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How does glue work?

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Quick answer

Glue works by adhesion and cohesion: it starts as a liquid that spreads into tiny surface pores, then hardens through cooling, evaporation, or chemical reaction. Adhesion sticks the glue to the surfaces, while cohesion holds the glue itself together, forming a bond that resists pulling apart.

Glue works through adhesion, where liquid glue wets surfaces and hardens to hold them together. It also relies on cohesion, the internal strength of the glue itself, to resist separation.

Step by step

  1. Glue is applied as a liquid or soft solid so it can flow and cover the surfaces.
  2. It wets the surfaces, spreading into microscopic cracks and pits to maximize contact area.
  3. The glue hardens by losing solvent, cooling, or undergoing a chemical reaction.
  4. Adhesive forces between the glue and the surfaces create a strong bond.
  5. Cohesive forces within the glue keep it from tearing apart under stress.

Key things to understand

  1. Glue must first be a liquid or soft solid to spread and make close contact with surfaces.
  2. It wets the surfaces, meaning it flows into microscopic pits and grooves to increase contact area.
  3. The glue then hardens by cooling, evaporating, or chemical reaction, locking the surfaces in place.
  4. Adhesion is the force between the glue and the surfaces, created by intermolecular attractions like van der Waals forces.
  5. Cohesion is the internal strength of the glue, which must hold together to keep the bond intact.

Labelled diagram

  1. Liquid glue

    Applied as a fluid that can spread and fill gaps.

  2. Wetting

    Glue flows into microscopic pits on the surfaces.

  3. Hardening

    Glue solidifies by cooling, drying, or chemical reaction.

  4. Adhesion

    Intermolecular forces attract glue to both surfaces.

  5. Cohesion

    Glue molecules stick to each other, holding the bond together.

How glue bonds two surfaces

Real-world examples

  • Wood glue seeps into the pores of timber and hardens as water evaporates, creating a bond stronger than the wood itself.
  • Cyanoacrylate super glue polymerizes instantly on contact with moisture in the air, bonding skin or metal in seconds.
  • Hot glue melts and flows into fabric or paper, then solidifies as it cools, forming a flexible but strong hold.
  • Epoxy resin mixes two liquids that react chemically, curing into a rigid bond that resists heat and solvents.

Common mistake

✗ Myth: Glue works only because it is a thick, sticky liquid.

✓ Fact: Glue works through intermolecular forces and close surface contact, plus mechanical interlocking in porous materials. It must wet the surfaces and then harden, creating adhesion and cohesion that resist separation.

People also ask

What makes glue sticky?
Glue's stickiness comes from adhesion, the attraction between glue molecules and the surface molecules. When glue wets a surface, it forms weak bonds like van der Waals forces across a large area. These many weak bonds add up to a strong hold, especially once the glue hardens.
How does glue stick to paper?
Paper is porous, so liquid glue soaks into its fibers. As the glue dries or cures, it hardens within the fibers and on the surface, creating a mechanical interlock plus chemical adhesion. This makes the bond strong because the glue is anchored inside the paper.
Why does glue not stick to some plastics?
Many plastics, like polyethylene and Teflon, have low surface energy. Glue cannot form strong adhesive forces with them because their molecules do not attract glue molecules well. The glue's cohesion is stronger than its adhesion, so it peels off easily.
Why does glue not stick to its container?
A sealed container stops glue losing solvent or meeting moisture, so it stays liquid, and containers use low-surface-energy plastics such as polyethylene that glue grips poorly. The glue's cohesion is stronger than its adhesion to the container, allowing it to be poured or squeezed out.
How does super glue dry so fast?
Super glue cures through a chemical reaction called anionic polymerization, triggered by moisture in the air or on surfaces. This rapid chain reaction turns the liquid into a solid plastic in seconds, creating a strong bond.
Does glue work in space?
Yes. Inside a spacecraft cabin there is air, so most glues cure normally. In the vacuum outside, solvent-based glues dry badly and give off vapours, so engineers rely on adhesives that cure by chemical reaction, such as epoxies.
What makes glue stronger than tape?
Glue forms a continuous layer that fills microscopic gaps, creating many more contact points and stronger intermolecular forces. Tape has a pre-formed adhesive layer that may not conform as closely, so its bond is usually weaker.

Part of the guide: Everyday Science: Light, Heat & Matter →