poise
Dynamic viscosityexact by definition
The poise is the CGS unit of dynamic viscosity, one dyne second per square centimetre, which is exactly 0.1 pascal seconds. The relation is exact because both systems are decimal. It is now used almost entirely in its hundredth, the centipoise.
| 1 P | 0.1 Pa·s |
Named for Jean Léonard Marie Poiseuille, the French physician who in the 1840s studied the flow of liquids through narrow tubes while investigating blood circulation. The relation he established between flow rate, pressure drop, tube radius and viscosity is still called the Hagen-Poiseuille equation.
About the poise
The poise commemorates work that began as physiology rather than physics. Poiseuille was studying blood flow in capillaries and needed to know how pressure drop related to tube diameter, so he ran careful experiments on water in fine glass tubes. The result, that flow rate is proportional to the fourth power of the radius, is the single most consequential fact in pipe and vessel sizing: halving a tube's diameter cuts its flow by a factor of sixteen at the same pressure. It explains why a small amount of arterial narrowing has a large effect on blood flow and why undersized tubing cripples a hydraulic circuit.
The unit itself is now mostly encountered as the centipoise, because a poise is inconveniently large for ordinary liquids: water is 0.01 P. The relation \(1\ \mathrm{P} = 0.1\ \mathrm{Pa\,s}\) is exact, since a dyne is exactly 10⁻⁵ N and a square centimetre is exactly 10⁻⁴ m². The kinematic counterpart in the same CGS family is the stokes, named for George Gabriel Stokes, equal to 1 cm²/s or 10⁻⁴ m²/s, and it is the unit whose centistokes form is so easily confused with the centipoise.