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

Abstract

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.

Long-range transport and directed assembly of charged colloids under aperiodic electrodiffusiophoresis

“…The model solves the transient equation for conservation of ionic species. 31,45,46 Diffusion, migration and convection determine the fluxes, j i , of ionic species. The reaction term, R i , accounts for the equilibrium reaction of water and dissociation of SNARF within the liquid domain.…”

“…11,13,25,36 However, with some exceptions, 33,48 the effect of faradaic processes, and the ensuing gradients of electroactive species, have not been an integral part of the physical description for the forces acting on charged particles under ac electric fields. 7,31,36 An important barrier has been the need to measure concentration and fluxes of ionic species with high spatial and temporal resolution. The transport model reproduces the basic features of the experiments.…”

“…Recent experimental observations highlight the potential of coupling electric fields with induced gradients of electroactive species to control the assembly and transport of charged particles. 31,[37][38][39] In a recent work, we confirmed that electrodiffusiophoresis (EDP)-phoretic motion under diffusiophoresis and electrophoresis-was the mechanism responsible for the focusing and levitation of charged particles under uniform dc fields. 40 With the aid of confocal microscopy and ratiometric analysis, we observed that the passage of current through water induces significant pH gradients.…”

“…First, an increasing number of experiments use frequencies near 1 kHz, or combine steady and oscillating potentials, to control collective dynamics and propulsion. 7,11,13,25,31 By balancing hydrodynamic and dipolar interactions, these experiments produce 2D colloidal materials with rich structural and dynamical behavior. Second, with some exceptions, 32,33 effects associated with faradaic reactions, such as gradients of electrolytes near electrodes caused by the depletion/generation of electroactive species do not feature extensively in the mechanistic description of colloidal electrokinetics.…”

Visualization of Concentration Gradients and Colloidal Dynamics under Electrodiffusiophoresis

“…45 Although existing theories describe EDP for single particles with small or large zeta potentials, [24][25][26]37 possibilities for fundamental contributions exist in the context of dynamic assembly and manipulation of colloids near electrodes. 46 For example, pH gradients generated in the vicinity of planar electrodes led to the patterning of particles. 47 In another key example, Silvera Batista et al observed that when charged particles of low zeta potential were subjected to dc fields in dimethyl sulfoxide (DMSO), particles focused tens of microns away from both electrodes.…”