Viscosity

Illustration of viscosity; The violet fluid at the bottom has a higher viscosity than the white one at the top

Viscosity is a physical property of fluids. It shows resistance to flow. In a simple example, water has a low viscosity, as it is 'thin'. Syrup or tar, on the other hand has a high viscosity, as it is 'thick'. A way to test for viscosity is the speed at which the substance runs down a slope. Syrup would reach the bottom very slowly, whereas water would be a lot quicker.

There are two types of viscosity: dynamic viscosity, measured in pascal seconds, and kinematic viscosity, measured in metres per second squared.

Viscosity is used as a way to predict when volcanoes erupt. When the lava comes out very thickly (viscous), there is more chance that it will erupt violently. This is because the lava has a hard time getting out and may burst out when it can. If the lava is thin (low viscosity), then it just flows out like water.

The word viscous comes from the Latin viscum, meaning sticky.

References

Continuum mechanics

Laws Conservation of mass Conservation of momentum Conservation of energy Entropy inequality Solid mechanics Solids Stress · Deformation Compatibility Finite strain · Infinitesimal strain Elasticity (linear) · Plasticity Bending · Hooke's law Failure theory Fracture mechanics Frictionless/Frictional Contact mechanics Fluid mechanics Fluids Fluid statics · Fluid dynamics Surface tension Navier–Stokes equations Viscosity: Newtonian, Non-Newtonian Rheology Viscoelasticity Smart fluids: Magnetorheological Electrorheological Ferrofluids Rheometry · Rheometer Scientists Bernoulli · Cauchy · Hooke Navier · Newton · Stokes