Viscosity
When we apply force on a matter, it will respond to the force differently depending on their intrinsic properties. In the case of liquids, they tend to flow upon the applied force. Because all the liquids are composed of molecules interacting with each other, they develop a flow resistance caused by internal friction. Intuitively, we can conclude that the higher the internal friction, the higher the viscosity. Interestingly, the viscosity values could differ depending on the types of liquids, and various factors such as temperature, how strong the force you applied to, and time. Let me explain how the viscosity differs upon the factors mentioned above in another post. :)
Kinds of viscosity
There are various kinds of viscosity, and sometimes we could confuse each terminology and their definitions. So I'm going to introduce some types of viscosity and their common usages in scientific research in this writing. I categorized various viscosity in three types; shear viscosity, intrinsic viscosity, and extensional viscosity.
Shear viscosity
Apparent viscosity
Apparent viscosity is the viscosity value that we can get from the rheometers. Rheometry is the measuring technology used to determine rheological properties, and a rheometer is a kind of machine that we can perform for the rheometry.
Dynamic viscosity
The synonym of dynamic viscosity is absolute viscosity and true viscosity. The dynamic viscosity is the 'true' viscosity of the matter, and generally, we cannot find out the exact values.
Kinematic viscosity
The definition of the kinematic viscosity involves the relationship between viscous forces and inertial forces in a fluid. Note that kinematic viscosity is always determined if gravitational force or the weight of the sample is the driving force, for example, flow cups, falling-ball, and capillary viscometers!
Intrinsic viscosity (e.g., IV)
We usually use intrinsic viscosity when we want to describe the viscous behavior of dilute polymer solution. In other words, we could know how the specific type of polymer contributes to the solution viscosity. For your information, when we can neglect the interaction between solutes, we call the system the dilute polymer solution. I'm going to deal with two cases for further understanding; a single component and multi-component system, respectively.
Case 1: A single component system
Relative viscosity
We can get relative viscosity by dividing the apparent viscosity value by the solvent viscosity. So it is a dimensionless value. The relative viscosity is a kind of normalization, so that the effects of dissolved polymers on the solution viscosity have been removed. We usually calculate the relative viscosity to differentiate the interaction between particles from the collective contribution of all solution components, e.g., solutes and solvent.
Specific viscosity
We can get specific viscosity by dividing the difference between the apparent viscosity and solvent viscosity by the solvent viscosity. Sometimes it is called relative viscosity increment, and it is also a dimensionless value.
Case 2: The multi-component system
Reduced viscosity
Reduced viscosity is the ratio of specific viscosity to the mass concentration of the polymer that is an interest. Recalling that the specific viscosity is a dimensionless value, the unit of reduced viscosity is mL/g.
Inherent viscosity (e.g., IHV)
Inherent viscosity is the ratio of the natural logarithm of the relative viscosity to the mass concentration of the polymer. The unit of inherent viscosity is the same as that of reduced viscosity; mL/g.
Extensional viscosity
When the direction of the applied force is normal to the surface, we should use other types of viscosity called extensional viscosity. Specifically, the extensional viscosity is used for the situation when the extensional deformation occurs.
According to Trouton's ratio, for a Newtonian fluid, the extensional viscosity value is exactly three times the shear viscosity.
