Rheology of Complex Fluids
Introduction
Rheology is the study of the flow of matter, primarily in a liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force. It applies to substances that have a complex molecular structure, such as muds, sludges, suspensions, polymers and other glass formers (e.g., silicates), as well as many foods and additives, bodily fluids (e.g., blood) and other biological materials or other materials that belong to the class of soft matter.
Basic Principles
The word rheology originates from the Greek words 'rheo-' translating to 'flow' and '-logia' meaning 'study of'. The term was coined by Eugene C. Bingham, a chemist, and a physicist, Markus Reiner. The study of rheology is concerned with elucidating the 'laws of flow and deformation' for these complex fluids.
Rheological Models
There are several models used in rheology to describe the flow characteristics of complex fluids. These include the Newtonian model, the Power-law model, the Bingham plastic model, the Herschel-Bulkley model, and the Casson model. Each of these models has its strengths and limitations, and their applicability depends on the specific characteristics of the fluid being studied.
Newtonian Fluids
Newtonian fluids are the simplest type of fluid, in which the shear stress is directly proportional to the rate of strain. This means that the fluid's viscosity, which is the ratio of shear stress to shear rate, is constant. Examples of Newtonian fluids include water and thin oils.
Non-Newtonian Fluids
Non-Newtonian fluids, on the other hand, do not follow Newton's law of viscosity. Instead, their viscosity changes with the rate of shear strain. Non-Newtonian fluids can be further classified into several types, including pseudoplastic fluids, dilatant fluids, plastic fluids, and viscoelastic fluids.
Rheometry
Rheometry refers to the experimental techniques used to determine the rheological properties of materials, that is the way they respond to deformation as a function of time, temperature, stress, and other conditions. The rheometer is the instrument used in these investigations.
Applications of Rheology
Rheology has a wide range of applications in both industrial and physiological systems. It is used in the food industry to help formulate products with desirable texture and stability. In the oil industry, rheology is used to predict how fluids will flow in reservoirs. In medicine, understanding the rheology of blood is crucial in diagnosing and treating cardiovascular diseases.