(126b) Electrification and Dispersion of Particles Using Mesh Electrode

Authors: 

Electrostatic forces cause spontaneous
movement in every charged particle. Thus, electrostatic technologies are attracting
attention in the actual processes of powder handling, i.e., separation, classification,
and collection. When conductive particles come in contact with any one of the inside
walls of parallel electrodes, the particles are charged to the same polarity as
that of the electrode by induction. If Coulomb forces acting on the charged
particles are larger than the adhesion forces between the contact surfaces, the
particles will levitate from the electrode. After the particles move and reach the
counter electrode, the polarity of the particles inverts, and then the particles
levitate again. As a result, the particles oscillate between the parallel electrodes.
Replacing the counter electrode with a mesh electrode allows the passage of the
charged particles through the mesh openings; then, the particles with the same
polarity as the first electrode are dispersed by mutual electrostatic repulsion.
However, since the particles are polarized in the external electric field, chain
agglomerates can be formed; thereby the particle behavior becomes more
complicated. In powder handling processes, dielectric particles are often used.
Such particles are accumulated and form multi-layers on the electrodes; this affects
the phenomena of particle charging and levitation. The phenomena for single
dielectric particles have been reported, but there are few studies focusing on
the induction charging and levitation for particle layers.

We present the charging, levitation,
and relevant behaviors for dielectric particle layers, which are obtained using
parallel electrodes consisting of a lower plate electrode and an upper mesh
electrode. First, multi-layers of particles are formed on the lower electrode. Next,
DC voltage is applied to the lower electrode and the upper electrode is grounded
to vertically form a high-intensity electric field. Then, the particles are charged
and levitated in the electric field. The particle motion around the lower
electrode is observed via a high-speed microscope camera, showing that
agglomerated as well as single particles levitate from the surface of the
particle layers, i.e., the particles are polarized in the electric field and
form chain agglomerates by mutual electrostatic interactions. When the specific
charge, i.e., charge-to-mass ratio of particles is small, single particles cannot
levitate because of small Coulomb forces. However, chain agglomerates can
levitate because of large Coulomb forces even though the specific charge is
small. The Coulomb forces of the chain agglomerates can exceed the adhesion
forces and gravitational forces acting on primary particles in the agglomerates.

The specific charge of each
levitating particle is estimated by fitting experimentally obtained particle trajectories
with calculated ones in simulated electric fields, showing that as the number
of primary particles of the chain agglomerate increases, their specific charge
decreases. In addition, it is found that as the adhesion force of each primary
particle increases, the number of primary particles of the chain agglomerate
increases.

Furthermore, a unique phenomenon is
observed around the upper electrode. Fig. 1 shows a chain agglomerate that was
dispersed. Agglomerates can be rotated in the dispersion process. The charges
of the agglomerates during the dispersion process are estimated using a simulated
non-uniform electric field around the upper electrode and the effect of the
particle charge on the dispersion process is studied.

Fig. 1. Dispersion of chain
agglomerate by electrostatic repulsion around upper mesh electrode.