The Physics of Non-Newtonian Fluids in Industrial Applications
Introduction
Non-Newtonian fluids are a class of fluids that do not follow Newton's law of viscosity. This law states that the shear stress between adjacent fluid layers is proportional to the negative gradient of their velocity. However, in non-Newtonian fluids, viscosity can change when under force to either more liquid or more solid. Non-Newtonian fluids have numerous applications in various industries due to their unique flow characteristics.
Classification of Non-Newtonian Fluids
Non-Newtonian fluids can be classified into several types based on their flow behavior. These include pseudoplastic fluids, dilatant fluids, plastic fluids, and rheopectic and thixotropic fluids.
Pseudoplastic Fluids
Pseudoplastic fluids, also known as shear-thinning fluids, exhibit a decrease in viscosity with an increase in shear rate. Examples of pseudoplastic fluids include ketchup, nail polish, and blood.
Dilatant Fluids
Dilatant fluids, also known as shear-thickening fluids, behave in the opposite manner to pseudoplastic fluids. Their viscosity increases with the increase in shear rate. Examples of dilatant fluids include quicksand and a mixture of cornstarch and water.
Plastic Fluids
Plastic fluids behave as a solid at low stresses but flow like a fluid when the stress exceeds a certain threshold value, known as the yield stress. Toothpaste is an example of a plastic fluid.
Rheopectic and Thixotropic Fluids
Rheopectic fluids are fluids whose viscosity increases over time when subjected to a constant shear stress. Thixotropic fluids, on the other hand, are fluids whose viscosity decreases over time under a constant shear stress. Certain types of clay and creams exhibit thixotropic behavior.
Physics of Non-Newtonian Fluids
The physics of non-Newtonian fluids is complex and involves understanding the behavior of these fluids under different conditions. The flow behavior of non-Newtonian fluids is typically represented using rheograms, which plot the shear stress against the shear rate.
Rheological Models
Several rheological models have been developed to describe the flow behavior of non-Newtonian fluids. These include the Power-law model, the Bingham plastic model, and the Herschel-Bulkley model.
Power-law Model
The Power-law model, also known as the Ostwald-de Waele model, describes the flow behavior of pseudoplastic and dilatant fluids. The model states that the shear stress is proportional to the shear rate raised to a power, known as the flow behavior index.
Bingham Plastic Model
The Bingham plastic model describes the flow behavior of plastic fluids. According to this model, a fluid behaves as a solid below the yield stress and flows as a Newtonian fluid above the yield stress.
Herschel-Bulkley Model
The Herschel-Bulkley model is a generalized model that describes the flow behavior of all types of non-Newtonian fluids. This model incorporates the yield stress, the consistency index, and the flow behavior index.
Industrial Applications of Non-Newtonian Fluids
Non-Newtonian fluids find numerous applications in various industries due to their unique flow characteristics.
Food Industry
In the food industry, non-Newtonian fluids such as ketchup, mayonnaise, and honey are commonly encountered. Understanding the flow behavior of these fluids is crucial for their processing and packaging.
Pharmaceutical Industry
In the pharmaceutical industry, non-Newtonian fluids are used in the formulation of various products such as creams, ointments, and gels. The flow behavior of these fluids plays a significant role in their manufacturing and application.
Cosmetic Industry
In the cosmetic industry, non-Newtonian fluids are used in the formulation of products such as nail polish, mascara, and lotions. The flow behavior of these fluids is important for their application and performance.
Oil and Gas Industry
In the oil and gas industry, drilling fluids, which are non-Newtonian fluids, are used in the drilling process. These fluids help in the removal of drill cuttings, cooling and lubricating the drill bit, and maintaining the pressure in the wellbore.
Conclusion
Non-Newtonian fluids, with their unique flow characteristics, have numerous applications in various industries. Understanding the physics of these fluids is crucial for their effective utilization in these applications.