Mosquitoes sitting on water

In rainy reason, diseases like dengue and malaria spread by mosquito breeding on fresh stagnant water. They do not sink in water due to surface tension. At the points where the legs of the mosquito touch the liquid surface, the surface becomes concave due to the weight of the mosquito.

The surface tension acting tangentially on the free surface acts at a certain angle to the horizontal. Its vertical component acts upwards. The total force acting vertically upwards all along the line of contact of certain length balances the weight of the mosquito acting vertically downward.

Excess pressure on concave side of a spherical surface

If the surface is plane, the surface tension on the two sides tangential to the surface balances and the resultant tangential force is zero. However, if the surface is convex or concave, the forces due to surface tension acting across the sides will have resultant force towards the center of curvature of the surface

Thus, whenever the surface is curved, the surface tension gives rise to a pressure directed towards the center of curvature of the surface. This pressure is balanced by an equal and opposite pressure acting on the surface. Therefore, there is always an excess pressure on the concave side of the curved liquid surface.

Detergents and surface tension

Detergents can remove oil stains from clothes. Water is used as cleaning agent. Soap and detergents lower the surface tension of water. This is desirable for washing and cleaning since high surface tension of pure water does not allow it to penetrate easily between the fibers of materials, where dirt particles or oil molecules are held up.

Surface tension of soap solution is smaller than that of pure water but the surface tension of detergent solutions is smaller than that of soap solution. That is why detergents are more effective than soap. A detergent dissolved in water weakens the hold of dirt particles on the cloth fibers which therefore, get easily detached on squeezing the cloth.

Wax-Duck floating on water

The surface tension of liquids decreases due to dissolved impurities. If you stick a tablet of camphor to the bottom of a wax-duck and float it on still water surface, you will observe that it begins to move randomly after a minute or two. This is because camphor dissolves in water and the surface tension of water just below the duck becomes smaller than the surrounding liquid. This creates a net difference of force of surface tension which makes the duck to move.