  We report surface hardening or crust formation, unlike caking, during drying
when a confined porous medium was heated from above using IR radiation. These
crusts have higher strength than their closest counterparts such as sandcastles
and mud-peels which essentially are clusters of partially wet porous medium.
Observed higher strength of the crusts is mostly due to surface tension between
the solid particles which are connected by liquid bridges (connate water).
Qualitative (FTIR) and quantitative (TGA) measurements confirm the presence of
trapped water within the crust. Amount of the trapped water was ~1.5% (this is
about 10 times higher than in the samples with caking) which was confirmed
using SEM images. Further, in the fixed particle sizes case, the crust
thickness varied slightly (10-20 particle diameters only for cases with
external heating) while with the natural sand whole porous column was crusted;
surprisingly, crust was also found with the hydrophobic glass beads.
Fluorescein dye visualization technique was used to determine the crust
thickness. We give a power law relation between the crust thickness and the
incident heat flux for various particle sizes. The strength of the crust
decreases drastically with increasing hydrophilic spheres diameter while it
increases with higher surface temperature.
