Density gradient is a spatial variation in

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

over a region. The term is used in the

natural sciences Natural science or empirical 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 ...

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 pa ...

, but can apply to any

quantity Quantity or amount is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value multiple of a u ...

whose density can be measured.

Aerodynamics

In the study of supersonic flight,

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

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 flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

, such a gradient is employed in solar ponds. In the case of salt water, sharp gradients can lead to stratification of different concentrations of

salinity Salinity () is the saltiness or amount of salt (chemistry), 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 dimensio ...

. This is called a Halocline.

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, Differential centrifugation, 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 Svedberg, sedimentation rate. A ...

. A blood donation technique called Pheresis involves density gradient separation.

Geophysics

The understanding of what is at the centre of the earth, the earth core, requires the framework of density gradients in which elements and compounds then interact. The existence of a fast breeder nuclear reactor 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 mechanical wave of acoustic wave, acoustic energy that travels through the Earth or another planetary body. It can result from an earthquake (or generally, a quake (natural phenomenon), quake), types of volcanic eruptions ...

. 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