Something is either a gas, a liquid or a solid, right? Well, not so fast. Ionized gas, or plasma, behaves differently enough to be thought of as a separate form of matter. And some everyday things can cross the line. Saliva, blood, even toothpaste are viscoelastic materials. They can have the vicious behavior of a fluid. Or the elastic behavior of a solid. And a simple experiment in the journal Physical Review Letters caught the material’s split personality.
Penn State researchers used a mixture of water, soap and an organic salt to make a viscoelastic material. They then passed a cylinder through the mix. If the cylinder moved slowly, the mix acted like a fluid. A little faster, though, and the cylinder actually cuts the mixture. Faster still and you get uneven tearing. Cutting and tearing spontaneously heal, but it takes hours. Connecting the “sort of” states of the material is the interesting fact that its strength when acting like a solid is basically the same as its surface tension when acting like a liquid. That blood is thicker than liquid water is thus a *solid* conclusion.