It was Herbert Max Finlay Freundlich
a physical chemist
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
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
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.