Home > Water strider

The Water strider, Gerris remigis, is a predatory insect that relies 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. They can move very quickly (up to 1m/s) on the surface of water. It is a member of family Gerridae ( Leach 1815).

1 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 vorticesFluid dynamics This is an article on the real vortex phenomena. For information on the fictional aliens from the Ecco the Dolphin video game franchise, see Vortex life forms . For the VORTEx project, see the Versatile Ocean Subsea Robot for Technical Expe in the water. These vortices carry sufficient backwards momentum to propel the animal forwards.

In a series of experiments, mathematicianA mathematician is a person whose area of study and research is mathematics. Roles Mathematicians not only study, but also research, and this must be given prominent mention here, because a misconception that everything in mathematics is already known is 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.

2 Nature of the hydrophobic legs of a water strider

Water striders can stand effortlessly on water due to their non-wetting legIn zoological anatomy, a leg is any one of the parts of an animal's body that (in most legged species and at most times) separate the rest of the body from the ground, and are used for locomotion. Legs are most common in one of the even-numbered quantities. Writing in NatureNature is one of the oldest and most reputable general-purpose scientific journals, first published on November 4, 1869. Although most scientific journals are now highly specialized, Nature still publishes articles across a wide range of scientific fields, biophysicistsBiophysics (also biological physics is an interdisciplinary science that applies theories and methods of the physical sciences to questions of biology. Biophysics research today comprises a number of specific biological studies, which do not share a uniqu 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 hairHair is also a musical: see Hair (musical) and Hair (movie Hair is the filamentous outgrowth of the epidermis found in mammals. Hair is a characteristic of all mammals, though in some species hair is absent at certain stages of life. Hairs" are also founds (microsetae) with fine nanogrooves". They go on to demonstrate that this physical structure is more important than the chemical properties of the waxWax has traditionally referred to a substance that is secreted by bees ( beeswax) and used by them in constructing their honeycombs. In modern terms, wax is an imprecisely defined term generally understood to be a substance with properties similar to bees coating of the legs.

Gao and Jiang calculate the maximal supporting force of a single leg to be is newtonThis article is about the SI unit of force. For other uses see Newton (disambiguation In physics, the newton (symbol: N) is the SI unit of force, named after Sir Isaac Newton in recognition of his work on classical mechanics. It was adopted by the Generals (152 dynes), 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 newtons (19.05 dynes): 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 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.