Drop (liquid)
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A drop 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 weight of the largest drop that can hang from the end of a tube of radius a is nearly
[mg = 3 \pi a \lambda \cos \alpha]
where λ is the surface tension of the liquid and α is the angle of contact with the tube. This relationship is the basis of a convenient method of measuring surface tension.
Due to the different refractive index of water and air, refraction and reflection occur on the surfaces of raindrops, leading to rainbow formation.
As a measurement unit, a drop was another name for a minim. The unit was used in Pharmacy to describe a volume equal to one 60th of a fluid dram or one 480th of a fluid ounce. This is equal to about 0.0616mL (U.S.) or 0.0592mL (Britain). Pharmacists have since moved to metric measurements, with a drop being rounded to exactly 0.05mL (that is, 20 drops per millilitre). In hospitals, intravenous tubing is used to deliver medication in drops of various sizes ranging from 10 drops/mL to 60 drops/mL. A drop is abbreviated gt, with gtt used for the plural. These abbreviations come from the Latin for drop, gutta.
A drop can also be used less formally as a unit of volume in recipes. According to some older kitchen references, 24 drops = 1/4 teaspoon. Using U.S. definitions, this makes the drop equal to about 0.051mL, making it quite comparable to the pharmacist's drop.
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