# Gak attack!

Halloween is right around the corner, and that means lots of candy, costumes and haunted houses! A good haunted house isn’t complete without… something gooey and slimy!

Gak (goo, slime, or whatever you wish to call it) is very easy to make, and with all the variations, it’s sure to keep the kids entertained for hours. It will definitely be a hit at any Halloween party!

To make the slime, a small amount of Elmer’s® glue is mixed with water and a saturated borax solution — or liquid starch. This combination of glue, water and borax produces a thick, gooey material.

The Elmer’s glue contains long, flexible molecules called polymers. The specific polymer found in Elmer’s glue is poly(vinyl acetate) (PVA). The addition of water disperses the PVA. When borax is dissolved in water, it forms hydrolyzed borate ions (a boron atom surrounded by four hydroxides with a net charge of -1). These ions connect the polymer molecules together in a process called cross-linking (ester linkage). When enough polymer chains are cross-linked, the solution turns from a liquid-like solution to a rubbery solid-like material, which we call gak!

You will notice, when you pull the gak slowly, it stretches, but if you pull the gak quickly, it breaks. Gak can slowly flow like a liquid, but it also can bounce like a solid. Materials with such behavior are called non-Newtonian fluids. Other non-Newtonian fluids include blood, ketchup and toothpaste.

Gak is also an example of a hydrogel. Hydrogels are jelly-like material made mostly of water. The water is held within the network of cross-linked polymer chains. The water aids the “flowing” of the polymer chains. Other common hydrogels include contact lenses, hair gels and skin grafts.

Gak can be colored with food dye. It can be made to sparkle by using commercial glue that contains glitter or simply by adding the glitter yourself. Gak can be made to fluoresce under a black light by adding small amounts of Glow Germ® (powder or liquid) or fluorescein solution. Magnetic Gak can be made by adding black iron oxide —mixture of iron(II) and iron(III) oxides — or made into Floam® by using poly-fil micro-bead filler. Along with being fun on Halloween, at the high school level with all its variations, the making of gak lends itself to be an inquiry lab.

## Fluorescent gak attack!

Concepts

• Hydrogels

• Non-Newtonian fluids
• Fluorescence

Materials

• Goggles

• Elmer’s glue (white or gel); do not use “school glue”; available in hardware and craft stores
• Water
• Borax (saturated solution); solid borax is available in the laundry section of most grocery stores; liquid starch can be used in place of the borax and is also available in most grocery stores
• Warm water — to dissolve the borax
• Measuring spoons and plastic cups
• Popsicle sticks; also known as craft sticks
• Plastic Zip-loc® sandwich bag

For Fluorescent/Glow-in-the-dark gak

• Glow Powder or Lotion (we’ve used Glo Germ® available from Educational Innovations, solid or solution, GLO-230, $17.95 US, powder or GLO-220,$19.95, lotion); you need very little for this activity.
• Black light

Safety

• Gak is not edible!
• Wear protective goggles/safety glasses and gloves
• Gloves are definitely required when blending the black iron oxide and fluorescein. The Glo Germ is actually intended for a person to rub on their hands. Then after they wash their hands, the Glo Germ reveals unwashed areas.
• Fluorescein is toxic. Avoid contact and ingestion.

• A saturated solution of borax can be made by adding enough borax into warm water (about 70 °C) until it stops dissolving and then allowed to cool. Some solid at the bottom is fine.
• If fluorescein is to be used to make the gak fluoresce, dissolve 1.0 g of disodium fluorescein in 100.0 grams of water to make a 1% solution.

Procedure for a small amount of fluorescent gak

Larger portions will require more of each substance. Keep the volume of water equal to the volume of glue used. Keep the amount of Glo Germ small. Add the borax solution a little at a time until you reach the desired consistency after thoroughly mixing.

• Measure 1 tbsp (tablespoon) of Elmer’s glue; pour into a small plastic cup.
• Add 1 tbsp of water to the Elmer’s Glue and stir vigorously with a Popsicle stick. (Stirring helps to disperse the PVA in the glue and facilitates the formation of the final product, gak.)
• Add ½ tsp (teaspoon) of Glo Germ powder or lotion and stir.
• Add 1 tsp of the saturated borax solution to the will begin to thicken. Add more borax in small quantities if the mixture has not fully thickened.
• Pick the material up with your hands and knead it for a few minutes. The gak will become “dryer” and more malleable over time.
• Place the gak under a black light to cause it to glow in the dark — fluoresce!
• Store the gak in a plastic bag.

Other variations

• Instead of Elmer’s glue, a PVA solution can be used. This would give a clear gak instead of white gak.
• Using food coloring, one can make various colors of gak. Simply add one or two drops of food coloring
• Glitter gak can be made by using Elmer’s glitter glue. It can also be made by using Elmer’s® white glue, and adding some free glitter into the water/glue suspension. Free glitter can also be added after the borax has been added, and folded in — this technique tends to be more glittery.
• Mixing in black iron oxide powder after the gak is completely formed can make magnetic gak. The black iron oxide powder is messy. We recommend only the teacher do the mixing. Gloves should be worn. We do the mixing in a tray that is placed over paper towels. Once the iron oxide has been added into the gak, it is safe for kids to handle. Use a neodymium magnet to demonstrate the magnetic properties of the gak.
• “Floam” can be prepared by adding Poly-fil Micro Bead Filler to the water/glue mixture.

Clean-up

• Gak can be disposed of in the trash.
• The excess borax solution can go down the drain or saved for future use.
• Elmer’s glue is washable.

## An aside

The prepared material can be placed in a plastic funnel. Over time, the material will slowly seep out, flowing downwards. This is a fun way to show that it is a non-Newtonian fluid.

*Kacey Hall is a graduate student in the Department of Chemistry, Duke University. She is investigating force-induced small molecule release from metal complexes.

**Kenneth Lyle is a lecturing-fellow in the Department of Chemistry at Duke University NC. <kenneth.lyle@duke.edu> (The Powell Family Trust, the Duke-Durham Neighborhood Partnership, and Bio-gen Idec – Research Triangle Park, fund the Duke Chemistry Outreach program.)