angle between the liquid and the tubing wall is less than 90 degrees (the liquid wets the tube wall), the liquid will be
higher in the tube than on the outside. When the contact angle is 90 degrees or greater (poor wetting and high surface
tension), the liquid will not rise above the outside level and may even be depressed. Capillary rise occurs when a liquid
wets the inside of a tube and the surface tension draws additional liquid into the wetted area. Figure 2-9 illustrates the
effects of contact angles and capillary action.
Figure 2-9. The Rise or Depression of Liquid in a Capillary Tube Depends upon the Contact Angle.
Penetrant Entry Into discontinuities.
The description of capillary action illustrates the basic principles by which a penetrant enters a small surface opening.
If one end of the tube is closed, such as occurs in the case of a flaw, the capillary rise is affected by compression of the
air trapped in the closed end. In addition, flaws are not capillary tubes since the sides are not parallel and are not
circular. These factors allow penetrant to enter a flaw even in an inverted position, such as on a lower wing surface.
The points to be remembered about penetrant entry into discontinuities are:
a. A high surface tension and small contact angle are desirable in a penetrant, however these are
conflicting properties. High surface tension tends to increase contact angle and decrease wetting ability,
but enhances drawing penetrant into wetted areas.
b. Capillary force increases with smaller flaws.
c. Viscosity does not affect the penetrating ability but it can affect the time required for penetration.
d. Shape of a discontinuity can affect penetrant entry.
e. Temperature affects the surface tension.
f. Roughness of the flaw walls affects penetrant entry.
g. Contamination in the flaw can affect penetrant entry.
h. Residual cleaning solution in the flaw can effect penetrant entry.