Deposit rheology may change with time, due to processes
such as (i) ageing, by cooking on a hot surface or
drying in air, or (ii) diffusion of water into the deposit. Diffusion
will occur relatively slowly but, by reducing viscosity,
and making the deposit easier to remove, may influence
slow removal e such as the slow cleaning in pipes as
opposed to rapid removal under the action of a jet. Removal
will thus depend on two time constants:
the removal rate of the ‘pure’ deposit, into which diffusion
has been negligible, and
the rate of diffusion of fluid into the material to create
a removable form.
If the removal rate of the deposit using water alone is
negligible, then chemical action is needed, and the mechanism
will be different.