Animal locomotion on the surface layer of water is the study of animal locomotion

Animal locomotion

Animal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including running, swimming, jumping and flying. Animals move for a variety of reasons, such as to find food, a mate, or a suitable microhabitat, and to escape predators...

 in the case of small animal

Animal

Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s that live on the surface

Interface (chemistry)

An interface is a surface forming a common boundary among two different phases, such as an insoluble solid and a liquid, two immiscible liquids or a liquid and an insoluble gas. The importance of the interface depends on which type of system is being treated: the bigger the quotient area/volume,...

 layer of water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, relying on surface tension

Surface tension

Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...

 to stay afloat.

There are two means of walking on water; the regime determined by the ratio of the animal's weight

Weight

In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...

 to the maximum vertical force that the surface layer can exert. Creatures such as the basilisk lizard (which can run on water) have a weight which is larger than the surface tension can support.

Surface living animals such as the water strider

Water strider

Gerridae is a family of true bugs in the order Hemiptera, commonly known as water striders, water bugs, magic bugs, pond skaters, skaters, skimmers, water scooters, water skaters, water skeeters, water skimmers, water skippers, water spiders, or Jesus bugs...

 typically have hydrophobic feet covered in small hair

Hair

Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....

s that prevent the feet from breaking the surface and becoming wet. Another insect known to walk on the water surface is the ant Polyrhachis sokolova

Polyrhachis sokolova

Polyrhachis sokolova is a species of ant recently discovered to be capable of surviving tidal inundations.Researchers at the James Cook University gained attention in 2006 after finding and reporting this behavior. Polyrhachis sokolova nests in mud in mangrove swamps, regularly inundated by rising...

. The pygmy gecko (Coleodactylus amazonicus) due to its small size and hydrophobic skin is able to walk on the water surface.

According to biophysicist

Biophysics

Biophysics is an interdisciplinary science that uses the methods of physical science to study biological systems. Studies included under the branches of biophysics span all levels of biological organization, from the molecular scale to whole organisms and ecosystems...

 David L. Hu, there are at least 342 species of water striders. As striders increase in size, their legs become proportionately longer, with Gigantometra gigas having a length of over 20 cm requiring a surface tension force of about 40 millinewtons.

Water striders generate thrust by shedding vortices

Vortex

A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...

 in the water: a series of "U"-shaped vortex filaments is created during the power stroke. The two free ends of the "U" are attached to the water surface. These vortices transfer enough (backward) momentum to the water to propel the animal forwards (note that some momentum is transferred by capillary wave

Capillary wave

A capillary wave is a wave traveling along the phase boundary of a fluid, whose dynamics are dominated by the effects of surface tension.Capillary waves are common in nature and the home, and are often referred to as ripples...

s; see Denny's paradox

Denny's paradox

In the study of animal locomotion on the surface layer of water, Denny's paradox refers to the apparent impossibility of surface-dwelling animals such as the water strider generating enough propulsive force to move...

 for a more detailed discussion)

Meniscus climbing

To pass from the water surface to land, a water-walking insect must contend with the slope of the meniscus

Meniscus

The meniscus is the curve in the upper surface of a liquid close to the surface of the container or another object, caused by surface tension. It can be either convex or concave. A convex meniscus occurs when the molecules have a stronger attraction to each other than to the material of the...

 at the water's edge. Many such insects are unable to climb this meniscus using their usual propulsion mechanism.

David Hu and coworker John W. M. Bush have shown that such insects climb meniscuses by assuming a fixed body posture. This deforms the water surface and generates capillary forces that propels the insect up the slope without moving its appendages.

Hu and Bush conclude that meniscus climbing is an unusual means of propulsion in that the insect propels itself in a quasi-static configuration; without moving its appendages. Biolocomotion is generally characterized by the transfer of muscular strain energy to the kinetic and gravitational potential energy of the creature, and the kinetic energy of the suspending fluid. In contrast, meniscus climbing has a different energy pathway: by deforming the free surface, the insect converts muscular strain to the surface energy that powers its ascent.

Marangoni propulsion

Many insects, including some terrestrial insects, can release a surfactant

Surfactant

Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...

 and propel themselves using the Marangoni effect

Marangoni effect

The Marangoni effect is the mass transfer along an interface between two fluids due to surface tension gradient...

. Hu and Bush report that Microvelia can attain a peak speed of 17 cm/s (twice its peak walking speed), using Marangoni propulsion.

Marangoni propulsion by a wetting

Wetting

Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...

 arthropod is precisely analogous to a soap boat but the situation for insects such as water striders is more complex: Hu and Bush state that "for nonwetting arthropods, the transfer of chemical to kinetic energy is more subtle, as the Marangoni stress must be communicated across the creature’s complex surface layer".

External links

• Meniscus-Climbing Insects (photos)