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Viscosity is the resistance to flow of a fluid and varies greatly between fluids due to the ease of molecule sliding. It is measured by dividing shear stress by velocity gradient. Kinematic viscosity is the dynamic viscosity divided by fluid density. Newtonian fluids maintain a constant dynamic viscosity, while non-Newtonian fluids like ketchup have high viscosity at low flow rates but become thin at high flow rates.
The dynamic viscosity of a fluid is defined as the applied shear stress divided by the velocity gradient obtained when a shear force is applied to a fluid. Viscosity varies greatly between fluids. It is important in the flow behavior of liquids.
The term used to describe the resistance to flow of a fluid is viscosity. Syrup is more viscous than water; molasses more viscous than syrup. Viscosity is a result of how easily molecules slide past each other and is a function of the chemistry, shape and temperature of the molecules. Gases also have variable viscosities. At the same temperature, oxygen is twice as viscous as hydrogen or ammonia, while the viscosity of nitrogen is somewhere in between.
The classic viscosity measurement comes from a test consisting of two smooth plates separated by a thin film of fluid. The bottom plate is fixed. The experimenter, using a known force, attempts to slide the top plate past the bottom plate. Within the liquid film, the molecules near the bottom plate have zero velocity. The fluid molecules near the top plate have the same velocity as the top plate.
While the fluid can be a gas or a liquid, it’s easier to think of liquids in this test. If the substance separating the plates is water, the top plate will slide sideways very easily. With molasses as a liquid, the top plate will move slowly with the same force applied.
A shear force is in the direction of the force, as opposed to tensile or compressive forces, which in this case would be the forces required to push the plates apart or together. Shear stress is the shear force divided by a unit area and is expressed as newtons per square meter (N/m2) or pascals (Pa). Viscosity calculations are usually done in metric units.
The velocity gradient is the difference in the flow velocity of the liquid near the bottom plate, which is zero, compared to the liquid near the top plate, which is equal to the velocity of the top plate. Dynamic viscosity is therefore the shear stress divided by the shear rate and is measured in pascal-seconds (Pa-s). In practice, viscosity is measured in centipoises (cPo). One centipoise equals one second millipascal.
Another measure of viscosity is kinematic viscosity. Kinematic viscosity is the dynamic viscosity divided by the density of the fluid. Kinematic viscosity is easier to measure than dynamic viscosity, so it is often used to determine dynamic viscosity.
A Newtonian fluid maintains a constant dynamic viscosity independent of shear rate. Ketchup is a non-Newtonian fluid. It has a high dynamic viscosity when the flow rate is low but becomes almost as thin as water when the flow rate is increased. This explains why people shake ketchup bottles to speed up the flow of ketchup, but then end up with too much on their plate.
