Explanation

In a steady flowing fluid, the ratio of shear stress to shear rate is called viscosity η Represent. viscosity is a measure of the internal resistance of a fluid, usually in Greek letters η (eta) to represent. Fluid under shear stress τ When acting on the flow, a velocity gradient (i.e. shear rate) is generated perpendicular to the flow direction (flow layer direction) y γ）， The shear stress is proportional to the shear rate, i.e τ = ηγ。η It is viscosity or viscosity coefficient, and its unit can be pashlu (Pa · s) or Newton per square meter (N · s/m 2）。 viscosity divided by fluid density is usually called dynamic viscosity, and its unit is square meters per second (m 2/ s) . Those who satisfy τ = ηγ Relationship and γ A fluid that is a constant is called a Newtonian fluid and can be（ τ, γ) The image is represented as a straight line, with a slope of η， therefore γ Also called absolute viscosity. Water, some low molecular liquids, and gases all belong to this type of fluid. However, some fluids, such as macromolecule solutions and melts, whose viscosity does not follow the above rule, can be used τ = K γ^ N to describe, where n may be greater than or less than 1, and k is the coefficient. These fluids are called non Newtonian fluids. For these fluids, apparent viscosity can be introduced η A. As γ Of the function of η，η A=k γ^ (n-1), that is, the apparent viscosity is γ The function of. The tangent slope d at any point on the curve η/ D γ It is called differential viscosity or consistency. If the shear force is sinusoidal, the viscosity obtained will become the complex viscosity η'， Consisting of real and imaginary parts, where the real part η' It is called dynamic viscosity. High viscosity fluids under tensile stress σ (T) When applied, the deformation rate s in the tensile direction will also change with time. tensile viscosity or Truton viscosity η_ T is defined as η_ T= σ/ s. Among them σ Is the stress, s is the deformation rate. Common instruments for measuring viscosity include capillary tube viscosity (for low viscosity liquid, the range is 0.5-1600 mm 2/ s) , falling ball viscosity (for medium viscosity liquid, the range is 0.5-106 mPa · s), sharp hole viscosity (commonly used in petroleum products, coatings and inks, the range is 1-1000 mPa · s), pressurized capillary tube rheometer (for macromolecule melts, the range is 100-100000 Pa · s), and rotary viscosity (there are many types, for different ranges, usually 2-109 mPa · s), some are used for routine testing and quality control, Some for scientific research can even measure the dynamic complex viscosity at the same time.

Classification