A drop or droplet is a small volume of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant drop. Drops may also be formed by the condensation of a vapor or by atomization of a larger mass of liquid.

Surface tension

Liquid forms drops because the liquid exhibits surface tension.

A simple way to form a drop is to allow liquid to flow slowly from the lower end of a vertical tube of small diameter. The surface tension of the liquid causes the liquid to hang from the tube, forming a pendant. When the drop exceeds a certain size it is no longer stable and detaches itself. The falling liquid is also a drop held together by surface tension.

Pendant drop test

In the pendant drop test, a drop of liquid is suspended from the end of a tube by surface tension. The force due to surface tension is proportional to the length of the boundary between the liquid and the tube, with the proportionality constant usually denoted $\backslash gamma$. Since the length of this boundary is the circumference of the tube, the force due to surface tension is given by

$\backslash ,F\_\{\backslash gamma\}\; =\; \backslash pi\; d\; \backslash gamma$

where d is the tube diameter.

The mass m of the drop hanging from the end of the tube can be found by equating the force due to gravity ($F\_\{g\}\; =\; mg$) with the component of the surface tension in the vertical direction ($F\_\{\backslash gamma\}\; \backslash sin\; \backslash alpha$) giving the formula

$\backslash ,mg\; =\; \backslash pi\; d\; \backslash gamma\; \backslash sin\; \backslash alpha$

where α is the angle of contact with the tube, and g is the acceleration due to gravity.

The limit of this formula, as α goes to 90°, gives the maximum weight of a pendant drop for a liquid with a given surface tension, $\backslash gamma$.

$\backslash ,mg\; =\; \backslash pi\; d\; \backslash gamma$

This relationship is the basis of a convenient method of measuring surface tension, commonly used in the petroleum industry. More sophisticated methods are available when the surface tension is unknown that consider the developing shape of the pendant as the drop grows.

Droplet

The term droplet is a diminutive form of 'drop' - and as a guide is typically used for liquid particles of <500 µm diameter. 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).