Ready for Next Season

Nature
Physics magazine explains the
secret for building the perfect sand castle – but only 7 people in the
world can understand it, and none of them has ever been to the beach…

Anyone
who has built a sandcastle recognizes that adding liquid to the sand
grains
increases
the overall
stability.
However, measurements of the stability in wet granular materials often
conflict with theory and with each other. The friction-based
Mohr-Coulomb model, distinguishes between granular friction and
interparticle friction, but uses the former without providing a physical
mechanism.
A frictionless model for the geometric stability of dry particles on
the surface of a pile is in excellent agreement with experiment. However,
the same model applied to wet grains overestimates the stability and
predicts no dependence on system size. Here we take a frictionless
liquid-bridge model and perform a stability analysis within the pile.
We reproduce
our experimentally observed dependence of the stability angle on system
size, particle size and surface tension. Furthermore, we account for
past discrepancies in experimental reports by showing that sidewalls
can significantly increase the stability of granular material.

from Nature Physics