Structured Fluids

Water, gasoline, pancake syrup, mucus, salad oil, cornstarch‐and‐water paste, Silly Putty, liquid dishwashing soap, egg white, glycerin, Vaseline, ketchup, model airplane glue, paint, toothpaste—this motley list of substances seems to defy scientific classification. Water, gasoline, salad oil and glycerin are clearly liquids. The other substances, strictly speaking, are also liquids, since they all flow on the application of stress. But the name “liquid” hardly captures the variety of behavior in these substances. The substances differ from one another in their mechanical responses almost as much as they differ from solids. In the first place, they differ enormously from one another in viscosity, the quantitative measure of a liquid's flow response. Furthermore, many of them have striking properties whose description requires variables besides viscosity. Syrup becomes sticky as it dries. Liquid dishwashing soap has the same consistency as syrup, but it does not become sticky on drying. Mucus, egg white and model airplane glue are springy and rubbery. Silly Putty and cornstarch paste flow on a tilted surface but shatter like a brittle solid on sudden impact. Dishwashing soap in water easily forms masses of long‐lived foam. Even the simplest flow properties of toothpaste are qualitatively unliquid‐like: It comes out of its tube as a plug rather unlike an ordinary liquid, which flows fastest in the middle of the opening.