Water strider

The Water strider, (also known as: Skater, Pond Skater, Jesus Bug, Water Skeeter, water scooter, water skater, and Skimmer) is any of a number of predatory insects in the family Gerridae that rely on the surface tension of water to walk on top of it. They live on the surface of ponds, slow streams, marshes, and other quiet waters and can move very quickly (up to 1 m/s) over the surface of water.

Aquarius remigis (formerly known as Gerris remigis) is one of the species in Gerridae known as a water strider.

Method of propulsion of a water strider

Animals such as water striders that live on the surface of water need to push something backwards to generate a reaction force (that is, Newton’s third law of motion).

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.

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.

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.

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.

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 micrometers down to a few hundred nanometers. Most of the setae were about 50 micrometers 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.