Asked by: Sue Watkins, Andover
Amazingly, scientists have only recently got to grips with this – despite the importance of understanding the behaviour of sand and similar ‘granular’ materials in civil engineering.
As every kid who’s played on a beach knows, dry sand refuses to form anything but a pathetic cone-shaped heap, while wet sand just slumps. Clearly there must be an optimum water content, and last year, researchers at the University of Amsterdam showed that this is around just 1 per cent. That’s enough to ensure the water molecules bridge the gaps between the sand grains, gluing them together by capillary attraction.
They’re amazingly effective at doing so, too. The team found that by using the optimum amount of water they could create a vertical column of sand that was 2.5m (8ft 2in) tall yet only 40cm (15in) in diameter before it collapsed. Formed into a traditional sandcastle shape with sloping sides, the tower could be even taller.
So, the best way to build a sandcastle is to add just a dash of water to your sand mix.