Gak! (and Physics)

Gak is a non-Newtonian fluid. In their natural state, glue molecules stick to each other only weakly, which is why glue flows smoothly and why it's easy to smear flat when you're using it to stick two other things together.

Much like the cornstarch used to make oobleck, glue molecules are long and slippery. Unlike oobleck, making gak involves a chemical change and the two ingredients cannot be separated out again.

We made the gak by adding boron atoms (in a water solution) to glue. These atoms crosslink with the sulfur atoms in the glue molecules, causing them to tangle and knot together. This happens immediately, with the glue wrinkling up as soon as boron is added and then forming into ropy webs when it's stirred.

The result is a substance that sticks to itself more than everything else. In fact, gak does so strongly enough that after I made the demo batch, I could clean my hands off by pressing the main ball of gak on my hands so it pulled in the errant bits.

The kids experimented with the gak for a while. Gak bounces like rubber. It rips when pulled quickly but can stretch to amazing lengths if the same is done more slowly. It's also completely non-toxic, so it's safe to eat (although I wouldn't recommend it with how often it fell on the floor). Pressed against interesting textures, it can be pulled off again to show the pattern.

If you do find gak stuck on fabric, vinegar reverses the reaction, breaking the gak back into boron and glue molecules that can be wiped off.

The standard slime gak is more liquid than the version we made. It's fun because it flows more easily but is more of a cleanup concern because it flows more easily into things like cloth and carpet, where it has to be washed out.

If you'd like to try making the slimier version of gak at home, it can be done by mixing equal parts glue and water (most recipies suggest one 8 ounce bottle of glue) and then adding dissolved borax - you can find borax in the laundry aisle as a powder. Add a teaspoon of that powder to a half cup of warm water, stir it up, and add the liquid to the glue. It should turn ropy on contact and tighten up as it's stirred until it's an semi-firm blob, at which point it can be squeezed and pounded into even consistency.

When we finished with the gak and discussing its properies, we moved on to our next unit, physics! We focused g-forces, which produce gravity-like effects. This is what allows people to spin a bucket full of water over their head without any of it falling out. It can also be felt by spinning around with your arms out, being spun around by someone else until you lift off the ground, or spinning in a chair and trying to keep your legs in. While it's mostly fun for fooling around here on Earth, it could be used to simulate gravity and allow humans to live in space indefinitely and even have children born and raised in space safely.



If your kids would like to try it at home, here's a more extensive demonstration video that also gets into Newton's Laws of Motion, which we'll go through in the next class.

Also, here's a video of a bullet fired into a water balloon!



Can your kid figure out what happened? We'll be discussing this and other aspects of physics next class.