If you've ever worked with glitter for an art project, you know that the sparkly stuff sticks to just about everything — and removing it from your skin, or really any surface, can be a real chore.
So why is something so whimsical also so tenacious? Turns out, there is no simple answer to the question — the physicists we contacted provided a variety of explanations, ranging from static electricity to a physics principle called van der Waals forces.
Glitter is made up of tiny bits of a polymer called polyethylene terephthalate, or PET, (trade name: Mylar). The pieces can be very small, at just a millimeter or so across, though glitter comes in several sizes. The shine comes from an ultrathin coating of metal. Both its composition and its small size make glitter ideal for sticking to all things oily and moist, and even to dry surfaces.
Read on to find out the funky physics of how all of this works.
The physics of glitter's stickiness
An informal survey of experts reveals that several different forces make glitter such a sticky problem (no pun intended). Among them are static, the viscosity of air and a phenomenon called van der Waals forces.
Static electricity occurs when two objects come into contact and one shuttles its negatively charged electrons to the other, making one object positively charged and the other negatively charged. Similar charges repel each other, and opposite charges attract. You can watch this happen by rubbing a balloon on your head and making it stick. [The Mysterious Physics of 7 Everyday Things]
Paul McCord, a senior lecturer in chemistry at the University of Texas at Austin, said that most of the time, when glitter sticks to dry surfaces, static is probably the culprit. "Plastic picks up electrons like crazy," he said. (You can observe this phenomenon when you buy a brand-new plastic vial of glitter: Even though the inside of the vial is dry, it is covered with a thin layer of glitter.)
It's in the air
Another phenomenon might also be at work: the viscosity of air. At very small scales, air is actually rather viscous, behaving more like a fluid than a gas.
Glitter particles are very flat, McCord said. If you put a flat, thin object on a very smooth table — a piece of paper, for example — you find it stays relatively stationary and is hard to pick up unless you can lift an edge slightly. That's because the paper pushes out all of the air underneath it. The weight of the air on top of the paper pushes it down, and you need to let some air under it to allow the paper to rise — the air underneath counteracts the pressure from above. Glitter particles could behave in a similar fashion, especially because they are usually so tiny and flat, McCord said. [Fun Science Experiments for Kids]
Van der Waals forces
But that doesn't explain the entire phenomenon. And that's where van der Waals forces come in — though they are a bit more mysterious, at least with respect to glitter, experts told Live Science. Van der Waals forces describe many different phenomena that involve certain attractive or repulsive forces between molecules. For example, van der Waals forces explain how geckos can walk up walls, as well as why water sticks to some substances and not others.
Glitter gets its stickiness from different kinds of van der Waals forces depending on whether it is adhering to water, oil or a dry surface:
With water, some of the effect is from surface tension, involving intermolecular attraction between water molecules — one set of van der Waals forces. Water is a polar molecule, meaning it has a positive end and a negative end. As such, water molecules attract each other at their positive and negative ends, like little magnets. This is why on some surfaces, water "beads up" and tries to make a droplet — the molecules are all butting up against one another's opposite ends. If you wet your hand and put sand on it, the sand sticks because as those water molecules attract each other, they trap the sand. The same phenomenon would make glitter — or any other small particle — stick to your fingers when moist. [The Surprisingly Strange Physics of Water]
This van der Waals force describes the phenomenon in which liquid sticks more strongly to the walls of a tube, for example, than it does to its neighboring liquid molecules. Capillary action explains why water gets soaked up by a paper towel: It is being pulled more strongly by the pores in the surface of the towel than it is to itself. Like paper towels, the little imperfections in the glitter's surface can allow for molecules with exposed positive and negative charges that attract the water.
Adhesion: This type of van der Waals force arises from the amount of surface area contacted between two surfaces, said Kellar Autumn,a professor of biology at Lewis & Clark College in Oregon who did groundbreaking experimental work on adhesion when he studied how geckos stick to surfaces. Glitter particles are flat, so they will tend to have a lot of surface touching whatever they are on, even on a dry surface.
Other forces involved
Yet another factor could be the metal that coats glitter. "If the glitter is coated with metal, then I suppose there is the capacity for a more formal electrostatic interaction," Dave Farina, a former teacher of chemistry at the Southern California University of Health Sciences and star of the "Professor Dave Explains" YouTube series on chemistry, told Live Science in an email.
The metal, which has free electrons in it, will be slightly charged; any positively charged molecule nearby will attract it, Farina said. "The metals make things funkier, as they don't fall into the category of generating van der Waals forces," he added. "[It] depends quite a bit on the metal, really, and how the electrons are distributed."
Glitter particles are so light and small that friction becomes significant relative to gravity, so if the glitter is on a smooth surface and you tilt it, it won't be heavy enough to slide off, Farina said.
Solving the mystery of which forces are the main culprits of glitter's stickiness will probably require some investigation by materials scientists. "If you want to really know how glitter adheres to skin, experiments are required!" Autumn said. "There are no easy answers when it comes to adhesion."
Cleaning up glitter
Perhaps the more important question is, how do you get glitter unstuck?
Picking glitter up from a dry surface is probably best done with a vacuum or a moist cloth or sponge (although you probably have to throw them out afterward). Removing the stuff off your skin is a bit different because skin almost always has at least a little moisture on it.
The best way to pick it up from a wet surface may involve using powder like talc or compressed air, said Joe Colleran, sales manager at Meadowbrook Inventions, the company that invented glitter as we know it today.