What is adsorption?

• A. The adhesion of developer molecules to the part surface creating a film
• B. The process by which developer enters a surface opening increasing the volume of the penetrant helping to leach it to the surface
• C. The evaporation of solvent
• D. A chemical reaction between developer and surface

Answer: (B)

Adsorption is the attraction of penetrant molecules to a surface, forming a layer at the interface rather than soaking into the bulk. In liquid penetrant inspection, after the penetrant has had time to enter surface openings, applying developer brings the penetrant to the surface by adsorbing its molecules at the defect openings. This increases the local concentration of penetrant at the surface and helps draw (leach) it out to the surface, making the indication visible. The description that the developer enters surface openings to increase the penetrant volume and aid its migration to the surface captures this surface-attraction behavior. Evaporation of solvent, a chemical reaction, or mere bulk absorption do not describe the adsorption process.