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TOOTHPASTE

Some things flow. Some don't. A rock doesn't flow, whereas water does. Some other things flow, but slowly. A glacier can be hard enough to walk on, but nevertheless flows, at the ``glacial'' pace of a few centimetres a year. Even honey pours out of a but bottle more slowly than water does -- it is more viscous. Tar, for example, before it becomes a solid, flows even more unwillingly.Think of the toothpaste. Toothpaste must flow when the tube which contains it is squeezed. Moreover, it must flow easily. In this it is not unusual, for even water in a tube would do the same. What makes toothpaste unusual is that the same toothpaste, once squeezed onto a toothbrush or even on top of the sink, ceases to flow. (Water would never do that.) So toothpaste then is
an everyday example of a very unusual fluid. Sometimes it flows; sometimes it doesn't. It is for us to figure out under what conditions does these unusual behaviour of the fluid is observed.

How does toothpaste do this? When you press at one end of the tube, this creates a pressure difference between the two ends of the tube. The toothpaste then feels a force which makes it flow. Further, when it flows through the tube, the molecules of the toothpaste which are closest to the wall of the tube are practically stationary, while those towards the centre of the tube move fastest. Thus, the speed with which the paste moves through the tube changes as you move from the centre of the tube towards its periphery. All fluids behave like this.Now liquids don't like different parts moving with different speeds. Liquids resist such relative motion and a measure of this resistance is the {\em viscosity}. When we say that honey is more viscous than water, we mean that when we apply the same pressure across a tube filled with honey or water, the honey will flow more slowly.

Toothpastes, as well as some other interesting fluids have the following unusual property -- when the difference in speeds in different parts is large, in this case between the walls of the tube and the center of the flowing column of toothpaste, the fluid actually flows easier {\it i.e.} it behaves as though it has a smaller viscosity. When this difference is small then the fluid moves very slowly or may even behave like a soft-solid. Very roughly, the reason for this is the following: Toothpaste contains long molecules called polymers. When polymers flow they can do so either in an untidy jumble or with the molecules oriented in the direction of the flow. When the molecules are oriented properly, flow happens more easily than when the molecules are all jumbled up. When the flow is at low velocities, the molecules are pointing in any odd direction; at larger speeds, they uncoil and align, stretching themselves like cats, to achieve a smoother, less viscous flow.

Toothpastes also contain abrasives, which keep teeth clean by grating against them. They include special chemicals which prevent tooth decay, attack gum infections and control tartar, as well as other chemicals which prevent the mouth from drying out and increase the pleasant mouth feel. But what ensures that these different ingredients stay together? These are other useful chemicals called binders, which provide consistency and shape.