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Adhesion and cohesion

Two different forces with one outcome. Whether a deposit holds to the surface or holds to itself decides whether you lift it off in one piece or in a hundred.

In detail

Adhesion is the attraction between a deposit and the surface under it. Cohesion is the attraction of the deposit to itself. Almost every practical difference between an easy deposit and a hard one comes from the ratio between the two. A deposit with high cohesion and low adhesion — chewing gum on glass, candle wax on a worktop, a peelable adhesive — comes away in one piece if you can get an edge under it, because it would rather hold on to itself than to the surface. A deposit with high adhesion and low cohesion — soot on stove glass, a dried milk film, polymerised oil — tears rather than lifts, so you remove it in layers and each pass looks like progress without finishing. Knowing which you have changes the tool. High cohesion rewards patience and a plastic edge; high adhesion rewards softening, dissolving or converting the deposit rather than pulling at it. It also explains a common frustration: on a rough or porous surface, adhesion rises sharply because the deposit has more contact area and mechanical keying into the texture, which is why the same mark wipes off gloss tile and refuses to leave grout.

Why

Sources for this page

Sources

  1. Kirk-Othmer Encyclopedia of Chemical Technology

    John Wiley & Sons · Reference work

    Relied on for: That interfacial forces between a deposit and a substrate, and within the deposit itself, govern how material is removed from a surface. (background and mechanism only)

    Verification: the work exists and is described correctly. Nobody has yet read the specific passage.

  2. Spot Solver: spot and spill removal procedures

    The Carpet and Rug Institute (CRI) · Trade or professional body

    Relied on for: That solids are broken up and removed mechanically before any solution is applied, and that semi-solids are scraped with a rounded edge. (states this directly)

    Verification: a person has read the passage relied on.

    Read on — Sprint 5-10: fifteen entries, stain by stain. Sprint 30: enumerated at 296 from the publisher's own sitemap, 134 more read plus 'Unknown'. Sprint 38: 56 more, chosen against stains the catalogue lacked. 192 of 296 read. Supports the claim as stated.

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