Physicists Find Clues to Adhesive Mystery with Computer Model
The findings could one day help scientists in their quest to produce the ultimate super-strong adhesives.
Logic suggests that the amount of energy needed to break an adhesive bond, for example the glue between two pieces of wood, should be equal to the number of molecules bonded together within the area of the glued surface, times the energy needed to break one molecular bond.
But it doesn't work that way.
"The initial framing mystery is, here you've got a bond; you make a simple guess as to how much energy it should take to break the bond, and then you go and measure it, and you are off by a factor of 1,000 or 100,000," said Mark Robbins, a Johns Hopkins physicist.
Somewhere along the way extra energy, a lot of extra energy, is being used to do some sort of work associated with breaking adhesive bonds. Understanding the nature of the discrepancy could help scientists make the ultimate glue; the more energy it takes to break bonds, the stronger the adhesive.
"So if you are out to make an adhesive, you would like to make this discrepancy as big as possible," Robbins explained.
Now Robbins and postdoctoral fellow Arlette Baljon have shed light on the extra-energy puzzle. Their findings will be detailed in a scientific paper to be published Jan. 26 in the journal Science.
Using a computer model, Robbins and Baljon have observed for the first time what happens at the atomic level when bonded surfaces are pulled apart. The model demonstrates that the extra energy is used in a three-stage process that could be called the Rice Krispies scenario, as the adhesive breaks apart with a "pop, crackle, snap."
In the computer simulation, an adhesive is placed between two plates and then the plates are pulled apart. As the plates are moved farther and farther apart, the stretching adhesive suddenly "pops," and holes open up at the weakest points of the bonded surface.
As the two plates are pulled yet farther apart, the holes grow in a series of little jerks, the "crackle" stage. Finally, as the plates are pulled even farther apart, only thin tendrils, or bridges of glue, hold the two plates together. These finally "snap" as the surfaces come apart.
The final stage can be seen when a piece of tape, or a Band-Aid, is pulled off of a surface. Thin tendrils of adhesive are visible as they stretch and finally snap as the tape is pulling away from the surface.
The most important revelation is that the model shows precisely where the extra energy is needed. Each of the three stages uses about one-third of the excess energy. And the energy is used very suddenly in each stage, when molecules quickly rearrange and atoms move swiftly past each other. That atomic motion produces heat. The faster the motion, the more heat is produced.
"People had conjectured that this was going on, but what's new here is the ability to actually show that there is no extra energy being given off as heat, except at discrete sets of sudden rearrangement -- this snap, crackle and pop corresponds to very rapid rearrangements that occur," Robbins said.
With the help of the National Science Foundation's Pittsburgh Supercomputing Center, the physicists used the model to study hypothetical adhesive molecules made up of a chain of 16 atoms. But in real life, the adhesive molecules would be long polymers consisting of perhaps 100,000 atoms. The next step in the research will be to see what happens when the molecular chains are lengthened and when other factors are changed. The work has been funded by NSF and the Petroleum Research Foundation.
Go to Headlines@HopkinsHome Page