Pond skating insects reveal water-walking secrets

1 November 2012 | Source: Physics World

This month’s special issue of Physics World is devoted to animal physics, and includes science writer Stephen Ornes explanation of how pond skaters effortlessly skip across water leaving nothing but a small ripple in their wake.

Physics World November cover

This month’s special issue of Physics World is devoted to animal physics, and includes science writer Stephen Ornes explanation of how pond skaters effortlessly skip across water leaving nothing but a small ripple in their wake.

As Ornes writes, our current understanding of the mechanisms adopted by the pond skater is down to the efforts of David Hu, who as a mathematics graduate from the Massachusetts Institute of Technology spent four years studying their behaviour.

Hu, along with his PhD supervisor John Bush, found that pond skaters use the middle of their three pairs of legs to “row” across the water. When a rowing boat’s oar slices the water, it creates swirling vortices just beneath the surface that twist away from the boat and move it forwards – the same vortices are created by tiny hairs that cover the pond skater’s legs.

The hairs are the only part of the insect’s body that penetrate the water and are covered in a waxy substance that keeps water out by allowing bubbles to attach to them. The hairs have subsequently drawn the attention of materials scientists looking for a permanent waterproofing material that doesn’t wash off.

The pond skater (also known as the water strider) is one of a tiny proportion of insects – around 0.1 per cent – that are able to stand and move on water. They stay afloat thanks to their small weight and the surface tension of water acting like a skin; however, according to Newton’s third law of motion, the pond skater must push against something to move forward, which is fairly tricky when the only thing available is water.

Previous theories had suggested that pond skaters created tiny ripples – known as “capillary waves” – with their legs which subsequently propelled them across water, but as biologist Mark Denny pointed out in 1993, this couldn’t be the case as infant pond skaters cannot move their legs faster than the phase speed of the capillary waves – a feat necessary to create them.

Hu and Bush came to their conclusions by filming a group of pond skaters using high-speed cameras as they moved across a body of water filled with colourful floating particles – this helped identify the swirling vortices that were being created.

David Hu from Georgia Institute of Technology's "laboratory for biolocomotion" will present a special online lecture at 15:00 GMT on Thursday 8 November 2012, which can be viewed by registering at http://physicsworld.com.

Also in this issue:

  • Meet the creatures with superpowers -- a set of stunning full-page photos of animals that use physics in quirky ways, including the mantis shrimp, the peacock and the Namib Desert beetle
  • Why lions roar like babies cry -- vocal scientist Professor Ingo Titze explains what these two sounds have in common and why they’re so hard to ignore
  • How the zebra got his stripes -- the stripy hides of these animals might have evolved to deter biting flies, which prefer all-white or all-black patterns
  • Lapping it up -- cats appear to drink more elegantly than dogs, but their drinking mechanisms have more in common than their owners might like to think
  • Fly away home -- how birds navigate using physics-related techniques based on geomagnetism, celestial mechanics, light and sound
  • Riding raindrops -- how do mosquitos survive being hit by raindrops more than 50 times their own body weight, which is equivalent to a human being hit by a black cab?
  • Vespan voltage -- how the humble hornet uses its in-built solar cell to generate electricity


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