We have found percolation to be useful for understanding how forest fires spread and how clusters of trees grow, but it can also be used to understand many other problems. One example is gelation, the process by which a liquid solution of gelatin (made of long-chain molecules---polymers) turns into a gel. Jello brand gelatin, a popular dessert, is a gel.
To make Jello, one starts with a solution of long-chain polymers and water. During the process of gelation, the polymers link together into larger chains. The chains become interconnected and form a connected path across the sample, they can act as ``bags'' to contain the water, creating a gel instead of a liquid. A liquid pours; a gel wiggles.
In the same way, if clusters of trees form a connected path across a forest, then there is a good chance that a fire will burn across the forest. Thus, connectivity is the key idea for both forest fires and gels.
What gives Jello its strange properties? It is similar in some ways to a liquid and in other ways to a solid. The ideas you have explored earlier in this chapter will help you to understand the characteristics of Jello, and gels in general. In particular, there is a critical concentration of gelatin molecules that makes Jello into a ``wiggly solid.'' The critical concentration is needed to create the bag compartments that contain the water, thus making jello into a wiggly solid. Below the critical concentration the bags do not form a connected path across the structure, the water is not contained, and the substance flows as a liquid.
In the laboratory, experiment with different concentrations of gelatin to see if you can find the lowest concentration that turns the liquid to a wiggly solid. How does raising the temperature affect your results?