Environmental Engineering Reference
In-Depth Information
Figure 6.12
The character of interaction of
two micrometer-sized particles in an ionized
gas. If the distance between particles exceeds
twice the size (2
field, these particles do not interact (a), and
if the total screening of the particle charges is
absent, that is a distance between particles of
less than 2
l
)ofactionoftheparticle
l
, the particles are repulsed (b).
which are in an electric trap that is created in the sheath of radio-frequency gas
discharge [105]. The interaction potential of two particles may be modeled by a
short-range interaction of particles, but the plasma electric potential between par-
ticles if the distance between them exceeds 2
l
differs from that far from particles.
This means that electron and ion fluxes to particles from distances where the par-
ticle fields are shielded differ from those toward one particle. The reason for this is
such that a large size is required for the region where the equilibrium number den-
sity of electrons and ions is restored. In this way particles influence the potential of
an ionized gas in which they are located.
Thus, one can model interacting particles inserted in an ionized gas as a system
of hard spheres (2.10) with the radius of screening of the particle Coulomb field
l
.
It is of importance that the radius of action of the particle field
l
or the radius of
the hard sphere model, may be varied by variation of the gas discharge current
and other parameters. This allows one to govern what structures can be formed by
these particles. The most beautiful property of a dusty plasma formed by insertion
of micrometer-sized dust particles into a gas discharge plasma is the possibility
to form a crystal structure under some conditions. Variation of gas discharge pa-
rameters may lead to melting of such a crystal. This possibility was proposed by
Ikezi [106] and was realized in 1994 by several experimental groups [20, 30-32]. Ev-
idently, the so-called plasma crystal, that is, the structure formed by dust particles
in a gas discharge plasma, attracted much attention to this problem and resulted
in much research in this area (e.g., [1, 4, 35-39]). Note that millimeter distances
between nearest neighbors of the structures formed by micrometer-sized particles
allow one to use simple visual methods for their observation. For this reason, a
