Definition and Measurement of Viscosity

 

Definition of viscosity

Viscosity refers to the resistance a fluid exhibits to flow. When a fluid (gas or liquid) flows and one part flows over another, it encounters resistance, which is the internal friction of the fluid. To make the fluid flow, a tangential force must be added in the direction of fluid flow to resist the resistance.

The upper boundary plate moves to the right at a constant speed, causing the liquid below to also move to the right. Since the bottom plate is stationary, the velocity of the liquid gradually decreases from top to bottom, resulting in shear stress inside the liquid. For liquids with low viscosity, the shear stress is small and the internal friction of the liquid is small; for liquids with high viscosity, the shear stress is large and the internal friction of the liquid is large. Therefore, viscosity is defined as the ratio of shear stress to the gradient of the liquid’s horizontal rightward movement velocity in the y-axis direction. In this example, if the distribution gradient of the horizontal rightward velocity of the liquid is fixed, then the viscosity of the liquid is proportional to the shear stress.
Viscosity μ is defined as the ratio of the shear stress to the unit velocity difference of the fluid when the fluid is subjected to shear stress. The mathematical expression is:
τ=μ*∂u/∂y
In the formula:
τ is the shear stress, ∂u/∂y is the velocity gradient along the y direction (perpendicular to the fluid velocity direction), also known as the shear deformation rate; μ is the proportionality constant, that is, the viscosity coefficient, which is equal to when the velocity gradient is one unit, The tangential force experienced by the fluid per unit area.
In the commonly used centimeter·gram·second system, the unit of viscosity coefficient is Poise.
The International System of Units uses Pa·s (1 poise=1 dyne·second/cm2=10-1 Pa·s). For most fluids, the commonly used unit is centipoise cP (10-3 Pa·s).

Rotational viscometer

A viscometer with a rotational measuring system. It has many types: according to the measurement parameters, it can be divided into torque measurement type (speed does not change) and speed measurement type (torque does not change); according to different device structures, it can be divided into coaxial cylinder type and cone plate type. For a torque-measuring viscometer with a coaxial cylinder structure, when the outer cylinder rotates at a certain speed, the viscosity can be expressed by measuring the torsional moment exerted on the axis of the inner cylinder. It can measure absolute viscosity, is suitable for high temperature and high pressure, and is commonly used in industrial production of chemical fibers, papermaking, resin polymers, etc.
Also known as a Brookfield-type viscometer, a torsion spring is used to measure the torque required to rotate a material’s central axis. Changing the spindle speed and size allows different viscosity ranges to be measured.
Come to find different types of rotational viscometer

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