Rheology is the study of flow properties of liquids. It addresses the viscosity characteristics of solution and colloidal systems.
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.
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