What if glue stuck to everything?

Glue sticks paper to paper. Tape sticks to a box.

But glue does not stick to everything. What if it did?

Glue holds your art project together and tape keeps boxes shut. Yet neither sticks to everything. Tape grips glass well but peels easily off some plastics and off non-stick coatings. Stickiness is choosy.

So what if glue stuck to everything it touched: the bottle, the brush, your fingers, the table? Would that be a super-glue dream or a sticky disaster? In this lab we follow three clues: how glue holds on, the invention of sticky notes, and the feet of geckos.

Every adhesive joint depends on two things: adhesion, the attraction between the adhesive and the surface, and cohesion, the adhesive's own internal strength. Researchers quoted by Scientific American explain that the stickiness of tape is a combination of the glue holding onto itself and holding onto the surface.

This thought experiment imagines an adhesive that bonds to every surface equally well. To reason about it, we need to know why ordinary adhesives are selective, how engineers sometimes want weak stickiness on purpose, and how geckos achieve a reversible grip without any glue at all.

Surfaces have tiny bumps, too small to see.

Glue flows into the bumps. Then it holds on tight.

Even a surface that feels smooth is covered with tiny bumps and dips. A good glue or tape starts out soft and runny enough to flow into them. That's one way it grips: like a key in a lock.

The other way is even smaller. When the glue's molecules get very close to the surface's molecules, they pull on each other weakly. One pull is tiny, but billions of them add up. Researchers at 3M, the company that makes many tapes, say a glue grips best when it can easily spread out over a surface.

Researchers from 3M's tape division, writing in Scientific American, describe two mechanisms. Mechanical adhesion occurs when the adhesive flows into the texture of the surface; specific adhesion includes electrostatic forces, van der Waals forces and acid-base interactions between adhesive and surface. A pressure-sensitive adhesive, the kind on tape, forms no chemical bond at all: it is a sticky, viscous, liquid-like material that adheres under pressure.

For good adhesion, the adhesive must flow out readily on the surface, and the degree of flow often determines the strength of the bond. With acrylic tape adhesives the bond typically grows over time, as the adhesive slowly flows into the surface texture.

Imagine glue that sticks to everything. It would stick to the bottle and the brush!

It would stick to your fingers, too. That would be a sticky mess.

Now imagine a glue that stuck to everything. At first it sounds great: it could fix anything. But think again. It would stick to the inside of its own bottle, to the brush, to your fingers and to the table. You might never get it out of the bottle at all!

Today, glues come in containers they don't stick to, and glues can't spread out well on non-stick coatings like Teflon. A glue that stuck to everything might make those tricks impossible. These are ideas, worked out from how glue works.

A universal adhesive would have to wet and bond to every surface, including those engineered to resist bonding. That immediately raises practical problems: it would bond to its own container and dispensing tools, and to skin, so storing and applying it might be impossible.

Selectivity is a feature, not a flaw. Teflon, used for non-stick coatings, is one of the low-surface-energy materials on which adhesives do not flow out well. Removable products depend on adhesives designed to grip weakly. A world where glue stuck to everything might lose both, along with the ability to choose where things stick. These consequences are reasoned, not observed.

Some surfaces are very slippery to glue. Glue can't spread out on them.

That's why tape won't stick well to a non-stick pan.

Glue sticks best when it can spread out and touch as much of a surface as possible. Some materials make that hard. 3M's researchers explain that tape spreads easily on metals, glass and some plastics, but not on plastics like polyethylene and polypropylene, or on Teflon, the slippery material used on some non-stick pots and pans.

Scientists describe this with something called surface energy. Surfaces with high surface energy let glue spread out; surfaces with low surface energy make it bead up instead. For glue to stick to everything, every surface would have to let it spread.

The flow of an adhesive over a surface depends on the difference between the surface energy of the material and that of the adhesive. According to the 3M researchers quoted by Scientific American, pressure-sensitive adhesives flow out on high-surface-energy materials, such as metals, glass, acrylic, polycarbonate and nylon, but not easily on low-surface-energy materials such as polyethylene, polypropylene and Teflon.

So a universal glue would need to wet even low-surface-energy polymers as well as it wets glass, and still hold together internally. Real adhesives are formulated for particular surface families, and surfaces are often treated before bonding.

Sticky notes stick, then peel off easily. Their glue was a happy surprise!

Geckos can walk up walls. Their feet have millions of tiny hairs.

Sometimes a weak glue is exactly what you want. In 1968, Spencer Silver, a scientist at 3M, made a glue of tiny spheres that held paper but peeled off easily. Years later his colleague Art Fry used it to make bookmarks that wouldn't fall out of his choir songbook. Sticky notes went on sale across the United States in 1980.

Geckos do something cleverer still, with no glue at all. In 2002, researchers at UC Berkeley reported that their toes are covered in millions of tiny hairs, each ending in about 1,000 even tinier pads. Weak pulls between molecules, added up over all those pads, let a gecko hold on, and then let go again.

The Lemelson-MIT Program describes Spencer Silver's 1968 adhesive as a high-quality 'low-tack' adhesive of tiny acrylic spheres that stuck only where they touched a surface, strong enough to hold papers together but weak enough to separate them. The National Inventors Hall of Fame notes that Art Fry later coated paper with it to make repositionable notes, introduced nationally in the United States in 1980.

Geckos achieve reversible adhesion without any adhesive. A 2002 UC Berkeley release reported that synthetic copies of a gecko seta tip ruled out water as the source of grip and pointed to van der Waals forces. Engineering professor Ron Fearing said it is geometry, not surface chemistry, that lets a gecko support its whole body on a single toe.