liquid, one of the three commonly recognized states in which matter occurs, i.e., that state, as distinguished from solid and gas, in which a substance has a definite volume but no definite shape.

Properties of Liquids

In general, liquids show expansion on heating, contraction on cooling; water, however, does not follow the rule exactly. A liquid changes at its boiling point to a gas and at its freezing point, or melting point, to a solid. The boiling point is especially important because, since liquids change their states at different temperatures, those in a mixture can be separated from one another by raising the temperature of the mixture gradually so that each component in turn undergoes vaporization at its boiling point. This process is known as fractional distillation.

Liquids, like gases, exhibit the property of diffusion. When two miscible liquids (i.e., they mix without separation) are poured carefully into a container so that the denser one forms a separate layer on the bottom, each will diffuse slowly into the other until they are thoroughly mixed. Liquids, like gases, differ from solids in that they are fluids, that is, they flow into the shape of a containing vessel. Liquids exert pressure on the sides of a containing vessel and on any body immersed in them, and pressure is transmitted through a liquid undiminished and in all directions. Liquids exert a buoyant force on an immersed body equal to the weight of the liquid displaced by the body (see Archimedes' principle and specific gravity). Unlike gases, liquids are very nearly incompressible, and for that reason are useful in such devices as the hydraulic press. Liquids are useful as solvents. No one liquid can dissolve all substances; each takes into solution only certain specific substances.

Molecular Structure of Liquids

The molecules (or atoms or ions) of a liquid, like those of a solid (and unlike those of a gas), are quite close together; however, while molecules in a solid are held in fixed positions by intermolecular forces, molecules in a liquid have too much thermal energy to be bound by these forces and move about freely within the liquid, although they cannot escape the liquid easily. Although the molecules of a liquid have greater cohesion than those of a gas, it is not sufficient to prevent some of those at the free surface of the liquid from bounding off (see evaporation). On the other hand, the cohesive forces between the molecules at the surface of a mass of liquid and those within cause the free surface to act somewhat like a stretched elastic membrane; it tends to draw inward toward the center of the liquid mass, to draw the liquid into the shape of a sphere, thus exhibiting the phenomenon known as surface tension.

A liquid is said to "wet" a solid substance when the attractive force between the molecules of the liquid and those of the solid is great enough to hold the liquid's molecules at the solid surface. For example, water "wets" glass since its molecules cling to glass surfaces, whereas mercury does not since the adhesive force between its molecules and those of glass is not strong enough to hold them together. Capillarity is an example of surface tension and adhesion acting at the same time.

Optoelectronic device used in displays for watches, calculators, notebook computers, and other electronic devices. Current passed through specific portions of the liquid crystal solution causes the crystals to align, blocking the passage of light. Doing so in a controlled and organized manner produces visual images on the display screen. The advantage of LCDs is that they are much lighter and consume less power than other display technologies (e.g., cathode-ray tubes). These characteristics make them an ideal choice for flat-panel displays, as in portable laptop and notebook computers.

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Substance that flows like a liquid but maintains some of the ordered structure characteristic of a crystal. Some organic substances do not melt directly when heated but instead turn from a crystalline solid to a liquid crystalline state. When heated further, a true liquid is formed. Liquid crystals have unique properties. The structures are easily affected by changes in mechanical stress, electromagnetic fields, temperature, and chemical environment. Seealso liquid crystal display.

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One of the three principal states of matter, intermediate between a gas and a solid. A liquid has neither the orderliness of a solid nor the randomness of a gas. Liquids have the ability to flow under the action of very small shear stresses. Liquids in contact with their own vapour or air have a surface tension that causes the interface to assume the configuration of minimum area (i.e., spherical). Surfaces between liquids and solids have interfacial tensions that determine whether the liquid will wet the other material. With the exception of liquid metals, molten salts, and solutions of salts, the electrical conductivities of liquids are small.

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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.

Gallery

See also

• Rain

References

External links

• Liquid Sculpture - pictures of drops
• Liquid Art - Galleries of fine art droplet photography
• Tvw Gallery of Drops - pictures of drops
• (Greatly varying) calculation of water waste from dripping tap: ,