Rheology in Physical Pharmacy

Study Notes

Rheology: The study of how materials flow and deform under applied stress.
Importance in Pharmacy: Influences the stability, efficacy, and patient compliance of pharmaceutical formulations.

Viscosity: Resistance to flow.
- Measured in Pascal-seconds (Pa·s) or centipoise (cP).
- Example: Syrup is thicker than water due to higher viscosity.
Shear Stress: Force applied per unit area.
Shear Rate: Rate of deformation under shear stress.
- Example: Increasing shear rate during mixing can reduce viscosity in paint.
Thixotropy: Time-dependent shear thinning, becoming less viscous over time.
- Example: Thixotropic gels become easier to spread when stirred.
Yield Stress: Minimum stress required to initiate flow.
- Example: Creams with high yield stress maintain shape until force is applied.
Elasticity: The ability to return to original shape after deformation.
- Example: Elasticity maintains the form of ointments after application.

Newtonian Fluids: Constant viscosity regardless of shear rate (e.g., water, ethanol).
Non-Newtonian Fluids: Viscosity changes with shear rate.
- Pseudoplastic (Shear-Thinning): Viscosity decreases with shear rate (e.g., ketchup).
- Dilatant (Shear-Thickening): Viscosity increases with shear rate (e.g., cornstarch in water).
- Bingham Plastics: Require a yield stress to flow (e.g., toothpaste).

Formulation Development: Designing formulations that are easily administered and stable.
Quality Control: Ensuring product consistency and quality.
Stability Studies: Predicting formulation behavior under storage conditions.
Drug Delivery Systems: Influencing drug release rates from gels and ointments.