pH-Mediated Aggregation-to-Separation Transition for Colloids Near Electrodes in Oscillatory Electric Fields

Colloids in low-frequency (<1 kHz) oscillatory electric fields near planar electrodes aggregate in neutral pH electrolytes due to electrohydrodynamic (EHD) flow but separate in alkaline pH electrolytes. Colloid ζ-potential and electrolyte ion mobilities are thought to play roles in the underlying mechanism for this phenomenon, but a unifying theory for why particles aggregate in some electrolytes and separate in others remains to be established. Here, we show that increasing local pH near the electrode with an electrochemical reaction causes a colloidal aggregation-to-separation transition in oscillatory electric fields that induce strong attractive EHD flows. An electroactive molecule, para-benzoquinone, was electrochemically reduced at the electrode to locally increase the solution pH near the colloids. Superimposing a sufficiently large steady electrochemical potential onto an oscillatory potential caused a reversible aggregation-to-separation transition. Counterintuitively, decreasing frequency, which increases attractive EHD drag forces, caused a similar aggregation-to-separation transition. Even more interesting, multiple transitions were observed while varying the oscillatory potential. Taken together, these results suggested that the oscillatory potential induced a repulsive hydrodynamic drag force. Scaling arguments for the recently discovered asymmetric rectified electric field (AREF) showed that a repulsive AREF-induced electroosmotic (EO) flow competed with attractive EHD flow. A pairwise colloidal force balance including these competing flows exhibited flow inversions qualitatively consistent with experimentally observed aggregation-to-separation transitions. Broadly, these results emphasize the importance of AREF-induced EO flows in colloid aggregation and separation in low-frequency oscillatory electric fields.