Relative importance of electrostatic forces on powder particles

The phenomenon of electrostatic attraction between charged materials has been recognized since early Greek times. Despite its long history, the relative importance of electrostatic forces (compared with non-electrostatic forces) on powder particles is still a subject of debate among today's scientists. In this article, we attempt to present a systematic evaluation of the possible magnitudes of electrostatic forces and van der Waals forces, based on theoretical analysis and existing experimental data. For small particles (about 10 μm as used in electrophotographic toner), both van der Waals and electrostatic forces may play a significant role. Without setting up appropriate constraints (or conditions) for comparison, measurements of powder samples can show either dominant van der Waals forces or dominant electrostatic forces and therefore lead to controversial conclusions. Our mathematical evaluation indicates that the electrostatic adhesion on a charged particle under the condition that it can be detached by an applied electric field is likely to be greater than van der Waals forces. However, for particles of the same surface structures, the relative significance of van der Waals forces is expected to increase as the particle size is reduced. But many electrophotographic processes require that the net electrostatic force is sufficiently greater than the magnitude of van der Waals forces. Hence, it becomes a practical trend to use more surface additives on smaller toner particles desired in high quality electrophotographic printing, to effectively reduce van der Waals forces so as to ensure that the electrostatic forces become relatively dominant.