Wednesday, 14 January 2015

Science for kids - Slime

Everyone likes slime! And it's easy to make, in its various forms.

Cornflour Slime:
All you need is cornflour (made from actual corn - maize - not the "wheaten" version you can get in Australia, which is made from wheat), water, some colouring, a bowl and a spoon. The process is:
  1. Pour cornflour into a bowl,
  2. Stir in small amounts of water until the cornflour becomes a thick paste. I prefer to have the water coloured at this point, as it helps to more effectively spread the colour throughout the slime.
And that's it. Try stirring the slime slowly (should be easy) and then quickly (should be almost impossible). Then try punching it. I employed a professional hand model for this video.

    Cornflour slime is a shear thickening non-Newtownian fluid - the viscosity of the slime increases as the shear rate (how quickly you stir, how hard you punch) increases. This is because the slime is a suspension of corn starch. The corn starch molecules are large and jagged and when stirred slowly, the molecules have time to move past each other, lubricated by the water. When a large force is applied, the molecules are jammed quickly together and the liquid does not flow - it becomes solid-like. You can keep it for about a week in the fridge before it starts to smell a bit like vomit. You may need to add a bit of water each time you use it, and stir it in slowly (you won't have much choice).

    Packing Peanut Slime (Borax Slime)
    A less cool, but nonetheless fun to make, slime is made from packing peanuts - the stuff used to protect fragile objects when you mail them, move house etc. Packing peanuts are generally made from starch but also contain PVA - polyvinyl alcohol. Place about 5 mL of water in a cup and add about 8 packing peanuts and stir. They will start to dissolve. Continue to add the peanuts until it starts to become thick, and then some food colouring.

    At this point, you need to have gloves on (and eye-wear if you have some). Add 1 mL of 4% sodium tetraborate solution (borax) and stir. You may have some in your laundry as it is used for cleaning clothes. Your goo will now start to become slimey. Once you can make a solid ball out of it, start to wash it with water to wash away any extra borax. After doing this, you can handle it with bare hands.

    Polyvinyl alcohol is a polymer - a long molecule with repeated subunits (monomers). The borax cross-links the polymer chains; that is, it bonds one polymer chain to the next. As you add more borax, more crosslinking occurs making the slime thicker.

    There is a similar slime you can make with PVA glue, although in this case the PVA stands for polyvinyl acetate as opposed to polyvinyl alcohol. The process is the same - the borax provides the cross-linking between the PVA polymers, except in this case the monomer contains an acetate (CH3COO) rather than an alcohol (OH). More on this slime here.

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