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.