Sculpting sandcastles grain by grain: Self-assembled sand towers
LS13910E -
We study the spontaneous formation of granular towers produced when dry
sand is poured on a wet sand bed. When the liquid content of the bed
exceeds a threshold value, the impacting grains have a
non-zero probability to stick on the wet grains due to instantaneous
liquid bridges created during the impact. The trapped grains become wet
by the capillary ascension of water and the process continues, giving
rise to stable narrow towers. The growth velocity is determined by the
surface liquid content which decreases exponentially as the tower height grows. This self-assembly mechanism could theoretically as long as the capillary rise
of water is possible, however the structure collapses before reaching
this limit. The collapse occurs when the weight of the tower surpasses
the cohesive stress at its base. The cohesive stress increases as the
liquid content of the bed is reduced. Consequently, the highest towers
are found just above the sticking threshold value.
LS13910E - We study the spontaneous formation of granular towers produced when dry
sand is poured on a wet sand bed. When the liquid content of the bed
exceeds a threshold value, the impacting grains have a
non-zero probability to stick on the wet grains due to instantaneous
liquid bridges created during the impact. The trapped grains become wet
by the capillary ascension of water and the process continues, giving
rise to stable narrow towers. The growth velocity is determined by the
surface liquid content which decreases exponentially as the tower height grows. This self-assembly mechanism could theoretically as long as the capillary rise
of water is possible, however the structure collapses before reaching
this limit. The collapse occurs when the weight of the tower surpasses
the cohesive stress at its base. The cohesive stress increases as the
liquid content of the bed is reduced. Consequently, the highest towers
are found just above the sticking threshold value.
