Flow electrification in non-aqueous colloidal suspensions, studied with video microscopy

We observed flow electrification in concentrated binary suspensions of hydrophobized silica particles in chloroform. In this low polarity solvent, electrical charges on the large-particles surfaces manifest themselves via long-ranged forces. On shearing the suspension for prolonged time, the interactions between large particles were found to change from weakly attractive (due to the small particles) to strongly repulsive (due to acquired Coulomb interactions). A spectacular manifestation of the phenomenon was observed with video microscopy. After an initial aggregation stage, disintegration into mostly primary particles occurred. The flow electrification was further corroborated by control experiments, without flow or with antistatic agent: here the aggregates kept on growing. We also did experiments where an electric field was applied to the suspension. A similarity was found with the original experiment, suggesting that herein the glass wall acquired negative charges. Aggregates also disintegrated in electric fields, but now the result was a segregated fluid: a coexistence of regions enriched in large particles, and regions depleted of them. At rest these regions exist as isolated units, but in shear flow they merge into bands, a behavior which resembles shear banding.