Water tends to minimize the air-water interface. Water molecules at the surface experience a net inward force.
Consider the behavior of water drops on a glass plate. (Glass and soil particles -particularly quartz- have similar chemical compositions.)
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Without gravity, and neglecting adhesion to the glass plate, a volume of water forms a sphere due to internal cohesion and surface tension. |
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Gravity and adhesion (arrows) distort the sphere into roughly a hemisphere. |
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As the plate is tilted, cohesive force (viscosity) resists distortion from the hemispherical shape (flow), and adhesion prevents the water from sliding over the plate. (Remember, the gravitational force "downhill" is proportional to the sine of the angle of the plate.) |
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As the plate is tilted further, gravity overcomes viscosity and the drop begins to flow. The "tail" of the characteristic teardrop is due to adhesion (wetting). |
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