cohesion and adhesion
/ koh-HEE-zhun / ad-HEE-zhun /
Spill water on a freshly waxed car and it beads into round droplets that won't spread; spill it on a clean dry sponge and it soaks right in, sticking to everything it touches. The difference is a contest between two pulls: water sticking to itself, and water sticking to the surface. Those two pulls are cohesion and adhesion.
Cohesion is the attraction between molecules of the same substance — water clinging to water. Adhesion is the attraction between molecules of different substances — water clinging to glass, paper, or skin. Both come from the same intermolecular forces; the only difference is whether the partners are alike or different. Which one wins at a surface decides what the liquid does there.
Why it matters: the tug-of-war between cohesion and adhesion explains droplet shapes, whether a surface gets wetted or stays dry, which way a meniscus curves, and whether a liquid climbs or sinks in a thin tube. The honest caveat is that neither is a special new force — they are just the same molecular attractions, named by who the two partners are: same substance (cohesion) or different substances (adhesion).
On a waxy leaf, water's cohesion beats its weak adhesion to wax, so it rolls into beads; on clean glass, adhesion wins and the same water spreads into a thin film.
Whether a drop beads up or spreads is a cohesion-versus-adhesion contest.
Cohesion and adhesion are not new kinds of force — both are ordinary intermolecular attraction, just classified by whether the two partners are the same substance (cohesion) or different substances (adhesion). The balance between them governs wetting, menisci, and capillary action.