Viscosity

Viscosity is a measure of a <a href="/facts/Fluid/tMQ46Tmz">fluid</a>'s rate-dependent <a href="/facts/Drag_(physics)/vdl4jF7I">resistance</a> to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of thickness; for example, <a href="/facts/Syrup/sYS48lTx">syrup</a> has a higher viscosity than <a href="/facts/Water/0lkXnJPK">water</a>. Viscosity is defined scientifically as a force multiplied by a time divided by an area. Thus its <a href="/facts/SI_unit/S7YLglNz">SI units</a> are newton-seconds per square meter, or pascal-seconds.
Viscosity quantifies the internal <a href="/facts/Friction/d0108Rpu">frictional force</a> between adjacent layers of fluid that are in relative motion. For instance, when a viscous fluid is forced through a tube, it flows more quickly near the tube's center line than near its walls. Experiments show that some <a href="/facts/Stress_(physics)/ErAXDbbd">stress</a> (such as a <a href="/facts/Pressure/eyc37GRG">pressure</a> difference between the two ends of the tube) is needed to sustain the flow. This is because a force is required to overcome the friction between the layers of the fluid which are in <a href="/facts/Relative_motion/bVyzQ02h">relative motion</a>. For a tube with a constant rate of flow, the strength of the compensating force is proportional to the fluid's viscosity.
In general, viscosity depends on a fluid's state, such as its temperature, pressure, and rate of deformation. However, the dependence on some of these properties is negligible in certain cases. For example, the viscosity of a <a href="/facts/Newtonian_fluid/hpzNyDH5">Newtonian fluid</a> does not vary significantly with the rate of deformation. 
Zero viscosity (no resistance to <a href="/facts/Shear_stress/aWWnzIhE">shear stress</a>) is observed only at <a href="/facts/Cryogenics/cw7dHAkp">very low temperatures</a> in <a href="/facts/Superfluidity/QuePDujN">superfluids</a>; otherwise, the <a href="/facts/Second_law_of_thermodynamics/m7SJ2eeH">second law of thermodynamics</a> requires all fluids to have positive viscosity. A fluid that has zero viscosity (non-viscous) is called ideal or inviscid.
For <a href="/facts/Non-Newtonian_fluid/jX8ldWJa">non-Newtonian fluid</a>'s viscosity, there are <a href="/facts/Pseudoplastic/gX9dSHgP">pseudoplastic</a>, <a href="/facts/Plastic_flow/z1LEmkQX">plastic</a>, and <a href="/facts/Dilatant/scuKwVtq">dilatant</a> flows that are time-independent, and there are <a href="/facts/Thixotropic/SwtTPjlv">thixotropic</a> and <a href="/facts/Rheopectic/cCqE22aJ">rheopectic</a> flows that are time-dependent.

Viscosity open-in-new

Viscosity