SMALL PRINT:IF YOU WERE on a beach this summer and you agonised over how to build the perfect sandcastle, fret not: science is on hand to help. An open-access paper in Scientific Reports this month builds an argument for the moisture content of the sand and the base radius of the cylinder being key factors in keeping things stable.
“Just a bit of water enables one to turn a pile of dry sand into a spectacular sandcastle,” write the authors. “Too much water, however, will destabilise the material, as is seen in landslides.” Capillary bridges between grains of sand lend stiffness to the structure, and a moisture content of around 1 per cent seems to be favourable, they claim.
Plus, as every sandcastle builder learns, compacting the sand also helps – but how high can you go? To experiment, the researchers filled pipes with sand and compacted it, then they removed the pipes to leave the sand columns standing, or not.
"We find that the columns become unstable to elastic buckling under their own weight. This allows us to account for the maximum height of the sand column," they write in the study, describing a relationship between the maximum height and the base radius in their model. Now just watch out for the incoming tide. – Claire O'Connell
Another shaggy dog story?
IF YOU HAVE ever stood too close to a dog as it bounds out from the sea and dries itself off, you’ll know all about that twisty-shaky move it does to remove much of the water from its fur and transfer it onto hapless bystanders.
Most of us just vow to give the wet dog a wider berth next time, but a group at Georgia Institute of Technology in the US has gone all out to study how furry mammals dry off. They used high-speed video to analyse animals ranging in size from mice to bears as well as Labrador retrievers, and made a “wet-dog-shake” simulator so they could measure the fate of water on fur.
The shake is an effective trick that can remove as much as 70 per cent of the water from soaked fur in just a few seconds, according to the researchers. And the frequency of the movement seems to vary with size.
"In our study we found the largest mammals, such as bears, tigers, and large dogs, shake about four times per second while small mice shake at more than 30 times per second," they write on their website dickerson.gatech.eduwhere you can see videos of some of the shakers in action. "We found shaking longer or faster does not contribute to further drying. Therefore, mammals 'tune' their shaking to achieve dryness with the least effort.
Their new paper in the Journal of the Royal Society Interface points out that loose regions between fur and muscles could help too: "We also observe a novel role for loose mammalian dermal tissue: by whipping around the body, it increases the speed of drops leaving the animal and the ensuing dryness relative to tight dermal tissue." – Claire O'Connell