Slime: hidden patterns

Gooey Slime seems the opposite of pattern. Yet there is an underlying pattern called a polymer. Moreover, the polymers are crosslinked, forming yet a larger pattern.

Slime is in the ocean, in our bodies, in our yards. Our bodies make some kinds. Kids make a similar kind out of glue. Slime seems the opposite of pattern, but up close there is a fascinating pattern that makes slime what it is: slimy. Let’s examine the kind of slime kids make out of glue, such as Elmer’s glue. 

The glue contains molecules from which we can make polymers. A polymer, in turn, is a pattern made up of repeating molecules, or “monomers” that are connected by relatively strong covalent chemical bonds. 

In slime, the repeating unit is poly vinyl alcohol (PVOH). Modeling this with Lego makes features of this PVOH more tangible.

The red Lego blocks show Oxygen atoms, the brown blocks show the Carbon atoms. The small white block shows the Hydrogen atoms. 

Two repeating units of PVOH (separated to show how to build with Lego)

Two repeating units of PVOH (connected)

Linking lots of these PVOH units makes a chemical pattern called a polymer. Can you identify the repeating unit in the image below? A polymer is made of up of many, many of these units. (Yes, it is actually called a “repeating unit”).  It is a good exercise to give students a picture like this and have them try to identify and circle the repeating unit. 

Lots of these polymers, dissolved in water, float past each other, rather fluid. (The polymers can bend, a limitation of the Lego models, which don’t bend). 

 

Slime is made of lots of these polymers, but with with an additional feature, called crosslinking, to give it the sticky, slimy qualities. The PVOH molecules in slime are weakly crosslinked by another chemical (discussed below).  PVOH can be crosslinked because this polymer has regions of positive and negative charge. That is, PVOH molecules have “polarity.” In our Lego model, the little white rectangle (Hydrogen atom) has a net positive charge and the red rectangle underneath (Oxygen atom) has a net negative charge. The  reason the Oxygen has a net negative charge is because the Oxygen pulls on the electrons (negatively charged) it shares with Hydrogen. 

You could say the oxygen “hogs” the electrons. So each repeating unit in polymer has a negatively charged region and a positively charged region. Water (H20) itself is like this, with the oxygen hogging the electrons from the hydrogen.

In the case of water and PVOH, the Hydrogen atom has a net positive charge because its electrons are hogged by Oxygen. 

Because the PVOH molecules are polar, as is water, it dissolves in water. Another consequence of PVOH being polar is that some other polar molecules can crosslink the PVOH loosely, giving it a pliability, a reformable yet elastic quality, characteristic of slime.

The crosslinking makes a pattern of patterns. The weak crosslinking can break and reform over time, patterns of change, when we move it slowly (move it quickly and it hardens up).

We can show the weak crosslinking using magnets. In a very general and rough way the crosslinking attraction between PVOH molecules is weak and breakable, like the attraction between magnets, and is due to opposites attracting, plus and minus charges. This weak crosslinking is what makes slime pliable, sticky, and, well, slimy.

It’s easy to fit magnets in large Lego, or Mega Bloks. 

Tape magnets inside a block, or let them hold themselves together by magnetic attraction.

The white blocks (hydrogen) of the PVOH stick to the red block (oxygen) of the B(OH)4.

We can model that very crudely by having magnets inside the white block and inside the red block. They will stick until we pull a little, as in Slime. 

Why use Lego? The main reason it helps is because the Lego has eight nubs. The Carbon atoms (brown Lego blocks) and Oxygen atoms (white Lego block),  like to have eight electrons in their outer shell, and will link to other blocks (or molecules) to get the eight filled. The small white Lego block has two nubs a bit like the Hydrogen atom can have up to two electrons here. The Carbon and Oxygen atoms have with eight possible connecting nubs to connect (roughly analogous to eight possible electrons in the shell of these molecules, four of the eight which are available in each Carbon and six of the eight in the Oxygen).  See our discussion on Lego for chemistry near the bottom of our page on energy patterns

Here’s my daughter playing with Slime she made with my sister Monica (who is a Slime making expert). This is the kind of Slime we described above, with blue food dye added.

Below, Monica’s Slime students show off their multicolor specimens, made from glue.

Other Polymers kids use in making Slime, like Alginate gel, also depends upon crosslinking. Sodium alginate is a polymer which can be extracted from brown seaweed and kelps. It is one of the structural polymers that help to build the cell walls of these plants. It has some unusual properties and a wide variety of uses. When sodium alginate is put into a solution of calcium ions, the calcium ions replace the sodium ions in the polymer. Each calcium ion can attach to two of the polymer strands. This is called cross-linking.
(Photo of my four year old holding some Alginate Slime gel, after she and I made some). 
Alginate (usually sodium alginate) is used in food when chefs want to thicken the food, or create a texture something like fish eggs. Some foods that may include alginate are ice cream, fruit-filled snacks, salad dressings, and pudding.  To form a gel, sodium alginate needs to come into contact with divalent ions, such as calcium (Ca2+). As soon as sodium alginate is added to a solution of calcium chloride, a gel forms as the sodium ions (Na+) are exchanged with calcium ions (Ca2+) and the polymers become crosslinked. The calcium ions are able to crosslink the alginate polymers because they can form two bonds.
We will devote one Beautiful Discovery Box to explore the patterns underlying Slime.