In plasmas and electrolytes, the Debye length (Debye radius or Debye–Hückel screening length), is a measure of a charge carrier's net electrostatic effect in a solution and how far its electrostatic effect persists. With each Debye length the charges are increasingly electrically screened and the electric potential decreases in magnitude by 1/e. A Debye sphere is a volume whose radius is the Debye length. Debye length is an important parameter in plasma physics, electrolytes, and colloids (DLVO theory). The corresponding Debye screening wave vector for particles of density , charge at a temperature is given by in Gaussian units. Expressions in MKS units will be given below. The analogous quantities at very low temperatures () are known as the Thomas–Fermi length and the Thomas–Fermi wave vector. They are of interest in describing the behaviour of electrons in metals at room temperature.
The Debye length is named after the Dutch-American physicist and chemist Peter Debye (1884-1966), a Nobel laureate in Chemistry.
Physical origin
The Debye length arises naturally in the thermodynamic description of large systems of mobile charges. In a system of different species of charges, the -th species carries charge and has concentration at position . According to the so-called "primitive model", these charges are distributed in a continuous medium that is characterized only by its relative static permittivity, . This distribution of charges within this medium gives rise to an electric potential that satisfies Poisson's equation:
where , is the electric constant, and is a charge density external (logically, not spatially) to the medium.
The mobile charges not only contribute in establishing but also move in response to the associated Coulomb force, . If we further assume the system to be in thermodynamic equilibrium with a heat bath at absolute temperature , then the concentrations of discrete charges, , may be considered to be thermodynamic (ensemble) averages and the associated electric potential to be a thermodynamic mean field. With these assumptions, the concentration of the -th charge species is described by the Boltzmann distribution,
where is the Boltzmann constant and where is the mean
concentration of charges of species .
Identifying the instantaneous concentrations and potential in the Poisson equation with their mean-field counterparts in the Boltzmann distribution yields the Poisson–Boltzmann equation:
Solutions to this nonlinear equation are known for some simple systems. Solutions for more general systems may be obtained in the high-temperature (weak coupling) limit, , by Taylor expanding the exponential:
This approximation yields the linearized Poisson–Boltzmann equation
which also is known as the Debye–Hückel equation: The second term on the right-hand side vanishes for systems that are electrically neutral. The term in parentheses divided by , has the units of an inverse length squared and by dimensional analysis leads to the definition of the characteristic length scale
that commonly is referred to as the Debye–Hückel length. As the only characteristic length scale in the Debye–Hückel equation, sets the scale for variations in the potential and in the concentrations of charged species. All charged species contribute to the Debye–Hückel length in the same way, regardless of the sign of their charges. For an electrically neutral system, the Poisson equation becomes
To illustrate Debye screening, the potential produced by an external point charge is
The bare Coulomb potential is exponentially screened by the medium, over a distance of the Debye length: this is called Debye screening or shielding (Screening effect).
The Debye–Hückel length may be expressed in terms of the Bjerrum length as
where is the integer charge number that relates the charge on the -th ionic
species to the elementary charge .