A
drop or
droplet is a small volume of
liquid, bounded completely or almost completely by free surfaces.
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
where ? is the surface tension of the liquid, a is the angle of contact with the tube, and g is the acceleration due to gravity.[citation needed] This relationship is the basis of a convenient method of measuring surface tension, commonly used in the petroleum industry.
The term droplet is a diminutive form of 'drop' - and typically used for liquid particles of <500µm 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 (p.d3/6000 to convert µm into picolitres); thus a 50µm droplet represents a dose in 65 pl and a 500 µm drop represents a dose in 65 nanolitres (65,450 pl).
