Science: What supermodels and ketchup have in common

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BESIDES being useful in Scrabble, thixotropy is a word that links supermodels, tomato ketchup, non-drip paint and earthquakes.

Put scientifically, a "thixotropic" liquid is one where the viscosity depends on the shear force: the greater the force (the more you stir or shake the liquid) the lower the viscosity becomes, so that it flows more easily when it's being agitated than when it's left alone.

How does that involve supermodels? Because they use mascara - which, in its container, is solid until disturbed with the brush. For a while, it stays liquid, then, on the eyelashes, it solidifies again, not by losing moisture, but because there's no shear force on it.

What makes a thixotropic substance? Generally, complex structures which, when quiescent, hold large amounts of water bonded into a structure like a house of cards - with the water in between the stacked cards. The viscosity is thus very high. Shake the cards, and the water is freed, and the cards can move individually - the viscosity has dropped. Once the force moving the "cards" stops, the structure can reassert itself, bonding the water once more.

Think of what happens with tomato ketchup. You can hold it open over your food for ages, yet nothing happens. But shake it up and suddenly it is a liquid. Commercial ketchup behaves like a kind of gel with fine bits of solid tomato and seasonings dispersed through the liquid.

Non-drip paint? Probably you're getting the idea by now. It doesn't drip off the brush, yet you can spread it on to a surface, where it will remain quite happily.

The idea that earthquakes and thixotropy could be entangled may seem surprising, but the effect is one of the more dangerous ones. Clay can act as a thixotropic substance. In Alaska in 1964, an earthquake struck on Good Friday. In Anchorage, the underground clay turned instantly to mud, because of the shockwaves. Many well-built houses turned into wrecks.

Why? Because the matrix of clay particles, which held the water in place, became dislocated by the ground's movement. That's why city planners in earthquake zones take great care finding out what lies below the ground before they approve building plans.

Charles Arthur, Science and Technology Editor