Resistance of a fluid to a change in shape, or movement of neighbouring portions relative to one another. Viscosity denotes opposition to flow. It may also be thought of as internal friction between the molecules. Viscosity is a major factor in determining the forces that must be overcome when fluids are used in lubrication or transported in pipelines. It also determines the liquid flow in spraying, injection molding, and surface coating. The viscosity of liquids decreases rapidly with an increase in temperature, while that of gases increases with an increase in temperature. The SI unit for viscosity is the newton-second per square metre (N-s/m2).

This entry comes from Encyclopædia Britannica Concise.
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