Make Rheopectic slime in less than 15 minutes! (It is not Oobleck)

Step 6 What happenned? The scientific background

What we just did is a simple polymer synthesis. Most commercial glues are polymers. Adhesives like mucilage or white glue are made of polyvinyl acetate.

Borax is a salt of boric acid, also known as disodium tetraborate. Its formula is Na2(B4O5(OH)4) Making a borax solution produces the dissociated form of the salt: Na+ and ion tetraborate: B(OH4)-

When you add borax to a solution of mucilage (which is made of polyvinyl acetate), the tetraborate ions bind the large polyvinyl acetate chains with hydrogen bonds forming a more complex polymer that is like a tridimensional net. Imagine binding many rings or chain links to form a net.

This net has trapped water molecules which tend to escape by simple evaporation. This is why the slime tends to dry out and become harder. When this happens it loses its slimy characteristics, changing to a soft and plastic but rigid structure. (I think you can use your slime to make figurines, because when it is fluid, you can pour it in a mold and let it dry. I have to try this!)

You can control the texture of the slime by adding more or less borax. Borax acts as the limiting reagent in this reaction. If there are more tetraborate ions in the system, they can form more bonds. The formed "net" becomes much thicker. This affects the texture and rheology of the slime.

Now that you have seen what happens with the structure of the mucilage and how it becomes slimy, it's time to explain its rheology.

The slime we made is a non-Newtonian fluid. It is rheopectic, which means it shows an increase in its apparent viscosity with time under a constantly applied pressure (it is called shear force).

In other words, the more you play with it, the more viscous it becomes. You can even make a ball with it and it bounces. When you stop (when the shearing force is gone), it starts flowing. In the video, first you see it flow when no shear force is present. (You can also leave it on a table and see how it flows.) Then I apply a shear force on it and make it into a ball that bounces.

Differences in behaviour between a thicker and a more flexibe slime

By controlling the amount of borax in the solution, I get two different types of slime: one thick and one more fluid. The question is: Why are they different? They are made using the same ingredients, after all.

The answer to this question is that both slimes are rheopectic but their behavior is different. This is due its chemical structure:

If you make a slime using a small quantity of borax, you obtain a very soft and elastic slime that flows faster than a thicker one. You can make a ball of it, but it is not going to bounce because it starts losing its form very fast and starts flowing again. This kind of slime likes to flow more than a thicker one. However it is much more elastic than a thicker one, and it forms filaments easily (remember it has less borate bindings). The soft version looks more "slimy," as you can see if you compare the video below with the one above.

What you see in the videos and pictures are only a few simple examples of what can you do with your slime. Experiment with it and have fun!

What you saw is only a simple example of what can you do with your slime, so experiment with and have fun at the same time.