Wetting and surface tension

In detail

Water's molecules attract each other strongly, which pulls a droplet into the smallest surface it can manage and is why water beads on a waxed car or a greasy plate. That is surface tension, and it is the first obstacle in any cleaning process: a liquid that beads is not in contact with most of the surface, so nothing it contains can act. Surfactants lower surface tension — the word is a contraction of surface-active agent — which lets the solution spread into the texture, the weave, the grout line and the corner. Wetting explains several practical things. It is why a drop of washing-up liquid in a bucket makes plain water work better even where no grease is involved. It is why a solution left to dwell outperforms one wiped straight off, since spreading takes time. It is why hydrophobic coatings on glass and paint work — they raise the contact angle so water rolls off carrying soil with it rather than drying in place. And it is why the glass trade specifies plenty of water: a well-wetted surface floats abrasive grit away from the tool instead of trapping it underneath.

Why

• Linear alkylbenzene sulphonate is explained by Wetting and surface tension
• Rinse aid is explained by Wetting and surface tension