It was Herbert Max Finlay Freundlich a physical chemist who coined the term thixotropy ('change by touching') in 1935 during his research on gels. The standard scientific definition of thixotropy is usually given along the lines of :-

A decrease of viscosity under constant shear stress or shear rate, followed by a time dependant recovery when the shear load is removed.

Hmmm... Like reading the unix man pages, you get a good definition, in a way that takes even more work to find out what the meaning of it is. An example here will be more illuminating I feel; tomato ketchup. The reason why it doesn't flow out of the bottle is because it is a thixotropic liquid. When its sitting on the shelf, the shear stress/load is zero and the molecules/components making it up take on a more ordered structure by forming inter-molecular bonds (usually through hydrogen bonding), this causes the viscosity to go up. Giving the sauce a quick shake breaks these relatively weak bonds, and the liquid becomes runny again. Quicksand is another example, athough a colloidal suspension of sand and water it behaves thixotropically; if you thrash around in it, it becomes less viscous, and down you go.

The study of thixotropic behaviour is important to a whole host of processes, many foodstuffs, cosmetics and explosives rely on accurate models to prevent settling during transportation. The behaviour of muscles can be said to be thixotropic, in that the stiffness of a muscle fibre in dependent on its recent activity. The higher stiffness of in-active muscles is actually useful in maintaining posture, the energy for keeping upright partly comes from the formation of intermolecular bonds in the muscle fibres; a very efficient mechanism really! Thixotrophy may offer an explanation as to why the blood of St. Januarius liquefies once a year...

The opposite behaviour to thixotropic is rheopectic or, where fluids become more viscous when you agitate them, an example of this is a solution of cornflour and water. This can be runny enough to flow through your fingers, but apply a rapid shear stress to it; say through hitting it, and it feels solid, it can even appear to snap like a solid.

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