CHAP. 10: TRANSPORT PROCESSES [CONTENTS] 337
10.3.2 Viscosity.
Viscosity is the measure of internal friction between molecules in a flowing fluid. The higher
the value ofηthe slower is the flow of the fluid (under otherwise identical conditions).
U Main unit:kg m−^1 s−^1 = Pa s.
Older unit:1 poise (P); 1 P = 10−^1 kg m−^1 s−^1 ; a hundred-times smaller unit, a centipoise, 1 cP
= 1 mPa s used to be in common use.
Note:Make sure that you do not confuse the terms viscosity and density. Mercury density
is much higher than that of honey or puree. Viscosity of honey or puree, on the contrary,
is much higher than that of mercury. Honey or puree would take much longer to flow
spontaneously out through a hole in the bottom of a pot as compared with mercury.
10.3.2.1 Dependence on state variables.
According to kinetic theory, viscosity in gases at low and medium pressures increases with tem-
perature and does not depend on pressure, see section10.5.3, relation (10.21). The description
of the viscosity of real gases by kinetic theory is relatively correct.
The viscosity of liquids is higher than that of gases. It increases (slightly) with pressure^1
and decreases with increasing temperature. The following empirical relation is often used for
the temperature dependence of viscositylnη=A−B
T
, [p] (10.10)whereAandBare adjustable constants.
Glass and macromolecular substances exhibit extremely high viscosity. For solid crystalline
substances, however, viscosity is effectively infinite.
The most widely spread helium isotope^4 He represents a special case. In the range of very
low temperatures, 0–2 K and in the pressure range from 0 to about 2.5 MPa, this isotope
forms asuperfluid liquidcalledhelium II(helium I is a liquid of common properties, stable
at temperatures above 2 K). The viscosity of superfluid helium II is zero; it is the only liquid(^1) Water is an exception: at low temperatures and moderate pressures, its viscosity decreases with increasing
pressure.