This statement may sound amusing, ignorant or confusing but the wetting ability of liquids in non-destructive testing is a real thing and very important in our craft. The cohesive forces between molecules in a liquid causes surface tension. An example of the influence of surface tension is the tendency of free liquid, such as a droplet of water, to contract into a sphere. In such a droplet, surface tension is counterbalanced by the internal hydrostatic pressure of the liquid. Take for instance the use of glass conditioners on one’s vehicle, before the conditioner is applied the water seems to just make “sheets” of water covering the entire windshield and slowly makes it’s
way down the slope of the windshield. This is an example of good wetting ability. After the application of a glass conditioner (e.g. Rain-x) the water beads up, no sheeting,and runs off the wind shield’s slope very quickly. This is an example of poor wetting ability. When the liquid comes into contact with a solid surface, the cohesive force responsible for the surface tension competes with the adhesive force between the molecules of the liquid and the solid surface. These forces jointly determine the contact angle between the liquid and the surface. If the angle is less that 90 degrees, the liquid is said to wet the surface, or to have good wetting ability; if the angle is equal or greater than 90 degrees, the wetting ability is considered poor.
Why is this important in NDT?
Simply put, the wetting ability of a liquid determines the effectiveness of the capillary action involved in liquid penetrant inspection techniques. Non-wet liquids will tend to stay out of the cracks and crevices (beading or bubbling up) we are typically searching for, whereas liquids with good wetting ability will flow into the areas of concern with actual force forming a meniscus, or convex shape, entering the opening.