Newtonian fluids and non-Newtonian fluids are two types of fluids that exhibit different flow behaviors and viscosity characteristics.
Newtonian Fluids:
Newtonian fluids are fluids that follow Newton's law of viscosity, which states that the shear stress (force per unit area) is directly proportional to the shear rate (velocity gradient) in the fluid. In other words, the viscosity of Newtonian fluids remains constant regardless of the applied shear rate or shear stress. Examples of Newtonian fluids include water, most gases, and some low-viscosity oils.
Non-Newtonian Fluids:
Non-Newtonian fluids are fluids that do not follow Newton's law of viscosity, and their viscosity can change with shear rate or shear stress. The viscosity of non-Newtonian fluids may increase (shear-thickening behavior) or decrease (shear-thinning behavior) with increasing shear rate, or may exhibit more complex behaviors such as yield stress, thixotropy, or viscoelasticity. Examples of non-Newtonian fluids include slurries, suspensions, emulsions, gels, pastes, and many biological fluids such as blood, saliva, and mucus.
Non-Newtonian fluids are further classified into several categories based on their flow behavior:
Shear-Thinning (Pseudoplastic): These fluids decrease in viscosity with increasing shear rate. Examples include ketchup, yogurt, and some paints.
Shear-Thickening (Dilatant): These fluids increase in viscosity with increasing shear rate. Examples include cornstarch mixed with water, quicksand, and some suspensions.
Bingham Plastic: These fluids exhibit a yield stress, which must be exceeded before they start to flow, and then exhibit Newtonian behavior once they are in motion. Examples include some toothpaste, clay slurries, and drilling mud.
Thixotropic: These fluids exhibit a time-dependent decrease in viscosity with continuous shear stress, and recover their original viscosity when shear stress is removed. Examples include some paints, printing inks, and certain gels.
Viscoelastic: These fluids exhibit both viscous (flow) and elastic (deformation and recovery) properties, and their behavior is time-dependent and frequency-dependent. Examples include polymer solutions, foams, and some biological fluids.
The flow behavior and viscosity characteristics of non-Newtonian fluids can be complex and varied, and may require specialized techniques and instruments, such as rheometers or Rotational Viscometers, for accurate characterization and measurement. Understanding the rheological properties of non-Newtonian fluids is important in various industrial, scientific, and medical applications, as it can affect their processing, performance, and behavior in different conditions.