Rheology in Pharmacy

Introduction to Rheology

• Rheology is the study of the flow of liquids and the deformation of solids, which affects the mixing and flow of materials, their packaging, and patient acceptability.
• Rheology influences physical stability and biologic availability.

Flow and Viscosity

• Flow occurs when stress is applied to a fluid, causing deformation.
• Stress is the force per unit area, while strain is the amount of deformation caused by the stress.
• Viscosity is the resistance of a fluid to flow due to shear stress, measured by the ratio of shear stress to shear rate.
• Fluidity is the inverse of viscosity, representing the ease of flow.

Types of Fluids

• Newtonian fluids have a linear relationship between stress and strain, with a constant viscosity at a given temperature.
• Non-Newtonian fluids do not have a linear relationship between stress and strain, and their viscosity changes with the shear rate.

Non-Newtonian Fluids

• Plastic fluids have a yield value, which is the minimum shear stress required to initiate flow.
• Pseudo-plastic fluids, or shear-thinning fluids, have a high viscosity at rest but flow easily when shaken, with a changing viscosity and mobility along the curve.
• Dilatant fluids, or shear-thickening fluids, have a high solid content and increase in viscosity with stress, often due to the formation of vacuum gaps between particles.

Thixotropy

• Thixotropic materials have a time-dependent, reversible viscosity, decreasing with stress and returning to their original state when stress is removed.
• This property is useful in pharmaceutical applications, such as sustained drug delivery systems, suspensions, and emulsions.

Factors Affecting Viscosity

• Intrinsic factors: molecular size, particle shape, and intermolecular forces.
• Extrinsic factors: temperature and additives.
• Viscosity of liquids decreases with temperature, while viscosity of gases increases with temperature.