Glow in the Dark Gak

As Halloween is approaching, we have updated this activity to include a more cost-effective way to have gak "glow in the dark".

  

Gak, more commonly known as slime, is something almost everyone has made, but here are some instructions on how to make glow-in-the-dark gak.  At the end of the instructions, explore some of the science behind gak and glow-in-the-dark toys.

For this experiment, you will need:  

• PVA glue - we find that Elmer's school glue works best
• Borax Hand Soap
• Water
• Measuring cups and spoons
• spoons for mixing
• 2 plastic containers for mixing
• Airtight container for storage
• Green phosphorescent pigment (the most affordable we found is at www.douglasandsturgess.com). 
• Alternatively, use highlighter markers to make slime glow under ultraviolet (UV) light or a blacklight (try with clear glue)

Add 1/4 water and 1 tsp. of borax to one container.

Add 1/4 cup water and 1/4 cup glue to a second container.

Make sure that both mixtures are fully combined and dissolved.

Add 1/2 tsp. green phosphorescent pigment to the glue and water mixture.  Stir well.

Add the water and borax mixture to the white glue, pigment, and water mixture.

Stir quickly to combine the two.  Strands should begin to form as the mixture thickens.

Stop stirring once all the water is dissolved.

The resulting mixture is gak.  This elastic, putty-like goo is fun to squish around and play with.

Place it under a bright light, then turn off the lights. See it glow! 

Be sure to store it in a plastic bag or an airtight container so it doesn’t dry out. 

Glowing Gak

This gak is special because it glows bright green in the dark. To “charge” it, hold it under a bright light. Store it in a plastic bag or an airtight container so it doesn’t dry out. The glowing pigment can sometimes settle out, so knead it now and then to keep everything mixed together.

You’ve probably seen other glow-in-the-dark toys like plastic stars, key chains, or yo-yos. All of these work because they contain chemicals called phosphors.

Atoms, which make up everything around us, have a center (nucleus) of protons and neutrons, with fast-moving electrons around it. When you put a glow toy under a light, the electrons in the phosphor absorb energy and move farther away from the nucleus. Later, the electrons relax back down, releasing that energy as light. This is why glow-in-the-dark objects slowly fade but can be recharged under a light.

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Polymers and Gak

When water is added to glue, the long molecules inside (called polymers) can slide past each other, so the glue flows. Adding borax makes these polymers link together in a net-like structure. The result is stretchy, bouncy gak.

These polymer chains are held together by weak hydrogen bonds, which break and reform easily. That’s why gak can stretch, bounce, and be pulled apart without falling to pieces.

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Fluorescent vs. Phosphorescent Glow

The glow in this gak comes from phosphorescent pigments, which store energy from light and release it slowly. That’s why they keep glowing even after the light is gone.

But there’s another way to make things glow: fluorescent dye. Fluorescent materials don’t store energy the same way. Instead, they absorb invisible ultraviolet (UV) light (like from a blacklight) and instantly re-emit it as visible light. They only glow while the UV light is shining on them—once the light is off, the glow stops right away.

So, if you swapped the phosphorescent pigment for fluorescent dye, your gak wouldn’t glow in the dark by itself. Instead, it would light up brightly under a blacklight, making it look like it’s glowing with neon colors.

You can even make your own fluorescent dye by pulling the ink tube out of a highlighter marker and soaking it in a little water to release the glowing liquid. Most highlighter ink is non-toxic, but it’s still important not to drink it or get it in your eyes—just use it for glowing science fun!