Viscosity
Viscosity describes a fluid's resistance to flow. Liquids such as water that flow easily, have a lower viscosity than do liquids such as ketchup. Viscosity has little effect on the ability of a
penetrant material to enter a defect but it does have an effect
on the speed at which the penetrant fills a defect. The equations for the fill times of a cylindrical void and an elliptical void are shown below:
Cylindrical Void
Fill time = (2l^{2}m)/
rcosq s_{LG}
Elliptical Void
Fill time = [(2l^{2}m)
/ s_{LG}cosq]
* [a^{2}+b^{2} / (a+b)ab]
Where: l = defect depth
m = viscosity
r = radius of the crack opening
s _{LG} = liquidgas surface
tension
q = contact angle
a = flaw width
b = flaw length
From these equations, it can be seen that fill time
is directly proportional to penetrant viscosity. While it has
no real bearing on this discussion, it should be noted that the
two equations do not take into account entrapped gas that could
be present in a closed end capillary.
Reference:
Deutsch, S. A, Preliminary Study of the Fluid Mechanics of Liquid
Penetrant Testing, Journal of Research of the National Bureau
of Standards, Vol. 84, No. 4, July  August 1979, pp. 287291.
