Robots And The Science Of Surface Tension :: Symblogogy

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“Aquabots” take advantage of the properties of water surface tension and the ability to mimic water walking insects.

 The adhesive bonding property of water molecules allows for the formation of water droplets Caption & Image Credit: © 2004 Edward Tsang via PhysicalGeography.net

Scientists have figured out how insects, like the Water Strider, not only walk, but more recently, bounce on water. The critical dynamic force that allows water skimming insects to bounce on water comes down to the speed at which the insect approaches the surface of the water.

Water has a very simple atomic structure. This structure consists of two hydrogen atoms bonded to one oxygen atom. The hydrogen side of the water molecule has a slight positive charge. On the other side of the molecule a negative charge exists. This molecular polarity causes water to be a powerful solvent and is responsible for its strong surface tension. Caption & Image Credit: PhysicalGeography.net

Water has a high surface tension, water is adhesive and elastic, and tends to aggregate in drops rather than spread out over a surface as a thin film. This phenomenon also allows water to stick to the sides of vertical structures despite gravity's downward pull.

The following illustration shows how water molecules are attracted to each other to create high surface tension. This property can cause water to exist as an extensive thin film over solid surfaces. In the illustration example above, the film is two layers of water molecules thick. Caption & Image Credit: PhysicalGeography.net

Applications for this new found understanding include robots that can move about on lakes and reservoirs to monitor water quality, spy or explore.

This excerpted from the Telegraph (UK) -

Scientists crack how insect bounces on water
By Roger Highfield, Science Editor - Last Updated: 7:01pm GMT 07/12/2007

Walking on water may seem like a miracle to humans. But it is a humdrum achievement for the little water strider, which is able to bounce up and down on water too.

Scientists have already solved the mystery of how their six slender, stilt-like legs evenly distribute their scant body weight over a relatively large area so that the "skin" formed by the surface tension of the water supports them, so four millimetre across dimples form under each foot as they skim about.

But scientists remained puzzled by how they could jump up and down upon the surface of water.

Now a team in South Korea is about to report that it has at last explained the water strider's baffling ability to leap onto water without sinking, in a forthcoming issue of the journal Langmuir.

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Ho-Young Kim and Duck-Gyu Lee of Seoul National University note that scientists already have discovered the water-repellent, hairy structure of the water strider's legs and how they enable the creatures to scoot along ponds and placid lakes.

They solved the mystery of how the insects jump onto or "bounce" off liquid surfaces by dropping a highly water-repellent sphere onto the surface of water at different speeds, carefully tracking its motion with high-speed cameras.

Footage revealed that the ball must be traveling within a narrow range of velocities in order to bounce off the water's surface. The sphere may sink if it goes too fast and won't bounce back if it is too slow.

This explains why water striders have extremely water repellent - superhydrophobic - legs, and how they touch down at just the right speed not to sink, said Dr Kim. "Application of our study can be extended to developing semi-aquatic robots that mimic such insects having the surprising mobility on water."
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The real thing is extraordinariliy mobile. Some water striders can propel their bodies across the water surface at nearly 3.5 feet, or 100 times their body length, per second. A six-foot-tall human swimming at a comparable speed would achieve around 400 mph.

The reason that the insects can skim around on water is because water molecules at the surface are strongly attracted to each other and those beneath, unlike the air above. The result is surface tension, a skin-like effect that these insects exploit. The strider's legs can support 15 times the insect's weight without it sinking, according to calculations.