Background

Colloids

Some materials do not conform neatly to the designations for solids, liquids, and gases. For example, a colloid is a material that consists of one substance suspended within another. The suspended material is comprised of particles so small that they don’t sink to the bottom of the second substance. Yet they are too large to be considered to be a homogeneous solution. Colloidal particles range in size from 1 nm to 1 µm and usually are made up of more than one molecule stuck together. Mixtures involving only homogeneously mixed single molecules are called solutions; every part of the mixture has an equal amount of each component of the mixture.

Colloids display properties unlike those of their separate components. Some examples of colloids include smoke (a solid suspended in a gas), fog (a liquid suspended in a gas), meringue (a gas suspended in a liquid), protoplasm, homogenized milk, synthetic rubber, and mayonnaise. The Oobleck is an example of a colloid of starch and water.

Polymers

Long chains of molecules formed from repeated units are called polymers. The repeating units are called monomers. Polymers have very different chemical and physical properties when compared to the monomers that make them up. PVC pipes are made by polymerizing vinyl chloride, a small molecule that is highly toxic, flammable and carcinogenic. And yet, the pipes made by stringing these molecules together are solid, durable, and chemically unreactive. Nylon is a fiber useful for making things from high quality clothing to parachutes. It is made by creating a polymer of two foul-smelling liquids (hexamethylene diamine and sebacoyl chloride for one version). The molecules in the starting liquids are perfectly useless for making bike shorts or evening gowns but when these small molecules are strung together they make a startlingly useful material.

Oobleck

Starch molecules are long chains of simple sugar molecules. Starch is a naturally occurring polymer made by many living organisms and it is used as cellular food storage since the sugar monomers it is made of can be broken off the long chain and used for energy. By making the sugars form a long chain the cell can reduce osmotic pressure, which would otherwise swell the cell to the bursting point. In water the long starch chains assume a globular shape. The globular starch molecules can become entangled when the they are suspended in water. This entanglement leads to the odd properties of the Oobleck. Technically, the Oobleck is a colloid made up of a polymer suspended in water.

Glurch

The Glurch is based on an artificial polymer. White glue is a polymer of a chemical called vinyl alcohol. The polymer is called polyvinyl alcohol. The long chains of polyvinyl alcohol can flow around and over each other, albeit slowly. Think of it like a bowl of spaghetti noodles with oil on them. You can stir the noodles around but they do tend to cling together. When you add borax solution to the polymer it causes what is called cross-linking. This makes it harder for the long chains to flow past each other. The stickiness which slows their movement is caused by hydrogen bonds: chemical bonds that have a certain strength but which are easily broken and reformed elsewhere.

You will be investigating the properties of these two strange substances. The properties you should investigate are viscidity (stickiness), resiliency (elasticity), and fluidity (ability to flow). You will be asked to come up with your own tests for these properties. Some suggestions will be given.

Safety

Don’t eat any of the materials.

Procedure

1. As always, remember to write carefully in your lab notebook what you do in the lab.
2. Glurch
   1. Measure 15 mL of the glue into the cup, add the water; mix thoroughly. (Vary the amount of added water from 5 to 20 mL to achieve different outcomes: more water makes a gloppier gel and less water makes a bouncier gel).
   2. Add one or two drops of food coloring, if desired; stir completely!
   3. Only after all other ingredients are fully mixed: Add the borax solution to the cup; stir the mixture to make the slime; take it out of the beaker and knead with your hands to complete the mixing
   4. As soon as you have finished getting the Glurch out of the beaker please clean the beaker immediately: it will be much harder to clean later.
   5. Glurch can be kept in a zip-top baggie when you are not using it and kept for a long time
   6. Try the following:
      • Devise a way to gauge the viscidity (stickiness) of the Glurch. Is it sticky enough to pick up a piece of paper? A pencil? Try to keep the surface area that touches an object the same for each test.
      • Does the Glurch bounce? Can you pull it apart? Try to devise a few tests to discover the elasticity of the Glurch.
      • How can you test for fluidity in the Glurch? Try placing it on something with a small hole in it. See whether it will run between your fingers. Hang it from something.