Definitions

# Drop (liquid)

A drop or droplet is a small volume of liquid, bounded completely or almost completely by free surfaces.

## Surface tension

The simplest way to form a drop is to allow liquid to flow slowly from the lower end of a vertical tube of small diameter. When the pendant drop exceeds a certain size it is no longer stable and detaches itself. Drops may also be formed by the condensation of a supercooled vapor or by atomization of a larger mass of liquid. The mass m (or weight mg) of the largest drop that can hang from the end of a tube of radius a can be found from the formula

$,mg = 2 pi a lambda cos alpha$

where λ is the surface tension of the liquid, α is the angle of contact with the tube, and g is the acceleration due to gravity. This relationship is the basis of a convenient method of measuring surface tension, commonly used in the petroleum industry.

## Droplet

The term droplet is a diminutive form of 'drop' - and typically used for liquid particles of <500 µm diameter (although this is for guidance rather than a 'rule'). In spray application, droplets are usually described by their perceived size (i.e., diameter) whereas the dose (or number of infective particles in the case of biopesticides) is a function of their volume. This increases by a cubic function relative to diameter (π.d3/6000 to convert µm into picolitres); thus a 50 µm droplet represents a dose in 65 pl and a 500 µm drop represents a dose in 65 nanolitres (65,450 pl).

## Optics

Due to the different refractive index of water and air, refraction and reflection occur on the surfaces of raindrops, leading to rainbow formation.

## Sound

The major source of sound when a droplet hits a liquid surface is the resonance of excited bubbles trapped underwater. These oscillating bubbles are responsible for most liquid sounds, such as running water or splashes, as they actually consist of many drop-liquid collisions.

## Shape

The classic shape associated with a drop (with a pointy end in its upper side) is actually an optical effect due to light reflections and the drops rapid movement. The shape of a drop falling through a gas is actually more or less spherical. Larger drops tend to be flatter on the bottom part due to the pressure of the gas they move through.

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