Charge transfer within electrostatic powder suspensions

Electrostatic powder suspension, also known as 'electrosuspension' is an area of study currently gaining some attention due to various industrial application prospects. Numerous different powders may be electrically levitated by the simple application of an intense electrostatic field in the order of >0.2 kV/mm applied across a powder bed, where one electrode is embedded in the powder, while the other is some distance above the bed. When the experiment is performed inside an electrically insulating vessel, the application of an appropriate DC-potential results in a dust cloud that is levitated above the bed and fills the void of the vessel. The present paper deals with some aspects of the electrosuspended particulate cloud, specifically with the mode of charge transfer between the electrodes. It was shown, that the small amount of current present whenever the electrosuspended cloud is formed, can be ascribed to three distinct charge transfer mechanisms, namely: (i) the flux of charged particles; (ii) to ion-drift between electrodes; and (iii) charge transfer by charge hopping between the levitated species. Experimental evidence is presented for these modes of charge transfer, including indirect evidence for the existence of charge-hopping within highly concentrated electrosuspensions. Additionally, a mathematical argument is presented as evidence of the logical necessity for this latter type of mechanism to take place.