At the Marine Science Laboratory (MSL) in North Wales, scientists spend a lot of time watching flies walk up a wall. No, they're not bored. They're interested in finding out how the flies appear to defy gravity and stick to the wall.
Why? Because any insight into zygology -- the science of joining things together, named from the Greek word for "yoke" -- can have very important practical applications.
Rivets, nails, screws, welds and threads play a large part. But where would we be without adhesives? Furniture would disintegrate, tiles would drop from walls, books and shoes would fall apart and we couldn't cap our teeth. Just imagine life without wallpaper, stamps, Scotch tape or Post-it notes!
Any investigation of stickiness really must begin at the microscopic level. So the MSL researchers first looked at the feet of flies. Each has two little claws at the tip that can be used to grip irregular surfaces. But flies can walk up and down surfaces smooth as glass with equal ease. Could there be some adhesive secretion playing a role?
They decided to see whether flies walking on glass left a residue. They did! When a glass slide over which flies had walked was treated with a stain that detects fatty materials, tiny footsteps appeared. Could this fat act as a glue?
An ingenious experiment was quickly concocted. A thin tether was attached to the back of a fly with Super Glue. Then a gauge was used to measure the force needed to lift the fly off the glass.
Next, the fly was made to walk a plank covered with filter paper soaked in hexane, a solvent that dissolves fatty materials. Once more, the fly was placed on a slide and hoisted into the air. This time, the force needed was only one-tenth as large. Flies really do glue themselves to walls.
But what is the chemical explanation? Why does gum stick to hair but not to Teflon? Why is a sugar solution sticky and vinegar not?
Stickiness is a very complex business. The basic principle is that any two surfaces brought close enough will adhere because of electrical attraction between any atoms or molecules.
Because the negatively charged electrons in an atom or molecule are not stationary, at any given moment there will be a negative region where a majority of electrons are found and a corresponding positive region where there are fewer. This arrangement is called a "dipole."
The negative end of one molecule can be attracted to the positive pole of an adjacent one. This is called "van der Waals" attraction, after Johannes Diderik van der Waals, the Dutch physicist who won the Nobel Prize in physics in 1910.
When two surfaces are brought so close that van der Waals forces can be exerted between their molecules, we have stickiness. Each attractive force is small, but there are billions and billions of dipoles on each surface.
So why can't we repair broken china by just fitting the pieces together?
In practice, it is virtually impossible to bring two solid surfaces so close that they will exert van der Waals forces on each other. No matter how smooth a surface may seem, there are tiny peaks and valleys. And van der Waals forces are exerted over very short distances -- a few tenths of a billionth of a meter, or about 11100,000th the width of human hair.
But if one of the surfaces is mobile, its molecules can flow into the valleys and cover the peaks of the other surface, bringing the molecules close enough to attract one another. Just think of what happens when a little honey is placed between two fingers.
Mobility alone, however, is not enough. After all, water flows and forms van der Waals bonds. But it isn't sticky. So there is another criterion. To act as an adhesive, a material not only must stick to both surfaces, but its component molecules also must form strong links to each other so they are not separated as the surfaces are pulled apart. That eliminates water, with its notoriously weak bonds between molecules.
Honey, by contrast, has sugar molecules strongly attracted to each other as well as to other molecules. It is therefore sticky. But of course, it's not sticky enough to use as an adhesive. For that, we need something like the flour-and-water paste once familiar to schoolchildren.
When wet, the stuff is mobile. But as it dries, the long starch molecules in the flour become intertwined with each other and thus very difficult to separate. Protein molecules can also do the job.
Remember hearing that old horses went off to the "glue factory"? Gelatin, a water-soluble protein, was extracted from the hooves and hide and turned into an adhesive. Today, instead of starch or gelatin, we often use a synthetic material, polyvinyl alcohol, which can be dissolved in water to make "white glue," a current household favorite.
An even better way to stick things together is to use small, very mobile molecules that, through a chemical reaction, link to form a matrix of giant molecules, or polymers. This is the way epoxy glues and Krazy Glue work.
Epoxies contain two separate compounds that react to form polymers. Super glues, such as Krazy Glue, are made of small molecules called cyanoacrylates. On exposure to moisture in the air, they join to make long intertangled polymeric chains.
The effectiveness of this type of glue actually depends on the amount of moisture in the air. So if someone asks you whether Krazy Glue works better in Miami or Phoenix, now you know.
But we still don't have the whole story. Calculations show that, even when a glue wets a surface effectively, van der Waals forces cannot account for all of the adhesive strength. There is another effect. As adhesives coat surfaces and harden, they trap tiny air bubbles in the microscopic crevices. These bubbles create a suction effect that must be overcome when the surfaces are separated.
Numerous glues are available on the market because surfaces vary tremendously and glues vary dramatically in strength. No single adhesive works on everything. And if one existed, how would we get the cap off?
New uses keep turning up. Cyanoacrylates, for example, have been used on cuts instead of sutures. They also have found application in gluing fissures in the esophagus and in treatment of cracked fingertips associated with eczema. They have been used in brain surgery to reinforce weak points in blood vessels.
But as with any other chemical, problems can arise. Just ask the patron of an Irish pub who, according to one press report, sat on a toilet seat that vandals had coated with Krazy Glue. He had a long time to contemplate the wonder of flies walking up and down the wall before being transported to a hospital with the seat still attached to his derriere.
Acetone, a common solvent found in nail polish remover, dissolved the polycyanoacrylate and freed the man from a very sticky situation.
ADHESIVES ON A ROLL
The Scots did not invent Scotch tape. But their reputation for thriftiness did spawn the name.
The story begins in the 1920s when two-tone cars were "in" and manufacturers had to address the problem of maintaining clean, crisp borders between the colors. They often glued newspaper to the cars because the paper had sharp, straight edges. This worked well, except for the fact that the newspaper would not come off easily.
At the time, 3M was selling sandpaper to car manufacturers. Its salespeople heard about the difficulties in the paint shop. There was great potential market here, it seemed, for a sticky tape that could peel off easily.
Chemist Richard G. Drew rose to the challenge. He knew that rubber cement was sticky yet could easily be peeled off a surface. He managed to coat one side of a paper strip with the material and was satisfied to see that, with a little pressure, the paper now stuck to surfaces but could be removed readily.
Drew figured that the tape could be produced cheaply, especially if the glue were applied only to the edges. After all, there seemed no need to waste glue on the whole width of the tape. The car painters thought the newfangled tape was a great idea -- until they started to use it.
In practice, there wasn't enough glue to hold the tape firmly in place. The salesman who had sold the tape was told unceremoniously to "take the tape back to those Scotch bosses of yours and tell them to put adhesive all over the tape!"
The problem was quickly remedied, but the stigma could not be eliminated. Workers still called the improved product "Scotch" tape, and 3M was stuck with the name.
When the glue was applied to clear cellophane, see-through Scotch tape was born. Today, there are more than 400 different varieties of pressure-sensitive tape. Various glues are used, but most fall into the "acrylic" family of polymers.
These are not designed to be removed as easily as, say, masking tape, and they adhere strongly because they can produce numerous microscopic suction cups when pressed on a surface.
That can be a problem, as in the case of adhesive bandages. Taking these off can be a painful experience. But there may be a solution in the offing.
Researchers in Britain have developed a new bandage with an adhesive that can be deactivated by light. When an opaque backing is peeled away, light initiates a reaction in which side groups on the polymeric acrylic molecules link, destroying the adhesive character. A camera flash gun removes the bandage -- in a flash.
INVESTING IN BONDS
In 1998, the American Chemical Society presented its award for creative innovation to Spencer F. Silver, 3M chief scientist. His name may not be a household word, but one of his inventions certainly is: Post-it Notes.
Thirty-one years ago, Silver was working on pressure-sensitive adhesives for 3M. These are glues that instantly bond to a surface but can be removed without destroying the surface.
Today, we are very familiar with such products; peel-off stickers are everywhere. In 1968, however, they were virtually unknown. Scientists did realize that certain polymers, such as natural rubber, could be peeled off under the right conditions. But they were not ideal.
So Silver went to work. He investigated various synthetic polymers and eventually found one that was a weak adhesive and could be pulled off a surface. But it would not always pull off cleanly. Silver lost interest.
Luckily, interest was rekindled by Arthur Fry, a chemical engineer working for 3M in the early 1970s. Luckily, Fry sang in a church choir. Luckily, Fry had tried to mark pages in his hymnal with pieces of paper that constantly fell out. Luckily, he remembered Silver's weak glue, put his hands on some and used it to mark his hymns with slips of paper that did not fall out and could be easily removed.
The prototype for Post-its was born!
Working out the bugs took about a year and a half. Fry developed a primer to glue the adhesive to the paper and ensure that it would not transfer to the surface to which it was applied.
Then he invented a machine to make the little pads now seen everywhere. But they were not instantly successful until a clever marketing trick was devised. Post-it Notes were given away free to offices in Boise, Idaho, and 90 percent of the users ordered more.
Today's Post-its and their kin employ highly sophisticated technology. They can be reused because the adhesive is contained within thousands of little bubbles of urea-formaldehyde resin that break under pressure. But they do not all break at the same time.
So how many times can a Post-it be reused? Who knows? Do the experiment. Leave yourself a reminder. On a Post-it, of course.
STAMPS OF APPROVAL
The 1995 season of "Seinfeld" ended with the death of George's fiancee. Poor Susan was poisoned. It seems that the prospective groom had purchased the cheapest envelopes for wedding invitations and that Susan was done in by licking hundreds of envelopes. The implication was that the glue was toxic and that tragedy could have been avoided had George not been so tight.
Could this episode have been based on a real-life event? Hardly.
Adhesives used on envelopes and stamps are subject to very stringent safety requirements. After all, some of the stuff may be swallowed, so it must be regulated as a food.
Gum arabic from the acacia tree, dextrin from cornstarch and a water-soluble resin called polyvinyl alcohol are the most commonly used. There also are additives for flexibility and spreading quality. These include glycerin, corn syrup, various glycols, urea, sodium silicate and emulsified waxes.
Preservatives such as sodium benzoate, quaternary ammonium compounds and phenols also are included. These substances may not taste great, but they are not poisons. In fact, cockroaches have been observed surviving for a long time on a diet of nothing but postage stamp glue.
MOLES AND MUSSELS
Moles are under attack in Britain. They eat virtually any crop and cause about $4 million in damage to agricultural products each year. Just about the only food they like better than crops are earthworms.
So the British Agricultural Ministry recently decided to take the mole by the worm. The idea was to glue a poison to the wriggling creatures and drop them into mole hills. But there was a problem: Moles are quite clever.
They draw the worms through their paws before dining and effectively remove any strange substances. So the moles happily ate away, even though a super glue or gum acacia had been used to apply the poison. It seems that a better glue is needed.
Maybe the researchers should look at mussels. These creatures can attach themselves to rocks and wharfs with such strength that knives must be used to pry them off. They secrete a "mussel adhesive protein" comparable in strength to epoxies. Obviously, it is waterproof. Tests have shown that it even works on glass, biological tissues and Teflon.
Application may even extend beyond moles. The substance shows potential as an adhesive in dentistry and orthopedics and as a replacement for sutures. Maybe one day, we'll even glue torn muscles with mussels.
Joe Schwarcz is director of the Office for Chemistry and Society at McGill University in Montreal.