Apparent viscosity

Apparent Viscosity

Diagram illustrating apparent viscosity in non-Newtonian fluids.

Apparent viscosity is a concept used in the study of fluid dynamics to describe the viscosity of a non-Newtonian fluid under specific conditions of flow. Unlike Newtonian fluids, which have a constant viscosity regardless of the applied stress, non-Newtonian fluids exhibit a viscosity that changes with the rate of shear or stress applied.

Definition

Apparent viscosity is defined as the ratio of the shear stress to the shear rate in a fluid. It is expressed mathematically as:

\( \eta_{app} = \frac{\tau}{\dot{\gamma}} \)

where \( \eta_{app} \) is the apparent viscosity, \( \tau \) is the shear stress, and \( \dot{\gamma} \) is the shear rate. This relationship highlights that apparent viscosity is not a fixed property of the fluid but depends on the conditions under which it is measured.

Types of Non-Newtonian Fluids

Non-Newtonian fluids can be classified into several types based on their flow behavior:

• Shear-thinning fluids: Also known as pseudoplastic fluids, these fluids exhibit a decrease in apparent viscosity with an increase in shear rate. Examples include ketchup and blood.

• Shear-thickening fluids: Also known as dilatant fluids, these fluids show an increase in apparent viscosity with an increase in shear rate. An example is a suspension of cornstarch in water.

• Bingham plastics: These fluids behave as a solid at low shear stresses but flow as a viscous fluid at high shear stresses. Examples include toothpaste and mayonnaise.

• Thixotropic fluids: These fluids decrease in apparent viscosity over time under constant shear. An example is paint.

• Rheopectic fluids: These fluids increase in apparent viscosity over time under constant shear.

Measurement

The measurement of apparent viscosity is typically performed using a rheometer, an instrument that applies controlled shear stress or shear rate to a fluid sample and measures the resulting flow behavior. The data obtained from rheometers can be used to construct flow curves, which graphically represent the relationship between shear stress and shear rate for a fluid.

Applications

Understanding apparent viscosity is crucial in various industries, including:

• Food industry: Apparent viscosity affects the texture and pourability of products like sauces and dressings.

• Pharmaceutical industry: The formulation of creams and gels requires precise control of apparent viscosity to ensure proper application and absorption.

• Petroleum industry: The transport of crude oil and drilling fluids depends on their flow properties, which are influenced by apparent viscosity.

Related Pages

• Viscosity
• Rheology
• Newtonian fluid
• Non-Newtonian fluid