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Surface tension

Video: Water drop on a glass surface
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Water molecules strongly attract each other. For them it is energetically favorable to be next to each other rather than being next to air molecules. Therefore, the water molecules residing in the water-air interface are pulled into the bulk of the water phase. Due to this force tthe area of the water-air interface is minimized. It is called “surface tension” because it acts as if the interface were a “skin” being pulled over the water surface. In absence of gravity the ideal shape of a water drop in air is spherical. Due to gravity it is getting somewhat flattened.

For water lying on a solid surface, the shape of the drop depends on the surface tensions of the respective materials. If the solid surface is easily wettable, the water molecules tend to stay close to the water-solid interface. The drop spreads out and becomes flat. If the solid surface is water-repellent, the water molecules tend to avoid to be in contact with the solid surface and the drop becomes therefore spheroidal. The water-air and the solid-air contact are then getting enlarged at the expense of the water-solid interface.

Illustration of surface tension

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