Viscosity

The official unit for viscosity is the Poise. It is named after the French physicist and physiologist Jean Leonard Marie Poiseuille.

1 Poise is 0.1 kg m^-1 s^-1 (water is 1 centiPoise (cP) at 20ºC )

One can better appreciate these units physically if one thinks of this in terms of pressure through time. Thus viscosity is sometimes reported as Pascal-seconds (Pa.s) as in your other reading.

The velocity of an object, v, depends on the force on the object, F, times the mobility, u.

v = uF

The mobility, u, is the reciprocal of the friction, f. For spheres f = 6πηr

Where η is the viscosity and r is the radius of the sphere.

For motion in water at the cellular and molecular level, drag is insignificant and thus ignored. This is motion at low Reynold’s number.

Different areas of science use different symbols so it is important to be aware of this and not be confused. As biologists, you should use the symbols used by biologists and biophysicists. Thus for this class you will use the symbols that I have specified here.