Rheology Definition

Rheology is basically the study of the flow behavior and the deformation of the materials, specially the unusual materials, like paints, mayonnaise, ketchups, foams etc. in the language of a chemical engineer, these materials, having unusual interesting flow behaviour and properties are called as non-newtonian fluids. These non-newtonian fluids have variable viscosity, upon the applied forces. So the rheology definition can be explained as the study of the fluids flow behavior.

Apart from the rheology definition itself, all materials do not possess same type of rheological properties. But the rheological properties of the fluids depend upon the viscosity behavior of the fluids. So the fluids can be divided into two kinds,

1)     Non-newtonian fluids, having time independent viscosities.

2)     Non-newtonian fluids, having time dependent viscosities.

This first type of newtonian fluid viscosity behavior is independent of the time. According to the non-newtonian fluid definition, this first type is further classified into three types:

• Pseudo plastic fluid  :  The viscosity of these fluids decreases with increased stress. Some of the examples are blood, syrups etc.

• Dilatant fluids              :  These are also named as shear thickening fluids. Their viscosity increases with increase in stress. The common dilatant fluid is sand and water mixture.

• Bingham plastic fluids :   The other name of these fluids are Newtonian fluids. Custard can be considered as the most common example of these Bingham fluids.

The second type of the fluids has the time dependent viscosity. There are further two classifications of these fluids.

• Rheopectic  : This newtonian fluid viscosity profile is directly proportional to the applied stress. And hence the viscosity shows increasing behavior with the increase in stress duration.

• Thixotropic : Their viscosity is inversely proportional to the applied stress, and hence the viscosity shows the decreasing behavior with the increased duration of the stress.

So, understanding the complete rheology definition, non-newtonian fluid definition, and types of fluids is essential to the study of the behavior of the fluids.