Monday, 1 October 2012

Science: Making and testing slime



The best slime in the world... maybe?

Before we start:
Keep the slime away from clothes as it can produce permanent stains. Store the slime in an air-tight container, like a plastic bag with a twist-tie. It is best to dip the slime in some water before storing, to keep it from drying out. Slime gets dirty from handling and may become mouldy after several days. When this happens you should throw it away. Do not put it down the sink because it clogs the drain always dispose of in the household waste bins.

Right now for some fun science...
You will need:
  • 15ml Borax
  • 80ml water
  • 15ml PVA - you could add the usual white PVA glue to make your slime opaque or clear to make it all snot like.
  • 100ml water
  • A container like a plastic disposable cup
Step one
Stir The borax and 80ml of water together in a container, we use disposable plastic cups, until the borax is dissolved.
Step two
Next, in another container, stir the PVA and 100ml of water until it is well mixed in.

Step three
Add some green dye to the PVA mixture if you want to give it the snotty effect - for the kids satisfaction, of course.   We used blue and of course you can use any colour you like.
Step four
Now, pour the borax solution into the PVA solution.

What happens?
It forms a gel.  This gel is sometimes known as a ‘slime’.

Place the slime in a plastic bag and mix and squeeze the mixture from outside the bag.

You can now test the slime! 
  • Pull the slime apart slowly. What happens?
       
  • Pull the slime apart sharply and quickly. What happens?
        
  • Roll the slime into a ball and drop it on to the bench. What happens?
        
  • Place a small bit of slime on the bench and hit it hard with your hand. What happens?
        
  • Write your name on a piece of paper with a water-based felt-tipped pen. Place the slime on top, press firmly, and then lift up the slime. What has happened to the writing and to the slime? Try the same again, this time using a spirit-based pen (a permanent marker.  Does this show the same effect?
Want to experiment some more with it? 

Place a very small piece of slime in a Petri dish. Add the dilute hydrochloric acid dropwise, stirring well after each drop. When you notice a change record the number of drops added and your observations.

Now add dilute sodium hydroxide solution to the same sample used above in 6, stirring after each drop. When you notice a change record the number of drops added and your observations.

Can the last two tests be tested over and over again with the same results?

You can buy slime from the toy shops in town, they are available under many different names but are really the same thing.

I found somewhere (I did a lot of cross referencing for this one) this information:

The cross-linking between the polymer chains of polyvinyl alcohol occurs by adding borax, Na2B4O7.10H2O (sodium tetraborate).  Borax forms the borate ion when in solution. If you can't get hold of polyvinyl alcohol, also known as Polyethanol, PVA glue contains so you can dilute it as we did in this experiment.

The borate ion can make weak bonds with the OH (hyrdogenoxide) groups in the polymer chains so it can link the chains together as shown below. This is called cross-linking.

This slime is when under stress, dilates or expands. Other well known stress-thickening materials are quicksand, wet sand on the beach, some printer’s inks, starch solutions and Silly Putty.  These materials are dilatant materials (definition of dilatant is: A dilatant (also termed shear thickening) material is one in which viscosity increases with the rate of shear. Such a shear thickening fluid, also known by the acronym STF, is an example of a non-Newtonian fluid).   They tend to have some unusual properties.  These are: 
  1. Under low stress, such as slowly pulling on the material, it will flow and stretch. If careful, you can form a thin film.
  2. Under high stress, (pull sharply) and the material breaks.
  3. Pour the material from its container then tip the container upwards slightly, the gel self siphons.
  4. Put a small amount of the material on a table top and hit it with your hand, there is no splashing or splattering.
  5. Throw a small piece onto a hard surface; it will bounce slightly.
  6. Adding acid to the slime breaks the crosslinking producing a liquid with lower viscosity. Adding alkali reverses the process and the slime should be regenerated.

You can turn your slime into magnet slime like this: Magnetic putty.

Other interesting facts about this material

There are various types of slime that have been manufactured, this is one type. In this investigation we use the polymer polyvinyl alcohol, which is cheap and is easy to get hold of from suppliers because it is used as a thickener, stabiliser and binder in cosmetics, paper cloth, films, cements and mortars. In ethanol solution polyvinyl alcohol solution dries to leave a thin plastic film that is useful in packaging materials, especially as it is biodegradable.

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