Rheology

Rheology

Introduction

Rheology is the study of flow properties of liquids. It addresses the viscosity characteristics of solution and colloidal systems. The flow of simple liq-uids can be described by viscosity, an expression of the resistance to flow. Liquids and solutions for which the flow characteristics can be adequately described by viscosity are called Newtonian fluids, and they are said to exhibit Newtonian flow characteristics. However, the flow of complex dispersions cannot be adequately described by viscosity. These fluids are termed non-Newtonian, and they are said to exhibit non-Newtonian flow characteristics.

Rheological properties are important considerations in the manufactur-ing, analysis, and use of several dosage forms, including solutions, emulsions, suspensions, pastes, lotions, suppositories, parenteral injectable drug prod-ucts, and intravenous infusions. Viscosity is important for the mixing and flow of materials, their packaging into containers, and their removal before use—whether achieved by pouring from a bottle, extrusion from a tube, or passage through a syringe needle. For example, pourability, spreadability, and syringeability of an emulsion are determined by its rheological properties.

In addition, viscosity is an important consideration as a critical material attribute (CMA) and critical (in-process) material attribute (CiMA) for pharmaceutical manufacturing. For example, viscosity of solutions of poly-mers and liquid ingredients such as polyethylene glycol is a CMA used for the quality control of incoming raw materials, while viscosity of suspension of polymer, colorant, and opacifier used for tablet coating is a CiMA dur-ing product manufacturing.