Density gradient is a spatial variation in

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

over an area. The term is used in the

natural sciences Natural science is one of the branches of science concerned with the description, understanding and prediction of natural phenomena, based on empirical evidence from observation and experimentation. Mechanisms such as peer review and repeatab ...

to describe varying density of

matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic partic ...

, but can apply to any

quantity Quantity or amount is a property that can exist as a Counting, multitude or Magnitude (mathematics), magnitude, which illustrate discontinuity (mathematics), discontinuity and continuum (theory), continuity. Quantities can be compared in terms o ...

whose density can be measured.

Aerodynamics

In the study of

supersonic Supersonic speed is the speed of an object that exceeds the speed of sound ( Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...

flight,

Schlieren photography Schlieren photography is a process for photographing fluid flow. Invented by the German physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects. C ...

observes the density gradient of air as it interacts with aircraft. Also in the field of Computational Fluid Dynamics, Density gradient is used to observe the acoustic waves, shock waves or expansion waves in the flow field.

Water

A steep density gradient in a body of water can have the effect of trapping energy and preventing

convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...

, such a gradient is employed in

solar pond A solar pond is a pool of saltwater which collects and stores solar thermal energy. The saltwater naturally forms a vertical salinity gradient also known as a "halocline", in which low-salinity water floats on top of high-salinity water. The ...

s. In the case of salt water, sharp gradients can lead to

stratification Stratification may refer to: Mathematics * Stratification (mathematics), any consistent assignment of numbers to predicate symbols * Data stratification in statistics Earth sciences * Stable and unstable stratification * Stratification, or st ...

of different concentrations of

salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

. This is called a

Halocline In oceanography, a halocline (from Greek ''hals'', ''halos'' 'salt' and ''klinein'' 'to slope') is a cline, a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water. Because salinity (in concert with temp ...

Biology

In the life sciences, a special technique called density gradient separation is used for isolating and purifying cells, viruses and subcellular particles. Variations of this include

Isopycnic centrifugation An isopycnic surface is a surface of constant density inside a fluid. Isopycnic surfaces contrast with isobaric or isothermal surfaces, which describe surfaces of constant pressure and constant temperature respectively. Isopycnic surfaces are s ...

Differential centrifugation In biochemistry and cell biology, differential centrifugation (also known as differential velocity centrifugation) is a common procedure used to separate organelles and other sub-cellular particles based on their sedimentation rate. Although o ...

, and

Sucrose gradient centrifugation In biochemistry and cell biology, differential centrifugation (also known as differential velocity centrifugation) is a common procedure used to separate organelles and other sub-cellular particles based on their sedimentation rate. Although o ...

. A blood donation technique called

Pheresis Apheresis ( ἀφαίρεσις (''aphairesis'', "a taking away")) is a medical technology in which the blood of a person is passed through an apparatus that separates out one particular constituent and returns the remainder to the circulation. ...

involves density gradient separation.

Geophysics

The understanding of what is at the centre of the earth, the

earth core The internal structure of Earth is the solid portion of the Earth, excluding its atmosphere and hydrosphere. The structure consists of an outer silicate solid crust, a highly viscous asthenosphere and solid mantle, a liquid outer core wh ...

, requires the framework of density gradients in which elements and compounds then interact. The existence of a

fast breeder nuclear reactor A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. Breeder reactors achieve this because their neutron economy is high enough to create more fissile fuel than they use, by irradiation of a fertile mate ...

system at the core of the earth is one theory by reason of density gradient. A popular model for the density gradient of Earth is one given by the Preliminary Reference Earth Model (PREM) that is based on the observed oscillations of the planet and the travel times of thousands of

seismic waves A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy. S ...

. Several models for density gradient have been built on the basis of PREM but they acknowledge that the "distribution of density as a function of position within the Earth is much less well constrained than the seismic velocities. The primary information comes from the mass and moment of inertia of the Earth and this information alone requires that there be a concentration of mass towards the centre of the globe. Additional information is to be found in the frequencies of the graver normal modes of the Earth which are sensitive to density through self-gravitation effects induced in deformation".

Urban Economics

In the study of population, the density gradient can refer to the change in density in an urban area from the center to the periphery. {{cite web , title = The Spatial Distribution of Population in 48 World Cities: Implications for Economies in Transition , url = http://www.bus.wisc.edu/realestate/pdf/pdf/Complete%20Spatial%20Distribution%20of%20Population%20in%2050%20World%20Ci.pdf , access-date = 2006-12-09 , archive-url = https://web.archive.org/web/20060905025111/https://www.bus.wisc.edu/realestate/pdf/pdf/Complete%20Spatial%20Distribution%20of%20Population%20in%2050%20World%20Ci.pdf , archive-date = 2006-09-05 , url-status = dead

References

Density Spatial gradient