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The defining equations for viscosity are not fundamental laws of nature, so their usefulness, as well as methods for measuring or calculating the viscosity, must be established using separate means. A potential issue is that viscosity depends, in principle, on the full microscopic state of the fluid, which encompasses the positions and momenta ...
The three viscosity equations now coalesce to a single viscosity equation = = because a nondimensional scaling is used for the entire viscosity equation. The standard nondimensionality reasoning goes like this: Creating nondimensional variables (with subscript D) by scaling gives
A simple and widespread empirical correlation for liquid viscosity is a two-parameter exponential: = / This equation was first proposed in 1913, and is commonly known as the Andrade equation (named after British physicist Edward Andrade). It accurately describes many liquids over a range of temperatures.
The Vogel–Fulcher–Tammann equation, also known as Vogel–Fulcher–Tammann–Hesse equation or Vogel–Fulcher equation (abbreviated: VFT equation), is used to describe the viscosity of liquids as a function of temperature, and especially its strongly temperature dependent variation in the supercooled regime, upon approaching the glass transition.
The viscosity of the sample is then calculated using the following equation: = ˙ where is the sample viscosity, and is the force applied to the sample to pull it apart. Much like the Meissner-type rheometer, the SER rheometer uses a set of two rollers to strain a sample at a given rate. [ 31 ]
The Huggins equation is valid when [] is much smaller than 1, indicating that it is a dilute solution. [2] The Huggins coefficient used in this equation is an indicator of the strength of a solvent. The coefficient typically ranges from about 0.3 {\displaystyle 0.3} (for strong solvents) to 0.5 {\displaystyle 0.5} (for poor solvents).
Relative viscosity (a synonym of "viscosity ratio") is the ratio of the viscosity of a solution to the viscosity of the solvent used (), =. The significance in Relative viscosity is that it can be analyzed the effect a polymer can have on a solution's viscosity such as increasing the solutions viscosity.
From this equation the molecular weight of a polymer can be determined from data on the intrinsic viscosity and vice versa. The values of the Mark–Houwink parameters, and , depend on the particular polymer-solvent system. For solvents, a value of = is indicative of a theta solvent.