Eww... Why is it so sticky?

InfiniteNow · 06-19-2007

Per Jay's suggestion, this topic deserves it's own thread.

How does adhesive work? What makes sticky... sticky?

This post touched on the topic in response to recent advance with nanotubes acting like gecko feet:

http://hypography.com/forums/technol...tml#post178835

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Originally Posted by InfiniteNow

Straight Dope Staff Report: How does glue work?

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A good adhesive has excellent properties of adhesion (the ability to stick to the surfaces to which it's applied) and cohesion (the ability to stick to itself). When you pull apart something that's been glued together and the glue comes right off the pieces, that's an adhesive failure. If the glue itself splits apart, leaving glue on either side of the joint, then that's a cohesive failure:

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There's no universally accepted theory, and given the variety of adhesives more than one process may be at work. It's generally agreed that adhesion occurs at the molecular level, the chief processes involved being Van der Waals forces, ionic bonding, covalent bonding, and metallic bonding.

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According to recent research, Van der Waals forces explain how the lizards known as geckos can stick to so many surfaces in a seemingly impossible manner. Specifically, the tiny hairs on the gecko's feet (called setae) are split at the microscopic level into "as many as 1,000 branches, whose spatula-shaped tips are only 200 nanometers wide." As a result, even though the Van der Waals forces acting on an individual tip is small, the adhesion of a billion or so tips adds up to enough force to let the gecko stick to anything.

Persuasive as all this sounds, other scientists doubt that Van der Waals forces alone fully explain how glue works. They attribute at least some of the strength of adhesives to mechanical bonding, also known as mechanical locking. The idea here is that all surfaces, even seemingly smooth ones, are actually rough at the molecular level, and that an adhesive achieves some of its strength by flowing into the hidden valleys and imperfections of the surfaces to be bonded...In mechanical bonding as with Van der Waals forces, an adhesive needs to be able to spread finely into the microscopic roughness of the surfaces, wet the tiny cracks properly, and allow trapped air to escape. The main difference is that mechanical bonding will work at distances that would render Van der Waals forces useless.

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Still another type of adhesion involves the presence of tiny air bubbles--in 1999, French researchers presented a theory in Discover magazine that instant adhesives (like that used in Scotch-brand tape and Post-It notes) actually work by creating numerous microscopic bubbles each having a partial vacuum in them, which act as suction cups.

So, what is it? Why IS it so sticky?

Jay-qu · 06-19-2007

Ok, one idea that keeps coming up is this Van der Waals force. To be clear I think it would be prudent to define a Van der Waals force.

A Van der Waals force is an intermolecular force - ie between molecules. They can be one of two types: Dispersion and polar forces. Both these forces are due to electrostatic attraction between charges within the molecules.

The charges in molecules are not always symettric, random fluctuations can happen that incidently give rise to temporaty polar molecules - such as from electrons orbiting. While it can also happen that electrons are more stongly attracted to one particular part of the molecule. This gives rise to a polar molecules, these molecules experiece much stonger intermolecular bonding, than non-polar molecules that only experience dispersion forces that are much weaker in comparrison.

So that said, this Van der Waals force is often attributed as the culprit of adhesive sustances, any objections?

InfiniteNow · 06-19-2007

Per the article I shared above, many scientists question that it's just VDW force. Many suggest that there's a degree of mechanical bonding involved.

Also, something about bubbles and air pressure.

What are the key elements involved? Of course it's different things all contributing, but what does what, when, and why?

Jay-qu · 06-19-2007

Yeah I dont think it is entirely due to VDW in all cases - but it appears that in the carbon nanotube case it might be, because they seem to think it will work find in vacuum.

TheBigDog · 06-29-2007

I work for an adhesive company, with teams of researchers inventing new adhesives all the time. Unfortunately I am not an adhesive scientist, so all I can do is add some basic definitions to the discussion.

There are several ways that an adhesive is characterized. Not just stickiness, but what it sticks to, how long it sticks, can it be removed and restuck, etc.

Substrate is what you stick an adhesive to. An adhesive actually lies between two substrates, its job is to bind them together.

In the label industry adhesives fall into two major categories; permanent and removable.

A permanent adhesive means that its bonds with both substrates is stronger than the label material, so the label cannot be removed without destroying it, and it is difficult to remove the gummy residue. This is especially important in the medical industry where the labeling needs to be stronger than the forces that would destroy what is being labeled.

A removable adhesive has a stong bond with the label, but a weak enough bond with the substrate that it will peel off, ideally cleanly. The best example of this is Post-It notes which have one of the best removable adhesives around.

Ironically the Post-It adhesive was not being made to be removable, but to never degrade. It was a success in this regard (it has the ability to stick over and over again before its weight becomes stronger than the adhesion), but it was not nearly as sticky as the scientist was shooting for. It turned into one of the most profitable accidents in recent history (for our competitor, 3M).

There are special adhesives built to survive in wet environments. Beer labels must not come loose even when submerged in water for very long periods of time. Labels for milk need cartons (dairy label) have a very specific adhesive for that environment. And labels for fruits that will stick to the skin, resist falling off, but peel off cleanly without leaving residue are custom made to the properties of what you are labeling. I guess they need to be non-toxic as well.

We even have an adhesive that "turns off" when exposed to water of a certain temperature. We sell this to industries that need a permanent adhesive for marking containers, but want to reuse those containers over and over in different applications. Using hot water the label just falls off. When it dries it is sticky again.

Stamps have their own special adhesive to insure they don't come loose and to protect from forgery.

Some of our adhesives are very versatile; put it on thin and it is a good removable, put it on thick and it has a permanent quality. Of course the quality of the bond varies with the substrate you are attaching to, and the environmental conditions when you make the attachment. Some adhesions need direct pressure to be most effective. Others stick just as well virtually regardless of the pressure used to apply them.

Tape is an interesting product. The substrate of tape has different characteristics front and back. The adhesive is applied to one side or the other as the tape is wound (varies by product and manufacturing method), it gets squeezed between the two sides, and it sticks permanently to one side, and is removable from the other - thus you can peel off tape from the spool, but it is always sticky on the proper side. The tape degrades a bit each time you wrap and unwrap it. How much depends upon the adhesive and the qualities of the back side of the tape.

I will inquire at work about "what makes it sticky" and see if I can get an answer to that.

Bill

max4236 · 06-29-2007

I made a greenhouse using transparent duct tape. It's handy stuff. It sticks well to pvc pipes and it doesn't degrade in the sunlight. Because it's transparent? I'm not sure what makes it stick but I love that stuff. I was reading somewhere that duct tape doesn't work in space, either the adhesive freezes or something about being in a vacuum. You'd think it would be the temperature. I'm not sure though. I don't get into space much.

Eww... Why is it so sticky?

Hypography?

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