv / dx). This is the basic characteristic of flow. Due to the existence of the speed gradient, the slower liquid layer resistance The flow of the liquid layer is relatively slow, so the liquid generates motion resistance. In order for the liquid layer to maintain a certain speed gradient motion, a reverse force must be applied to the liquid layer opposite the resistance.

This kind of force exerted on the unit liquid layer area is called the shear stress (N / m2).
Shear stress and shear rate are two basic parameters that characterize the rheological properties of the system. Newton uses the pattern in Figure 4-1 to define the viscosity of a fluid. Two different plane but parallel fluids have the same area "A", separated by a distance "dx", and flow in the same direction at different flow rates "V1" and "V2". Newton assumes that the force to maintain this different flow rate is proportional to the fluid's Relative velocity or velocity gradient, ie:
= dv / dx = D (Newton's formula) where is related to material properties, we call it "viscosity".
Definition of viscosity: two plates with an area of 1m2 are immersed in a liquid, and the distance between the two plates is 1 meter.If a 1N shear stress is added to make the relative velocity between the two plates 1m / s, the viscosity of the liquid is 1Pa .s.

The above three conditional viscometry methods are commonly used in Europe and the United States. Except for the dark lubricating oil and residual oil, which are measured by Enshi viscometer, the other two viscometers are rarely used. The three conditioned viscosity expression methods and units are different, but the relationship between them can be converted by the chart. At the same time, Engelson viscosity and kinematic viscosity can be converted, which is much more convenient and flexible.

There are many methods for measuring viscosity, such as the barrel method, falling ball method, damped vibration method, cup viscometer method, capillary method and so on. For fluids with lower viscosity, such as water, ethanol, carbon tetrachloride, etc., capillary viscometers are commonly used for measurement; for fluids with higher viscosity, such as castor oil, transformer oil, motor oil, glycerin, etc. Falling ball method measurement; For liquids with a viscosity in the range of 0.1 to 100 Pa · s, the drum method can also be used for measurement. The principle of measuring the viscosity in the laboratory is generally derived from the Stokes formula and Poiseuille formula about the viscosity coefficient, and the viscosity coefficient is obtained.

The viscosity depends on the nature and temperature of the liquid. As the temperature increases, the viscosity will decrease rapidly. Therefore, to determine the viscosity, it is necessary to accurately control the temperature change. The measurement of viscosity parameters has important guiding value for predicting the process control, conveyability and product operability during product production, and has important significance in many industries such as printing, medicine, petroleum, and automobiles.

In 1845, the British mathematician and physicist Stokes (GGStokes (1819-1903)) and French Navier (CLMHNavier) and others deduced the most basic system of equations in viscous fluid mechanics, namely Navier-Swed Tox's equation lays the foundation for traditional fluid mechanics.

In 1851, Stokes deduced the formula for calculating the resistance to a solid sphere when it moves slowly in a viscous medium, and found that under a given force (gravity), the resistance is proportional to the flow velocity and the viscosity coefficient. That is, the stoes formula for resistance.

The Navier-Stokes equation is one of the most difficult nonlinear equations in mathematics. It is very difficult to find its exact solution. To this day, there are only about 70 exact solutions, and only about one hundred special solutions have been solved, which is one of the most complicated and world-class mathematical problems that have not yet been completely solved.

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