A Basic Introduction to Rheology

There are a number of rheometric tests that can be performed on a rheometer to determine flow properties and viscoelastic properties of a material and it is often useful to deal with them separately. The first part of this introduction will focus on flow and viscosity and the tests that can be used to measure and describe the flow behavior of both simple and complex fluids. In the second part deformation and viscoelasticity will be discussed.

Rheometry refers to the experimental technique used to determine the rheological properties of materials; rheology being defined as the study of the flow and deformation of matter which describes the interrelation between force, deformation and time.  The term rheology originates from the Greek words ‘rheo’ translating as ‘flow’ and ‘logia’ meaning ‘the study of’, although as from the definition above, rheology is as much about the deformation of solid-like materials as it is about the flow of liquid-like materials and in particular deals with the behavior of complex viscoelastic materials that show properties of both solids and liquids in response to force, deformation and time. There are a number of rheometric tests that can be performed on a rheometer to determine flow properties and viscoelastic properties of a material and it is often useful to deal with them separately.  Hence for the first part of this introduction the focus will be on flow and viscosity and the tests that can be used to measure and describe the flow behavior of both simple and complex fluids.  In the second part deformation and viscoelasticity will be discussed.

Viscosity

There are two basic types of flow, these being shear flow and extensional flow. In shear flow fluid components shear past one another while in extensional flow fluid components flow away or towards one other.  The most common flow behavior and one that is most easily measured on a rotational rheometer or viscometer is shear flow, and this viscosity introduction will focus on this behavior and how to measure it.

Shear Flow

Shear flow can be depicted as layers of fluid sliding over one another with each layer moving faster than the one beneath it.  The uppermost layer has maximum velocity while the bottom layer is stationary. For shear flow to take place a shear force must act on the fluid. This external force takes the form of a shear stress (σ) which is defined as the force (F) acting over a unit area (A) as shown in Figure 1. In response to this force the upper layer will move a given distance x, while the bottom layer remains stationary. Hence we have a displacement gradient across the sample (x/h) termed the shear strain (γ). For a solid which behaves like a single block of material, the strain will be finite for an applied stress – no flow is possible. However, for a fluid where the constituent components can move relative to one another, the shear strain will continue to increase for the period of applied stress. This creates a velocity gradient termed the shear rate or strain rate ( ) is the rate of change of strain with time (dγ/dt).

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