All normal or Newtonian fluids (air, water, oil, honey) follow the same scientific laws. On the other hand, there are also fluids that do not follow the Newtonian flow laws. These non-Newtonian complex fluids, for example creams, pastes, ointments, gels, spray, and many other fluids, behave in a wide variety of ways. The science of studying these types of unusual materials is called rheology. The term came from Greek rheo meaning to flow and logos means science. Rheological properties are studied with rheometry.
Examples of Complex Fluids in Pharmaceuticals
- Gels (creams, particle precursors)
- Emulsions (creams)
- Aerosols (nasal sprays)
Many pharmaceutical processes such as new ingredient selections, formulation preparations, material packaging, and shelf storage are associated with a complex flow of materials. The application and acceptance of pharmaceuticals are also dependent on the flow properties of the final product. Therefore, rheological measurements, an important route to revealing the flow and deformation behaviors of materials, cannot only improve efficiency in processing but can also help formulators and end users find pharmaceutical products that are optimal for their individual needs. In general, rheological measurements on pharmaceutical materials are performed for the following reasons:
- to understand the fundamental nature of a system;
- for quality control of raw materials, final products, and manufacturing processes such as mixing, pumping, packaging, and filling; and
- to study the effect of different parameters such as formulation, storage time, and temperature on the quality and acceptance of a final product.
Pharmaceutical materials range in consistency from fluid to solid. Semisolid products are the most difficult materials to characterize rheologically because they combine both liquid and solid properties within the same material. The majority of pharmaceutical materials are ointments, creams, pastes, and gels—all semisolids.
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