what is viscosity
Viscosity is an important property of fluids, it describes the liquid's resistance to flow and is related to the friction inside the fluid. A common type of flow behavior is shear flow, in which layers of fluid move relative to each other in response to shear forces. This external force takes the form of shear stress, which is defined as the force acting on a unit area of fluid, and produces a velocity gradient known as the shear rate across the thickness of the sample. The shear viscosity or dynamic viscosity in relation to this is given by the ratio of shear stress to shear rate as shown below.
non-newtonian fluid
Many simple fluids are classified as Newtonian, meaning that their viscosity is independent of the applied shear force. Examples include water and simple hydrocarbons. As the complexity of the fluid increases (e.g., through the inclusion of gas bubbles, droplets, particles or polymers), the fluid may exhibit more complex behavior and exhibit a non-Newtonian response, where the viscosity depends on the applied shear force. These types of fluids are often referred to as structured or complex fluids, and their behavior can be better described using a rheometer that can be used over a wider range of shear rates, shear stresses, and temperatures than a simple viscometer Viscosity ranges are measured within the range.
This non-Newtonian behavior is common in many industrial and commercial products, including toothpaste, mayonnaise, paints, cosmetics, and cement, which are typically shear-thinning fluids, although it can occur in some thickened structured fluids Shear thickens, but viscosity decreases with increasing shear rate.
Application of Viscosity
For most products, higher viscosity is required at low shear rates to prevent settling or collapse, but thinner at higher shear rates to facilitate application or processing. Therefore, a single viscosity measurement is insufficient to describe the viscosity of such materials, and viscosity should be measured over a range of shear rates or stresses, or at least at shear rates relevant to the process or application of interest. Non-Newtonian fluids may also exhibit other phenomena, such as yield stress, thixotropy, and viscoelasticity, which can have a significant impact on the properties of materials and product performance.
Other viscosity parameters related to dispersions are relative viscosity, specific viscosity, and intrinsic viscosity, which measure the contribution of a solute or dispersion to the viscosity of a solution or dispersion. These parameters are most easily determined using a differential viscometer, such as used with an OMNISEC gel permeation chromatography (GPC) system.
While a simple viscometer is usually sufficient to characterize the behavior of Newtonian fluids over a narrow range or shear rate, for non-Newtonian fluids and for applications requiring evaluation of a wider range of shear rates, more specialized viscometers and rheometers are required.
