Rheology and Viscosity in Pharmaceutical Formulations

Rheology and Viscosity

• Rheology is the study of the flow and deformation properties of matter.
• Viscosity is the resistance of a fluid to flow or movement.

Poiseuille's Law

• For a liquid flowing through a capillary tube, Poiseuille's law states that the viscosity of the fluid is proportional to the pressure difference, radius of the capillary, and length of the capillary, and inversely proportional to the volume of the liquid.
• The law can be simplified to η = (k * P * L) / (8 * V * t), where η is the viscosity, k is a constant, P is the pressure difference, L is the length of the capillary, V is the volume of the liquid, and t is the time of flow.
• Poiseuille's law can be used to compare the viscosity of two fluids, where the ratio of the viscosities of the two fluids is equal to the ratio of their flow times.

Viscosity Measurement

• The viscosity of a fluid can be measured using a viscometer, which consists of a capillary tube and a pressure difference.
• The flow time of a fluid through the capillary tube is proportional to its viscosity.

Types of Fluids

• Newtonian fluids have a constant viscosity, which means their shear rate is directly proportional to the shear stress.
• Non-Newtonian fluids have a viscosity that changes with the shear stress, and their behavior can be described by different types of flow, including plastic, pseudoplastic, and dilatant flow.

Non-Newtonian Fluids

• Plastic or Bingham flow: the fluid does not flow until a minimum stress (yield value) is applied, and the relationship between shear stress and shear rate is not linear.
• Pseudoplastic flow: the fluid flows as soon as the shear stress is applied, and the viscosity changes with the shear stress, making it a "shear thinning" fluid.
• Dilatant flow: the fluid's viscosity increases with the shear stress, making it a "shear thickening" fluid.

Key Concepts

• Shear stress (σ) is the tangential force applied per unit of surface area, measured in N/m2.
• Shear rate (γ) is the rate at which the velocity of flow decreases, measured in s-1.
• Dynamic viscosity (η) is the coefficient of proportionality between the shear stress and shear rate, measured in Pa·s.
• Apparent viscosity is the viscosity of a non-Newtonian fluid at a defined shear stress value and temperature.
• The power law can be used to describe the behavior of non-Newtonian fluids, where the apparent viscosity is proportional to the shear stress raised to a power (n).

Test your knowledge on rheology, viscosity, and their importance in pharmaceutical formulations. Learn about Newtonian and non-Newtonian fluids and their behavior. Join the Vevox quiz to assess your understanding of these concepts.