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| [Note Guidelines] Photographer's Note |
In a pond in Bangalore University...
The family Gerridae contains insects commonly known as water striders, magic bugs, pond skaters, skaters, skimmers, water scooters, water skaters, water skeeters, water skimmers, water skippers or Jesus bugs. These are predatory insects which rely on surface tension to walk on top of water. They live on the surface of ponds, slow streams, marshes, and other quiet waters. They can move very quickly, up to 1.5 m/s.
It was originally thought that water striders transferred momentum to the water by the creation of capillary waves on the surface. However, biophysicist Mark Denny showed that to do this, some object must move faster than about 0.25 m/s — far faster than a water strider can move its legs. This apparent contradiction is known as Denny's paradox.
Water striders beat Denny's paradox by generating not capillary waves but hemispherical vortices in the water. These vortices carry sufficient backwards momentum to propel the animal forwards.
In a series of experiments, mathematician David L. Hu and coworkers showed that during the rowing stroke, water striders drive their middle legs backwards without penetrating the surface, and can attain speeds of up to 1.5 m/s.
Besides skating on the surface of water, adult water striders can also use wings (e.g. to fly to another body of water).
Water striders can stand effortlessly on water due to their non-wetting legs. Writing in Nature, biophysicists Xuefeng Gao and Lei Jiang show that the water resistance of the legs is due to the "special hierarchical structure of the legs, which are covered by large numbers of oriented tiny hairs (microsetae) with fine nanogrooves". They go on to demonstrate that this physical structure is more important than the chemical properties of the wax coating of the legs.
Gao and Jiang calculate the maximal supporting force of a single leg to be is 1.52 millinewtons (152 dynes or 0.011 poundal), which is about 15 times the total body weight of the insect. This shows that the surface of the leg is strikingly water repellent.
Gao and Jiang went on to calculate that the contact angle with water on a real strider's leg would be greater than 150° (and described this using the neologism 'superhydrophobic') and, using a sessile water-drop showed that the contact angle of the insect's legs with water was 167.6° ± 4.4°.
They went on to infer that the observed superhydrophobicity was due to microstructures on the legs and, using a scanning electron microscope, showed that the legs were covered in many needle shaped setae, with diameters ranging from 3 micrometres down to a few hundred nanometres. Most of the setae were about 50 micrometres long and were at an angle of about 20° from the surface of leg. Each microseta also had nanoscale grooves, contributing to the hierarchical structure of the leg.
Gao and Jiang used Cassie's law to show that air is trapped in spaces in the microsetae and nanogrooves, forming a cushion at the leg–water interface. This cushion prevents the legs from being wetted.
(From Wikipedia) |
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