We experimentally study the drying of loosely packed wet glass beads at
low initial water content. The drying rate is found to decrease at the
start, corresponding to the decreasing rate period controlled by vapor
diffusion, followed by a deviation in drying rate from the diffusion
limited evaporation. The propagation of drying front associated with a
sharp saturation gradient is identified through both image analysis and
magnetic resonance imaging technique. The drying-induced collapse of
granular medium is observed and quantified. The concentrated collapse at
the end of drying suggests the existence of liquid in the form of liquid
bridges in the apparent dry region until the end of drying process.
Collapse event is found to be local, i.e., a clear boundary can be
identified for each collapse event, below which the loosely packed
medium remains intact. This indicates the existence of a saturation
gradient in the apparent dry region. The drying dynamics and collapse
statistics suggest that the observed transition of drying regimes is due
to Kelvin effect. This work demonstrates for the first time the drying
enhancement phenomenon due to Kelvin effect even for grains with size of
hundreds of micrometers, and provides insights on the drying process of
partially saturated granular materials, especially near the final period
of evaporation.