drop(redirected from hanging drop)
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a small volume of liquid that is bounded in a state of equilibrium by a surface of rotation.
Drops form when a liquid flows slowly out of a small opening or runs off the edge of a surface, as well as when a liquid is atomized or emulsified. Drops also form when vapors condense on solid nonwettable surfaces and on condensation nuclei (ions, dust particles) in gaseous media; water droplets in the atmosphere arise in this way in the formation of dew, fog, and clouds.
The form of a drop is determined by the effect of surface tension (tending to decrease the surface of the drop) and external forces (above all, gravity). Microscopic drops for which gravity is not a decisive factor, and drops under the conditions of weightlessness have the form of a sphere, which is a body with minimum surface for a given volume. Large drops under earth conditions take a spherical form only when the densities of the drop and of the surrounding medium are equal. Falling raindrops, affected by gravity, the pressure of the opposing air current, and surface tension, assume the shape of a bun (broader across than vertically). Drops assume the shape of a flattened sphere on nonwettable surfaces, and they spread out on wettable surfaces.
The form and size of drops breaking away from the end of a capillary tube (pipette) depend on the diameter of the tube, the surface tension o~, and the density of the liquid. This relationship forms the basis for the determination of the surface tension of liquids from the weight of the drops leaving a vertical cylindrical tube (stalagmometer) and from the shape of the drops suspended from the end of a tube or resting on a flat surface.
IU. N. DROZHZHIN