Metals present extremely high energy, reactive surfaces to the environment. When mechanically or chemically cleaned, they rapidly oxidize and adsorb contaminants such as organic vapors. Polymers present surfaces that are less reactive towards their surroundings. When cleaned by abrasion processes they also show rapid changes due to oxidation and adsorption, but these changes tend to be of lower magnitude.
Successful bonded repair of aircraft structures involves creating a small area of carefully controlled surface composition on metallic or polymeric surfaces. This area to be bonded is located within a larger area of material that may be contaminated with a variety of soils picked up during normal aircraft operation: organic and inorganic soils, fuel, hydraulic fluids, etc.
Because of the reactivity of freshly prepared surfaces and the proximity and mobility of contaminants in the surrounding area, cleaning of these surfaces sufficiently to obtain reliable adhesive bonds can be particularly difficult in field situations. Furthermore, because the difference between a well-cleaned surface and a contaminated one may only be a few molecular layers, it can be difficult for the technician to establish when the surface has been properly prepared.
Measurement of the geometry of a liquid drop deposited onto the surface can be done extremely rapidly and form the basis of a sensitive check of surface cleanliness and consistency in a repair depot or in challenging field situations. This paper discusses the use of these rapid wetting measurements for quality assurance of surface treatments for adhesively bonded repairs.