Tuesday, October 11, 2005

Walking on water explained

S Ananthanarayan
Insects skid on water with the help of hair on their feet which keep their feet dry.

Scientists at Massachusetts Institute of Technology have found out how tiny insects use molecular forces on the surface of water to zip about like speedboats. David Hu and John Bush have reported in Nature that insects move at 30 body lengths a second when they move up the short slope that marks the edge of a pond or puddle, without even moving their legs!

Water molecules consist of two positively charged hydrogen atoms linked to one negatively charged oxygen atom, but the way the atoms are oriented is not symmetrical. Thus, at short distances, water molecules can show polarity, like magnets with N and S poles, and exert powerful electric forces.

When well within a body of water, where other molecules effectively surround a molecule in all directions, there is no net effect of these forces. But at the surface, where there is a mass on water on one side and nothing on the other, the surface molecules feel a strong inward pull. The surface of liquids, water particularly, is thus like a tight membrane, which resists anything that may create a gap in the surface.

Rigid Surface

The surface can thus support a reasonable weight and the surface of ponds or puddles supports a whole universe of tiny, millimetre-scale life-forms, which find the water surface as rigid as any other.

This is so long as the insects' feet do not get wet, with the water sticking to the feet due to molecular forces. The distinct separation between the feet and the mass of water then disappears and the feet would sink. We may have seen that a drop of water on a glass sheet that is just a little greasy does not spread out, but forms a little bubble, as it tries to pull itself into a ball, its smallest surface. But if the glass is clean, then the forces between the water molecules and the glass are as strong as the force of the water mass and the drop spreads out.

Similarly if the insects' feet are dry, the surface of water does not break and the insects can ride the surface like a sledge over snow, using the fore and rear legs as support and the middle legs as paddles. The secret of these tiny ones is that their feet are covered with the finest hair imaginable and the hair traps air, to keep the water away from the feet!