How can a water strider walk on water?
Topic: Surface Tension
Burning Question: A water strider walks on the surface of water.
How is this possible?
With an entire town present, a man walks across the surface of a pond. How could this be possible? The answer to this riddle is quite obvious when one considers the wording of the question. The question does not make reference to season; therefore the man simply walks upon ice. Perhaps a few small substitutions would increase the riddle's intrigue. A small insect now walks upon the surface of the same pond, in liquid water form, without submerging. Unlike the previous riddle, this puzzle's solution is based in science and not word manipulation. The insect, a Hydroconisae (water strider), actually exists and does possess the ability to "walk on water". This phenomenon has intrigued many researchers whom have obtained the solution to this riddle. The answer involves both the anatomical structure of the insect, as well as the behavioral characteristics of the water.
The H2O molecule plays a critical role in the development and sustenance of every living organism on the earth. Its unique molecular structure allows for peculiarities such as an expanded solid state, top to bottom solidification, and surface tension. Surface tension, the reason the water strider's weight can be supported, can be explained by examining Van der Waals attractive forces. These forces are actually small electrical charges caused mainly by asymmetrical distribution of electrons about the molecule. Normally surrounding molecules neutralize this charge; however the surface molecules, lacking any more molecules above them, do not become neutralized and experience a net force inward. For this reason water beads on hydrophobic surfaces such as glass and wax, resisting both stretching and breaking. The resulting elastic membrane has the ability to support small objects, only producing small depressions upon the water surface. Surface tension not only explains why the water surface is capable of supporting an insect, but also why "dimples" are generated around each of the strider's feet. The water strider's reliance on surface tension can be clearly illustrated by performing a simple experiment involving water and detergent. Easily standing upon the water's surface, the strider will only begin to sink as the detergent is added. At approximately a 0.005 M solution of sodium docedyl sulfate, the ingredient in detergent which causes surface tension to be lowered, the insect will become completely submerged. Thus it is apparent that a water strider would be unable to stand on a surface of water if it were not for surface tension.
To fully understand how the water strider stands, moves, and lives upon the surface of the water one must examine its anatomical structures and adaptations. The water strider is the general name for the insect family Gerridae, all of whose members are know for their ability to walk on the surface of water. Each strider has six legs, four of which are directly involved with the movement of the insect. The middle pair of the water strider's legs are used for propulsion, while its hind legs are used for steering. These four limbs are generally twice the length of the insect's body, becoming useful in distributing the weight of the strider over a larger area. With its weight evenly distributed around a large surface area there is less chance that the strider will break surface tension, thus increasing its survival. The largest possible size of a water strider would by approximately that of a human fist. Anything larger would weigh too much and submerge. Each of the water strider's four limbs is covered with tiny hydrophobic hairs. These water-resistant hairs reduced the amount of contact the strider has with the water, in turn causing a new problem. It would appear that without having contact with the water the strider would be unable to move. However, there must be an error in this reasoning since many water striders are capable of reaching speeds of one meter per second. The striders actually propel themselves using two techniques: "rowing" and "slapping". The rowing propulsion system works in a way similar to that of an oar. By pushing the dimples on the surface of the water towards its rear during the backward strokes, and lifting its feet off the water during the forward stroke the strider produces drag which drives it frontward. During the gallop the strider's feet slap the water and are momentarily submerged beneath the water. Both these methods of motion result from structural adaptations, which allow the water strider to stand, walk, gallop, and live on the surface of the water.
The family Gerridae is unique in having the ability to travel along the surface of ponds, lakes, and even the ocean. Surface tension is the critical behavioral characteristic of water, which allows the water striders to be supported on the water's surface. By developing hydrophobic legs the water strider has successfully reduced friction with the water's surface to near zero, while still keeping motility. The true answer to riddle of walking on water lies in the balancing of three important areas: the strength of surface tension, the utilization of hydrophobic limbs, and the ratio of weight to surface area on which it is distributed.