Water strider

Aquarius

Nature of the hydrophobic legs of a water strider

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.

Related Topics:
Leg - Nature - Biophysicists - Xuefeng Gao - Lei Jiang - Hair - Wax

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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.

Related Topics:
Millinewton - Dyne - Poundal

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For comparison, Gao and Jiang made a hydrophobic 'leg' from a smooth quartz fibre that was similar in shape and size to a strider's leg. Its surface was coated with a thin layer of heptadecafluorodecyltrimethoxysilane (FAS-17), whose contact angle with water is 109°. However, this artificial leg only supported a force of only 0.19 mN (19 dyn or 0.000014 pdl): this would be just about enough to support the strider at rest, but not to enable it to dart around rapidly on the surface.

Related Topics:
Hydrophobic - Quartz - Fibre - Heptadecafluorodecyltrimethoxysilane - Contact angle - Water

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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°.

Related Topics:
Neologism - Superhydrophobic

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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.

Related Topics:
Scanning electron microscope - Needle - Micrometre - Nanometre

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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.

Related Topics:
Cassie's law - Air

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