Definition of Coefficient of Viscosity
The coefficient of viscosity (often denoted by μ or η), also known as dynamic viscosity, is a quantitative measure of a fluid's resistance to gradual deformation or flow due to shear stress. It represents the internal friction within a fluid, indicating how 'thick' or 'thin' a fluid is. A high coefficient means a fluid resists flow strongly (like honey), while a low coefficient indicates easy flow (like water).
Newtonian vs. Non-Newtonian Fluids
For a Newtonian fluid, the coefficient of viscosity is constant, regardless of the shear rate applied. Its flow behavior can be fully described by a single viscosity value. In contrast, non-Newtonian fluids have a variable coefficient of viscosity that changes with the shear rate. Examples include ketchup (shear-thinning, becomes less viscous when stirred) or cornstarch suspension (shear-thickening, becomes more viscous under stress).
Measuring the Coefficient
The coefficient of viscosity is typically measured using instruments called viscometers or rheometers. Common methods involve measuring the force required to move one layer of fluid past another (rotational viscometers), or timing how long it takes for a fluid to flow through a narrow tube or for a sphere to fall through the fluid (capillary and falling-sphere viscometers). These devices quantify the shear stress and shear rate to determine the viscosity.
Importance and Applications
Understanding the coefficient of viscosity is vital across many fields. In engineering, it's crucial for designing lubrication systems, pipelines, and hydraulic machinery. In biology, it's relevant to blood flow dynamics and the movement of bodily fluids. Industrially, it impacts the processing of paints, polymers, and food products, ensuring consistent quality and performance. Its value is often highly dependent on temperature and pressure.