Viscosity

thumb|300px|The formation of a spherical drop (liquid)|droplet of liquid [[water minimizes the surface area, which is the natural result of surface tension in liquids.]]

When two fluid layers move relative to each other, a friction force develops between them and the slower layer acts to slow down the faster layer. This internal resistance to flow is described by the fluid property called viscosity, which reflects the internal stickiness of the fluid. In liquids, viscosity arises from cohesive molecular forces, while in gases it results from molecular collisions. Except for the case of superfluidity, there is no fluid with zero viscosity, and thus all fluid flows involve viscous effects to some degree.

Irish mathematician and physicist (1819–1903)

fluid in which the viscous stresses arising from its flow, at every point, are linearly proportional to the local strain rate—the rate of change of its deformation over time; the simplest mathematical model of fluid that account for viscosity

fluid that does not follow Newton's Law of Viscosity, viscosity (the measure of a fluid's ability to resist gradual deformation by shear or tensile stresses) of non-Newtonian fluids is dependent on shear rate or shear rate history

material that behaves as a rigid body at low stresses but flows as a viscous fluid at high stress

scientific phenomenon

measurement of property of a thermoplastic polymer as a means of quality control

material property relevant for characterizing fluid flow

measure of solute's contribution to viscosity

a tensor used in continuum mechanics