The stokes is a rare example of a word in the English language where the singular and plural forms are identical. One square meter per second is equal to one million centistokes. 1 cm 2/s =Įven this unit is a bit too large, so the most common unit is probably the square millimeter per second or the centistokes. One square meter per second is equal to ten thousand stokes.

A more common unit of kinematic viscosity is the square centimeter per second, which is given the name stokes after the Irish mathematician and physicist George Stokes (1819–1903). This unit is so large that it is rarely used. The SI unit of kinematic viscosity is the square meter per second, which has no special name. Capillary viscometers will be discussed in more detail later in this section. When two fluids of equal volume are placed in identical capillary viscometers and allowed to flow under the influence of gravity, the more viscous fluid takes longer than the less viscous fluid to flow through the tube. It is frequently measured using a device called a capillary viscometer - basically a graduated can with a narrow tube at the bottom. Kinematic viscosity is a measure of the resistive flow of a fluid under the influence of gravity. The other quantity called kinematic viscosity (represented by the Greek letter ν 'nu') is the ratio of the viscosity of a fluid to its density. The quantity defined above is sometimes called dynamic viscosity, absolute viscosity, or simple viscosity to distinguish it from the other quantity, but is usually just called viscosity. There are actually two quantities that are called viscosity. Ten poise equal one pascal second making the centipoise and millipascal second identical. The most common unit of viscosity is the dyne second per square centimeter, which is given the name poise after the French physiologist Jean Poiseuille (1799–1869). The pascal second is more rare than it should be in scientific and technical writing today.

Despite its self-proclaimed title as an international system, the International System of Units has had little international impact on viscosity. The SI unit of viscosity is the pascal second, which has no special name. Or if you prefer calculus symbols (and who doesn't)… F

The similarity to Newton's second law of motion ( F = ma) should be apparent. The more usual form of this relationship, called Newton's equation, states that the resulting shear of a fluid is directly proportional to the force applied and inversely proportional to its viscosity. (dynamic) viscosityįormally, viscosity (represented by the symbol η 'eta') is the ratio of the shearing stress ( F/ A) to the velocity gradient ( ∆ v x/∆ y or dv x/ dy) in a fluid. Fluids resist the relative motion of immersed objects through them as well as to the motion of layers with differing velocities within them. Informally, viscosity is the quantity that describes a fluid's resistance to flow.

Viscosity