Wetting
Wetting turns up in 2 fields: adhesives, welding. Here is what it means to the people in each one.
Wetting in adhesives
How well a liquid adhesive spreads over and makes intimate contact with a surface.
Adhesion needs molecular contact, so the adhesive must flow into every valley of the surface. Wetting is judged by the contact angle a droplet makes: low angles mean good spreading, while angles above 90 degrees mean the liquid beads. Contamination, low surface energy plastics and high-viscosity or partly gelled adhesive all reduce wetting.
"Look at the bead on that part, it's not wetting at all, sitting up like water on a waxed car."
Wetting in welding
Molten filler spreading across a clean surface and bonding to it, needed for a sound braze or solder.
Ability of molten braze or solder to spread over and bond to the base metal instead of balling up. It requires an oxide-free surface, provided by flux, protective atmosphere or vacuum, plus the right temperature. A low contact angle and a smooth, concave fillet show good wetting; filler beading up like water on wax shows poor cleaning, the wrong flux or underheating.
"Alloy's balling up and not wetting, the fitting's still oxidized, clean it and use more flux."