Most of us could do without stickiness a lot of the time. Sure, it has its uses, and none of us would be here today if it wasn’t for the various types of sticky mucus that lubricate, cleanse and protect our inner workings. But it quickly loses its charm when you’re trying to find the end of the Sellotape after opening a jar of honey.
Why are substances sticky?
Stickiness is the product of the forces of adhesion (the attraction of different substances to each other, through chemical bonds, electrical charges or entanglement) and cohesion (the attraction between the particles of a single substance).
Some things are stickier than others. Dewdrops are sticky enough to hang off an orb web but they won’t catch the spider any flies. Water scores high on cohesion and low on adhesion (which is why it forms drops on windows). An effective glue, too, needs to be cohesive, but it must also be, well, adhesive.
Nature came up with adhesives long before technology did. Indeed, the first postage stamp glue was made from the gum that oozes from wounds on acacias and other trees. Gum and similar substances, such as latex and resins, help defend the tree against herbivores by clogging their mouthparts or even engulfing them completely. It’s no coincidence that amber, a gem as renowned for the perfectly preserved ancient life-forms it contains as for its beauty, is fossilised tree resin.
Stickiness is also deployed offensively. Vampire squids (which can also turn themselves inside out) trail strings of tacky mucus behind them to catch food. Then there are the sticky traps of carnivorous plants such as sundews and, of course, spiders. Velvet worms, members of an ancient lineage of soft-bodied, millipede-like animals, entangle prey with a gelatinous fluid squirted at a distance from specialised mouthparts.
Stickiness also has its uses in locomotion. The mucous trails of slugs and snails are sticky enough to adhere to overhanging surfaces but not so sticky that the molluscs are glued to the spot. Geckos cling to pretty much any surface at any angle, without any glue at all, instead relying on close contact with the surface by means of the nano-structure of their feet.
What is the stickiest substance?
But the stickiest of all natural substances might be a glue produced by the freshwater bacterium Caulobacter crescentus to anchor itself to rocks. The scientists who discovered it in 2006 worked out that a 1cm² patch would be sufficient to suspend a 680kg weight – an average African buffalo, perhaps. But you’d need a very strong ceiling. And don’t try it after handling honey.
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Main image: a sundew plant. Credit: Getty