Thus F must be the x -component of the force exerted across A by the fluid below A on the fluid above. This derivation shows plainly why does not depend on n . Higher gas densities yield a higher flux of particles to carry excess momenta , but the distance that they carry before suffering a collision diminishes as n increases. The dependence on therefore cancels out in the produce . We see from the expression for that the viscosity is proportional to .
Experiments show that the viscosity increases faster than .
This is due to the fact that molecules are not rigid spheres. At higher temperatures, the molecules have more kinetic energy and come closer during collision. Therefore the effective radius, a , decreases with increasing temperature. This increase of viscosity with temperature is a property of gases. The viscosity of liquids, in common experience, decreases with temperature.