  We present an optical study that reveals the bulk dynamics of the
stratification process in drying colloidal dispersions. Terahertz time-domain
spectroscopy has been used to measure in situ solventborne and waterborne paint
layers as a function of drying time. The dynamic behavior of the dry top layer
and wet bottom layer thickness, as well as the bulk thickness, reflect the
principal processes of the established drying mechanism. In addition, the
results demonstrate stratification only when the drying process is in the
evaporation controlled regime, whereas the coating is shown to remain a single
layer for diffusion controlled drying.
