If you poured some water down a slope, it would flow freely and quickly, but what about if you repeated the process with honey? Can you imagine how honey would flow down a slope? Sticky oozy honey would probably move quite a bit slower than the water. This is because honey has a different viscosity to water.
Viscosity is the measure of resistance of a fluid to flow. A fluid that is highly viscous has a high resistance (like having more friction) and flows slower than a low-viscosity fluid. To think of viscosity in everyday terms, the easier a fluid moves, the lower the viscosity. Using our earlier example, which fluid has a greater viscosity? Honey would move slower than water, so honey would have a greater viscosity.
Pressure and temperature are also considered when talking about the viscosity of a liquid. If the viscosity does not change with pressure, we describe something as being a Newtonian fluid (named after Sir Isaac Newton). However, if the viscosity does change as stress or temperature changes, we describe something as being a non-Newtonian fluid.
Viscosity is also related to a fluid’s density. Density describes the space (volume) that is occupied by a substance’s particles and is measured in kilograms per cubic metre (kg/m3). Note that this does not describe a substance’s weight.
Viscosity is usually measured in pascal-seconds (Pa-s), a measure of force equalling 1 kilogram per metre per second (kg/(m·s)).
While viscosity is most commonly associated with fluids, gases also have viscosities.