ABSTRACT
In the first falling rate-drying period, a thin liquid
layer covers the unsaturated region of gel pores. There-
fore, the outside surface of the gel does not change
immediately so long as liquid flux is comparable to
evaporation rate, and this state may be continuously
preserved. However, as evaporation goes on, distance
from the solid outside surface to drying front increases,
the pressure gradient decreases and so does the flow
rate. Hence, distribution of liquid on the outside sur-
face becomes discontinuous, and drying enters the
second falling rate period. In this period, evaporation
completely occurs inside the gel body. The solvent
evaporation rate is no longer sensitive to external con-
ditions, the liquid near the external surface of the gel
pores appears discontinuous, and transfer of liquid
from the gel pores to the outside is taken by flow
with diffusion as predominant. The force exerted on
the gel is significantly mitigated during the second
falling rate-drying period. Therefore, the gel body
may dilate slightly. Because compressive stress on the
nondrying side of skeleton is larger than that on
the drying side, this may deform and collapse the gel
skeleton. In this period, the gel volume no longer
changes but its quality gradually decreases. Along,
with this excess part of solvent evaporation, various
degrees of collapse may appear, micropores vanish,
and the original skeletal structure shrinks into large
agglomerations.
