Scientists find formula for perfect sandcastle
By Roger Highfield
(Filed: 29/09/2005)

Scientists have used complex experiments and maths to create the perfect recipe for making a sandcastle.

Mixing eight parts sand to one part water provides the ideal building material, according to the American researchers.

	Ideal ratio: eight parts sand to one part water

The study, reported in the journal Nature Physics, and carried out by a team from the Massachusetts Institute of Technology (MIT), set about investigating how sand is stabilised by water.

They performed a set of measurements using transparent rotating drums partially filled with wet sand.

The drums were of different diameters and contained sand with varying grain sizes mixed with different liquids.

As each drum was turned, the sand within it reached a point of instability and collapsed in an "avalanche". The precise angle at which this happened was measured optically.

From these results, the scientists developed a mathematical model to describe the inherent stability of wet sand.

Sandcastles are possible because liquid added to dense granular material forms "bridges" at the contact points between the grains.

The surface energy of the bridges produces an attractive force holding the grains together, which keeps the castle standing.

The MIT team, led by Sarah Nowak and Arshad Kudrolli, showed that a ratio of eight to one was the most stable mix for sand and water.

In an accompanying article, Peter Schiffer from Pennsylvania State University, wrote: "The work is an important advance and opens the door to more systematic studies of how the addition of interstitial liquid affects the dynamics of such materials."

The findings have a wide range of applications besides building sandcastles, such as in wet milling processes and controlling debris flow.

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