Difference Between Rheology and Viscosity

The key difference between rheology and viscosity is that rheology is the study of the flow of matter, whereas viscosity is a measure of its resistance to deformation.

Rheology is a branch of physics or physical chemistry, while viscosity is a quantitative measurement that is useful in chemistry. These two terms are related to fluids such as liquids and gases.

1. Overview and Key Difference
2. What is Rheology
3. What is Viscosity
4. Side by Side Comparison – Rheology vs Viscosity in Tabular Form
5. Summary

What is Rheology?

Rheology is the study of the flow of matter primarily in liquid and gas state. However, this term can be used regarding soft solids or solids that are under conditions where the solids respond to the plastic flow rather than deformation that occurs elastically as a response to an applied force. This study area is a branch of physics that deals with the deformation and flow of materials regarding both solids and liquids.

Generally, rheology accounts for the behaviour of non-Newtonian fluids by characterizing the lowest number of functions required to relate stresses with the rate of change of strain or strain rates. The opposite phenomenon or rheology is rheopecty. Some non-Newtonian fluids show rheopecty where viscosity increases with relative deformation, and this is named shear thickening or dilatant materials.

Figure 01: Rheology of Time Independent Fluids

We can give the rheological behaviour as an experimental characterization that is named rheometry. However, the term rheology is interchangeably used along with rheometry by experimentalists. Practically, rheology involves extending continuum mechanics in order to characterize the flow of materials that can exhibit a combination of elastic, viscous, and plastic behaviour through properly combining elasticity and fluid mechanics.

What is Viscosity?

Viscosity of a fluid is the measure of its resistance towards deformation at a given rate. When considering liquids, it corresponds to the informal concept of thickness, e.g. the viscosity of syrup is higher than that of water.

We can show viscosity by quantifying the internal frictional force which arises between adjacent layers of fluid that occurs in a relative motion. For example, when we are forcing a viscous fluid through a tube, it tends to flow more quickly near the axis of the tube compared to the flow near walls. Experimentally, in this type of situations, the fluid requires some stress in order to sustain the flow through the tube.

Difference Between Rheology and Viscosity

Figure 02: Viscosity in a Diagram

Theoretically, we can observe zero viscosity of fluid only at very low temperatures in superfluids. A fluid having no resistance to shear stress is an ideal fluid or an inviscid fluid. According to the second law of thermodynamics, all fluids have a positive viscosity and these fluids are usually called viscous fluids or viscid fluids.

What is the Difference Between Rheology and Viscosity?

Rheology is a branch of physics or physical chemistry. Viscosity is a quantitative measurement that is useful in chemistry. These two terms are related to fluids such as liquids and gases. The key difference between rheology and viscosity is that rheology is the study of the flow of matter whereas viscosity is a measure of its resistance to deformation. The following infographic summarizes the difference between rheology and viscosity in a tabular form.

Below is a summary of the difference between rheology and viscosity in tabular form.

Summary – Rheology vs Viscosity

Reference:

1. “Rheology.” Wikipedia, Wikimedia Foundation, 27 Mar. 2021, Available here.

Image Courtesy:

1. “Rheology of time independent fluids” By g-sec – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Laminar shear” By Duk at the English-language Wikipedia (CC BY-SA 3.0) via Commons Wikimedia