The concept of viscosity

We often hear the word viscosity, but what is the real definition of viscosity?

Viscosity (Viscosity), also written as "viscosity". Two plates with an area of ​​1m2 are immersed in the liquid, and the distance between the two plates is 1 meter. If a shear stress of 1N is applied to make the relative velocity between the two plates 1m/s, the viscosity of the liquid is 1Pa.s. The flowing liquid is regarded as many liquid layers moving parallel to each other, and the speed of each layer is different, forming a velocity gradient (dv/dx), which is the basic characteristic of flow. Due to the existence of the velocity gradient, the slower flowing liquid layer blocks the flow of the faster liquid layer, therefore, the liquid creates resistance to motion. In order to keep the liquid layer moving with a certain velocity gradient, it is necessary to apply a reverse force opposite to the resistance to the liquid layer.

Viscous force is the resistance produced when the fluid is deformed by shear stress or tensile stress. In everyday terms, viscosity is like "thickness" or "friction within a fluid". Therefore, water is "thin" and has a low viscosity, while honey is "thick" and has a high viscosity. Simply put, a fluid with lower viscosity (low viscosity coefficient) will flow better.

How is viscosity tested?

Viscosity measurement has three methods: dynamic viscosity, kinematic viscosity and conditional viscosity.

(1) Dynamic viscosity: ηt is the resistance generated when the two liquid layers are separated by 1 cm and their area is 1 (square centimeter) and the relative moving speed is 1 cm/s, the unit is g/cm·s. 1 g/cm·s = 1 poise General: The dynamic viscosity unit in industry is represented by poise.

(2) Kinematic viscosity: at a temperature of t°C, the kinematic viscosity is represented by the symbol γ. In the International System of Units, the unit of kinematic viscosity is Si, that is, square meters per second (m2/s). Centistokes are commonly used in actual measurement, ( cst) means that the unit of centis is square millimeter per second (ie 1cst=1mm2/s). Kinematic viscosity is widely used to measure the viscosity of jet fuel oil, diesel oil, lubricating oil and other liquid petroleum products, dark petroleum products, used lubricating oil, crude oil, etc. The determination of kinematic viscosity adopts countercurrent method

(3) Conditional viscosity: refers to the viscosity expressed in conditional units measured by different specific viscometers. There are three types of conditional viscosity commonly used in various countries: Engler viscosity, Saybolt viscosity, and Raye viscosity.

What should be paid attention to when testing viscosity?

According to the measurement principles of different viscosity Testers, in order to obtain accurate and reliable measurement data, the following points need to be paid attention to when using a viscometer:

1. The performance indicators of the instrument need to meet the requirements of the National Metrology Verification Regulations. The instruments in use should be checked periodically, and if necessary (the instrument is frequently used or in a qualified critical state), an intermediate self-inspection should be carried out to ensure that its measurement performance is qualified and the coefficient error is within the allowable range, otherwise accurate data cannot be obtained.

Second, pay special attention to the temperature of the measured liquid. Many users ignore this point and think it doesn’t matter if the temperature is slightly different. Our experiments have proved that: when the temperature deviation is 0.5°C, the deviation of the viscosity value of some liquids exceeds 5%, and the temperature deviation has a great influence on the viscosity. Therefore, special attention should be paid to keeping the temperature of the liquid to be measured constant near the specified temperature point, and it is better not to exceed 0.1°C for accurate measurement.

Third, the choice of measuring container (outer cylinder). For the dual-cylinder Rotational Viscometer, it is necessary to read the instrument manual carefully. Different rotors (inner cylinders) match the corresponding outer cylinders, otherwise the measurement results will deviate greatly. For a single-cylinder rotary viscometer, in principle, the outer cylinder radius is required to be infinite, and the actual measurement requires that the inner diameter of the outer cylinder, that is, the measuring container, is not less than a certain size. For example, the laboratory standard Rotational Viscometer produced by BROOKFIELD requires a 600mL beaker for measurement. Experiments have proved that, especially when the No. 1 rotor is used, if the inner diameter of the container is too small, it will cause a large measurement error.

4. Correctly select the rotor or adjust the speed so that the torque percentage of the reading value is between 10% and 100%. If it is too low, the measured reading is invalid; if it is too high, the measurement is out of range and there is no reading.

5. The depth of the rotor immersed in the liquid and the influence of air bubbles. The rotary viscometer has strict requirements on the depth of the rotor immersed in the liquid, and it needs to be operated according to the instructions (some double-cylinder instruments have strict requirements on the amount of liquid tested, which needs to be measured with a graduated cylinder). When the rotor is immersed in the liquid, there are often air bubbles. Most of them will float up and disappear after a period of time after the rotor rotates. Sometimes the air bubbles attached to the lower part of the rotor cannot be eliminated. The existence of air bubbles will bring large deviations to the measurement data, so 45 Slowly dipping the rotor at an angle of 1° is an effective approach.

6. Cleaning of the rotor. The measuring rotor (including the outer cylinder) should be clean and free of dirt. Generally, it should be cleaned in time after measurement, especially after measuring paint and adhesives. Why are the same samples measured with different viscosities?

First of all, we need to understand that fluids are divided into Newtonian fluids and non-Newtonian fluids. Newtonian fluid refers to a fluid whose viscosity value does not change with the shear rate (rotational speed), but remains constant. For example, the standard oil we use for calibration and calibration is Newtonian fluid. In fact, most of the samples we come into contact with in production and life are non-Newtonian fluids (the viscosity will change with the change of shear rate or rotational speed). The rheological properties of non-Newtonian fluid samples are very complex, but they basically change with shear rate (rotational speed) and temperature, and some fluid samples are also related to shear time.

Secondly, we need to understand the viscosity measurement of the standard viscometer, which is a relative measurement method. If the same (or the same type) sample is measured with different range models, or with the same model but different measurement methods (spindle, speed, temperature, reading time, etc.), the viscosity measured between each other Results may vary significantly and vary.