Humans have always been able to figure out ways to stick things together. The ancient Egyptians made an adhesive from the connective tissue of animals and used it in building coffins for their pharaohs. Native Americans waterproofed their canoes with resin from the spruce tree. Today, we keep our bottle of glue and tape dispenser handy for repairing table legs, making holiday cards, or posting a reminder over our desk to pick up a pound of coffee on the way home.
All those adhesives are made of macromolecules – large molecules of proteins and sugars in the case of natural glues, and polymers in synthetic stickum. “Both glue and tape require interfacial bonds that that will adhere to the surfaces that you’re trying to stick together,” explains Niels Holten-Andersen, John Chipman Assistant Professor of Materials Science and Engineering at MIT. “You also need cohesive bonds, which hold the glue itself together.”
The intermolecular forces in glue are very strong, but they’re also brittle and irreversible. If an object breaks after the glue has set, it cannot be stuck back together without applying fresh glue. Two-component glues such as epoxies and elastomers reduce the change of rebreakage. “When you mix the two liquids, a chemical reaction takes place that induces the glue molecules to permanently bond with each other and also to the surface,” explains Holten-Andersen. Adding a third molecule – like triethylenetetramine (TETA) – to one of the components results in an even stronger bond.
The forces that hold tape in place involve much weaker bonds than those in glue. “On one side of the tape are pressure-sensitive adhesives,” says Holten-Andersen. “Squeezing them against a surface doesn’t create very strong bonds, but rather a large number of weak bonds that cumulatively result in a good adhesion.” Tape also stays stuck because of viscoelasticity. “When we press tape against a wall, we force the sticky substance to flow and adapt to the microscopic corrugation of the wall surface,” explains Holten-Andersen. “After you stop pressing, the tape resists flow and stays in place.” And unlike glue, tape is reversible and may be used multiple times before its polymer material fills with too much dust and detritus to work anymore.
The search for ever-better glue and tape continues. Holten-Andersen’s research includes the sticking power of aquatic organisms. “Mussels and shellfish produce fibers that allow them to stick to ships and rocks and anything they want to grow on,” he says. Understanding their secrets is the first step in creating bio-inspired synthetic glues that will work under water – and in medical applications like organ transplants or other surgeries.
“Such materials might function better than the ones we now have and be made in a way that they won’t harm the environment,” says Holten-Andersen. “Nature does a lot of things much better than we can.” –Sarah Jensen
Thanks to Patrick Chao of Shenzhen, China, for this question.