We made some slime on Sunday. In fact, the boys made rather a lot of slime, which they coloured blue with food dye. They followed a simple recipe, mixing PVA glue 50:50 with water and stirring until the mixture was an even, milky-white colour. Then they added a few drops of colouring. I had made up some saturated borax solution and they added that a little at a time whilst stirring vigorously. The PVA becomes thick and gloopy and then needs to be squeezed with the hands to make smooth slime. It can be stored in a sealed jar or in a closed plastic bag with a little water added.
PVA (or polyvinyl alcohol) is a long chain of carbon atoms called a polymer. Borate ions cross link these long molecules forming the slime. Adding a little acid causes these bonds to break. Adding an alkali neutralises the acid and makes the bonds reform.
Slime is an example of a dilatant non-Newtonian fluid (like oobleck made from cornflour and water) which means that its viscosity increases under stress. In other words, when you hit it becomes thicker. If you pull on it gently it stretches, but if you give it a sharp tug it snaps.
Slime is a shear thickening non-Newtonian fluid but there are other sorts. Tomato sauce is an example of a shear thinning fluid – it becomes runnier when put under stress. Honey is similar in that the longer you stir it the runnier it becomes – this is called being thixotropic. Cream, however, becomes thicker the longer you stir it, which is called being rheopectic.
Questions…
- If you wanted to test the effect of an acid on slime, what substance from your kitchen cupboards could you use?
- What could you find in the kitchen to neutralise the acid and restore the slime to its former glory?
- Cornflour is mostly starch. Starch is a long chain of glucose molecules. What process in the corn produced the glucose?
- The borax solution Hugo and Luke used was saturated. What does saturated mean in this context?
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