ABSTRACT
Rheology is the study of flow properties of liquids. It addresses the viscosity characteristics of solution and colloidal systems. 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. These fluids are termed non-Newtonian, and they are said to exhibit non-Newtonian flow characteristics. Viscosity of a liquid generally decreases as the temperature is raised. The temperature–viscosity relationships are critical for routine handling of liquids in pharmaceutical manufacturing under ambient temperature conditions and handling of drug products by patients. Thixotropy is the property of some non-Newtonian fluids to show a time-dependent change in viscosity. For dilatant (shear-thickening) thixotropic fluids, the longer the fluid undergoes shear, the more its viscosity. For pseudoplastic (shear-thinning) thixotropic fluids, the longer the fluid undergoes shear, the lower its viscosity.
