- density gradient
A column of liquid in which the density varies continually with position, usually as a consequence of variation of concentration of a solute. Such gradients may be established by progressive mixing of solutions of different density (as for example, sucrose gradients) or by centrifuge-induced redistribution of solute (as for caesium chloridegradients). Density gradients are widely used for centrifugal and gravity-induced separations of cells, organelles and macromolecules. The separations may exploit density differences between particles, or primarily differences in size, in which latter case the function of the gradient is chiefly to stabilize the liquid column against mixing.
Dictionary of molecular biology. 2004.