Viscosity unit

Viscosity is a physical quantity that measures the degree of viscosity of a fluid. Also known as the coefficient of viscosity or dynamic viscosity, denoted as μ. Newton’s law of viscosity states that in pure shear flow, the shear stress (or viscous frictional stress) between adjacent fluid layers is given by the formula, where dv/dy is the normal velocity gradient perpendicular to the flow direction. The viscosity is numerically equal to the shear stress experienced by a fluid under a unit velocity gradient. The velocity gradient also represents the rate of angular deformation in fluid motion; therefore, viscosity represents the ratio between shear stress and the rate of angular deformation. In the International System of Units, the unit of viscosity is pascal-second. Pascals or centipascals are also sometimes used (1 Pascal = 10-1 Pa·s, 1 centipascal = 10-2 Pascal). Viscosity is a property of fluids, and different fluids have different viscosity values. The viscosity of the same fluid is significantly dependent on temperature, while it is almost independent of pressure. The viscosity of gases increases as temperature rises, while that of liquids decreases. When the temperature is T＜2000 Kelvin, the viscosity of a gas can be calculated using the Sutherland formula: μ/μ0 = (T/T0)³/² × (T0+B) / (T+B), where T0 and μ0 are the reference temperature and corresponding viscosity, and B is a constant related to the type of gas; for air, B = 110.4 Kelvin ； Or use the power formula: μ/μ0 = (T/T0)^n, where the exponent n varies depending on the type of gas and temperature; for air, it can be taken as 8/ρ in the range of 90 K < T < 300 K. The viscosity of water can be calculated using the following formula: μ = 0.01779 / (1 + 0.03368t + 0.0002210t2), where t is the temperature in degrees Celsius. Viscosity can also be determined experimentally, such as by using a viscometer. In many formulas of fluid mechanics, viscosity often appears in the form of μ/ρ together with density ρ; therefore, v = μ/ρ is defined. Since the unit of v, namely meters squared per second, consists only of kinematic units, it is called dynamic viscosity. Viscosity is a measure of the internal frictional force that arises between molecules when a liquid moves under the action of an external force. Kinematic viscosity is a measure of the internal friction force experienced by a liquid as it flows under the influence of gravity; it is equal to the ratio of dynamic viscosity to density at the same temperature, and is expressed in meters squared per second in the International System of Units. *It is expressed in centistokes (cSt). 1 centistere = 10-6 m2/s = 1 mm2/s. Viscosity is a measure of a fluid’s flowability, indicating the magnitude of resistance resulting from the friction between molecules as they flow. There are three ways to express it: dynamic viscosity refers to the internal friction force that arises when two layers of fluid, each with an area of 1 m2 and separated by a distance of 1 m, move relative to each other at a speed of 1 m/s. Unit: Pa·s (pascals·seconds). Kinematic viscosity: the ratio of dynamic viscosity to the density P of the fluid at the same temperature. The unit is m2/s. Engler viscosity: at a certain concentration, it is the ratio of the time required for 200 ml of liquid to flow through an Engler viscometer to the time required for the same volume of distilled water to flow at 20°C. Viscosity index: The viscosity decreases as the oil temperature rises; oils with a high viscosity index are less likely to become thinner due to increased oil temperature. The VI value of synthetic oils is higher than that of mineral oils. Low Viscosity Index LVI: Generally refers to a viscosity index below 40. A high viscosity index HVI generally refers to a viscosity index in the range of 80 to 110. Absolute viscosity: In some cases it is also referred to as dynamic viscosity, as opposed to kinematic viscosity. Viscosity coefficient: The resistance within a fluid to flow, expressed as the ratio of the shear stress to the shear rate of the fluid. The unit is poise. Note: For Newtonian fluids, the ratio of shear stress to shear rate is constant and is known as Newtonian viscosity; for non-Newtonian fluids, this ratio changes with shear stress, and the resulting viscosity is referred to as the \"apparent viscosity\" at that specific shear stress. Plastics fall into the latter category.

Common unit conversions for viscosity: 1 centipoise (1cP) = 1 millipascal-second (1mPa.s); 100 centipoises (100cP) = 1 poise (1P); 1000 millipascal-seconds (1mPa.s) = 1 pascal-second (1Pa.s). Conversion between dynamic viscosity and kinematic viscosity: η = ν × ρ, where η represents the dynamic viscosity of the sample in mPa.s, ν represents the kinematic viscosity of the sample in mm2/s, and ρ represents the density of the sample at the same temperature as that used for measuring the kinematic viscosity in g/cm3. For liquids, the higher the pressure and the lower the temperature, the greater the viscosity； The lower the pressure, the higher the temperature, and the lower the viscosity. For gases, pressure has little effect ； The higher the temperature, the greater the viscosity; the lower the temperature, the smaller the viscosity.

Could that person please provide a detailed explanation of the Sutherland equation? \"The viscosity of a gas can be calculated using the Sutherland equation: μ/μ0 = (T/T0)^(3/2) * (T0+B) / (T+B). Here, T0 and μ0 represent the reference temperature and corresponding viscosity, while B is a constant related to the type of gas; for air, B = 110.4 Kelvin.\"”