Drop (liquid)

A drop refers generally to a form - on the other one, mostly small, fluid body. The liquid body has the ideal case (sleep, homogeneous fluid and the external medium ) a spherical shape. Only in the detachment of the drop from the larger body of liquid, ie, the droplet generation, short forms, the form of drops as unstable state.

Drop shape

In common parlance, teardrop shape called a spatial shape that is spherical on one side and tapers to a point on the other side. Contrary to the general assumption, however, has a drop of water just before the detachment of a body almost a " drop form ".

The teardrop shape is often a symbol of tears, blood and water drops and the like. In heraldry, the drop shape is also used as a symbol - see the article tears ( Heraldry ).

Physical Properties

A drop of a liquid body, which is separated by a phase boundary of the area and the shape of substantially determined by the interfacial tension. Due to the relatively low resulting from the surface tension forces drops are therefore the free fall, and other forms of gravity or under the conditions of terrestrial gravity on a large ratio of volume to surface area, and thus is limited to a small size, typically in the millimeter range. The environment can vacuum or gas, liquid or in a subspace also be a solid, as far as this is only partially wetted by drops. Within surrounding liquid, droplets only when the two liquids are immiscible, there may also be an emulsion. A space surrounded by a single phase drop is spherical at rest to the environment due to the surface tension, since the surface is reduced to reduce the surface energy. A lying between two fluid phases drop is limited for the same reason by two spherical caps. Disorders such as the detachment of a droplet of a larger body of liquid lead to oscillations about the equilibrium shape, such as between a abgeplatten and an elongated deviation from the spherical shape, the decrease of internal friction, however, quickly.

A moving through a surrounding medium droplet is flattened at the in movement direction side by the flow resistance. With increasing speed, the flattening is the indentation, so that a kidney-shaped cross section is formed. With further increase in speed, the front interface approaches on the rear, so that an umbrella -shaped structure is formed with a thickened hem, which is very fast instability: The screen is torn and the torus Hemming divides into several smaller drops.

Drop formation

If a drop begin to come off of a body of liquid, produced a constriction. But instead of simply taper so that a " teardrop shape " would arise, she retreats to the length. The result is a thread-like structure on the end of an almost spherical drop hangs. Where the " thread " runs into the drop, again forms a constriction. If the viscosity of the liquid is high enough ( higher than that of water), also this constriction pulls back into the length. The higher the viscosity, the more frequently repeats this process. Eventually it becomes unstable and the drop detaches from the filament. From the thread partially other, smaller droplets form.

Also from a jet of water, droplets. A jet of water runs from falling in the length and there are constrictions and bulges, which then contract to individual drops.

The detachment of a droplet can be observed without further technical aids well to a lava lamp ( liquid / liquid phase boundary ).

Raindrop

Rain is a form of precipitation, ie condensed water vapor. Even raindrops have no " drop form ". When a droplet size up to 0.5 mm, they are spherical. Normal raindrops 2-3 mm diameter and a weight of about 0.005 g hemispherical top and bottom indented by the air resistance. As an intermediate stage can be found drops that are flattened at the bottom. Large drops of rain storm ( max. 9 mm) are unstable and tear due to air resistance. When a droplet radius of 0.05 to 0.25 mm is called drizzle.

The droplet size within the precipitate is randomly distributed, with different rain intensities can be assigned a respective maximum.

Pressure

The drop internal pressure p is dependent on the surface tension (or more generally interfacial tension ) of the liquid (in fact the liquid / gas interface ) and the radius r, and the air pressure. In fact, p is the difference between the curvature of the capillary pressure and the static pressure acting from the outside. The capillary pressure curve is given by

So Small droplets have a high internal pressure. Is not spherical drop, you have the two mutually orthogonal and extremal radii and the surface element on which works, consider and receives

The drop as a unit

A drop that breaks away from a cannula or pipette is a unit of measurement inaccurate but widespread display for small amounts of liquid, for example, drugs or spices, because there are no other instruments for measurement are needed. The actual size of outgoing and incoming droplet depends (see above) strongly on the interfacial tension between the cannula and fluid ( eg reduced by surfactants ), the cohesion of the liquid (eg, gelling agent ) and the shape of the aperture during a dosage and adhesion of the droplet on the material of the tip of the metering device. For aqueous solutions is often about 15 to 20 drops are indicated as corresponding to one milliliter of a raindrop can contain up to one milliliter. In pharmacology, the gtt metric is defined with 50 microliters, so-called dropper for dosing of drugs found as 1 ml per 20 drops. As a historical unit weight of the pharmacist as a drop equivalent to approximately 0.05 grams.