Viscosity

Viscosity

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Discussion

definitions

Informally, viscosity is the quantity that describes a fluid's resistance to flow. Fluids resist the relative motion of immersed objects through them as well as to the motion of layers with differing velocities within them.Formally, viscosity (represented by the symbol η "eta") is the ratio of the shearing stress (F/A) to the velocity gradient (Δvx/Δzor dvx/dz) in a fluid.

|η |⎛ |F |⎞ |÷ |⎛ |Δv|⎞ |or |
|= |⎝ | |⎠ | |⎝ |x |⎠ | |
|A | | |Δz | |A | | |dz |
| | | | | | | | | |
| | |⇕ | | | | |⇕ | |
| | | | | | || | |
|F |= |m |Δv |or |F |= |m |dv |
| | | |Δt | | | | |dt |

The SI unit of viscosity is the pascal second[Pa s], which has no special name. Despite its self-proclaimed title as an international system, the International System of Units has had very little international impact on viscosity. The pascal second is rarely used in scientific and technicalpublications today. The most common unit of viscosity is the dyne second per square centimeter[dyne s/cm2], which is given the name poise[P] after the French physiologist Jean Louis Poiseuille(1799-1869). Ten poise equal one pascal second [Pa s] making the centipoise [cP] and millipascal second [mPa s] identical.

1 pascal second = 10 poise = 1,000 millipascal second
1 centipoise = 1 millipascalsecond

There are actually two quantities that are called viscosity. The quantity defined above is sometimes called dynamic viscosity,absolute viscosity, or simple viscosityto distinguish it from the other quantity, but is usually just called viscosity. The other quantity called kinematic viscosity(represented by the symbol ν "nu") is the ratio of the viscosity of a fluid to its density.

|ν =|η |
| |ρ |

Kinematic viscosity is a measure of the resistive flow of a fluid under the influence of gravity. It is frequently measured using a device called a capillary viscometer — basically a graduated can with a narrow tube at the bottom. When two fluids of equal volume are placed in identical capillary viscometers and allowed to flow under the influence of gravity, a viscous fluid takeslonger than a less viscous fluid to flow through the tube. Capillary viscometers are discussed in more detail later in this section.

The SI unit of kinematic viscosity is the square meter per second [m2/s], which has no special name. This unit is so large that it is rarely used. A more common unit of kinematic viscosity is the square centimeter per second[cm2/s], which is given the namestokes[St] after the Irish mathematician and physicist George Gabriel Stokes (1819-1903). Even this unit is also a bit too large and so the most common unit is probably the square millimeter per second [mm2/s] or centistokes[cSt].

1 m2/s = 10,000 cm2/s [stokes] = 1,000,000 mm2/s [centistokes]
1 cm2/s = 1 stokes
1 mm2/s = 1 centistokes

factors affecting viscosity

Viscosity is first and foremosta function of material. The viscosity of water at 20 ℃ is 1.0020 millipascal seconds (which is conveniently close to one by coincidence alone). Most ordinary liquids have viscosities on the order of 1 to 1000 mPa s, while gases have viscosities on the order of 1 to 10 μPa s. Pastes, gels, emulsions, and othercomplex liquids are harder to summarize. Some fats like butter or margarine are soviscous that they seem more like soft solids than like flowing liquids. Molten glass is extremely viscous and approaches infinite viscosity as it solidifies. Since this process is not as well defined as true freezing, some believe (incorrectly) that glass may still flow even after it has completely cooled, but this is not the case. At ordinary temperatures, glasses are as solid as true solids....