Surface Film



The surface film tension is the property of a liquid by which it acts as if its surface is a stretched elastic membrane. This tension allows insects to stand on the water’s surface. Surface film tension is caused by the attraction of water molecules to each other; consequently, the surface molecules are attracted back underneath by the underlying molecules.

The amount of surface film tension is affected by both temperatures and dissolved substances. An increase in temperature lowers the net force of attraction among molecules and likewise decreases the surface tension. An increase in dissolved organic substances lowers the surface tension. However, an increase in dissolved inorganic salts causes a rise in surface tension. So if you want to try to walk on water do so on a cold day in a salty lake.

Surface film tension is the property which allows items that are not wetted to float. This causes insects to float. It is also the reason why floatant is placed on our lines and flies; consequently, it prevents these objects

Surface Film

from being wetted. They will float in the surface film.

The surface film also reflects light; similarly, it reduces the amount of light entering the water. This influences both the anglers and the fish’s vision.

Emerging aquatic insects must be able to break through this surface tension to reach the air. At times when this surface tension is great, insects have difficulty emerging through the surface. They may become trapped in the surface film and vulnerable to fish predation. The cooler overcast days provides a longer

Surface Film

surface duration of insects hatching because they have more difficulty piercing the surface film. Being trapped at the surface, insects are forced to drift for a prolonged time.

Falling into the drink, the surface tension commonly traps terrestrial insects. An increase in surface tension makes it more difficult for these terrestrial insects to escape.

Hence cooler temperatures, enhances the angling production during emergence of midge, mayfly and caddis flies. It simply prolongs the insect’s appearance at the surface.

A fresh rain may cause an increase in dissolved

Surface Film

organic or inorganic substances in the water. If it’s an increase in inorganic salts the surface tension rises; on the other hand, if it’s increase in organic substances the surface tension lowers.

Warm temperatures favor the insect’s escapement by lowering the surface tension. During warmer temperatures hatching insects are not on the surface for long. Likewise, during extremely high surface tension times heavy objects such as snails may be found floating.

Surface Film

Best dry fly conditions are when the surface film tension is high. The trapped insects are prolonged at the surface and this elicits fish predation.


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