What keeps a sand castle together?

When dry sand is mixed with water, it acquires a visco-plastic texture, stiff enough for sculpting sand castles of astonishing stability. Scientists investigated why a change in moisture level from 1% to 10% barely affects the mechanical stiffness of
these moist granulates, whilst the shape of the liquid inside varies considerably.

X-ray microtomography of a cluster of liquid within a dense pile of tiny glass beads (0.8 mm in diameter) with 11% liquid (only the liquid is visible in the image) (Image credit: M. Scheel, copyright 2008 MPI DS Göttingen (Germany)).

Further information:

• ESRF News item: Morphological clues to the stability of wet granular piles
• Ref. M. Scheel et al., Nature Materials 7, 189-193 (2008).