Apparent viscosity is the ratio of stress to rate of strain of a liquid. It is used to describe liquids that have different viscosities depending on conditions. The equations typically used for calculating viscosity refer to Newtonian fluids, which are those that can be measured with a standard viscometer. Apparent viscosity is the calculation obtained from applying the Newtonian equations to non-Newtonian fluids.
The apparent viscosity is measured using a rheometer, which is a device that controls more parameters than a standard viscometer. There are two types of rheometers whose use are determined by the parameters that need to be controlled. The two types of rheometers are shear and extensional rheometers. Shear rheometers measure apparent viscosity by controlling the shear stress that occurs between different layers of the fluid. Extensional rheometers control the stress that occurs within layers.
Apparent viscosity can also be thought of as friction within a fluid. The causes of viscosity are the stickiness of different atoms and molecules within the fluid. Similar to situations with high friction between solids, high viscosity (both standard and apparent) results in less movement between different parts of the fluid.
The apparent viscosity is very important for oil drilling. The apparent viscosity of drilling fluid is measured in order to appropriately determine the drilling fluid to be used for the drill.