Ensemble average TIRM for imaging amperometry

Rock, Reza M.; Sides, Paul J.; Prieve, Dennis C.

doi:10.1016/j.jcis.2013.04.010

Cited by 5 publications

(9 citation statements)

References 28 publications

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“…We note that the height measured in this manner represents the "average" height of the particles, averaged over many particles that fluctuate in position due to thermal motion, rather than the "most probable height" one might intuitively inspect based on the minimum in a potential energy profile. 25,26 ■ RESULTS AND DISCUSSION…”

Section: ■ Experimental Methodsmentioning

confidence: 99%

Influence of Electrolyte Concentration on the Aggregation of Colloidal Particles near Electrodes in Oscillatory Fields

2016

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Micron-scale particles suspended in various aqueous electrolytes have been widely observed to aggregate near electrodes in response to oscillatory electric fields, a phenomenon believed to result from electrically induced flows around the particles. Previous work has focused on elucidating the effects of the applied field strength, frequency, and electrolyte type on the aggregation rate of particles, with less attention paid to the ionic strength. Here we demonstrate that an applied field causes micron-scale particles in aqueous NaCl to rapidly aggregate over a wide range of ionic strengths, but with significant differences in aggregation morphology. Optical microscopy observations reveal that at higher ionic strengths (∼1 mM) particles arrange as hexagonally closed-packed (HCP) crystals, but at lower ionic strengths (∼0.05 mM) the particles arrange in randomly closed-packed (RCP) structures. We interpret this behavior in terms of two complementary effects: an increased particle diffusivity at lower ionic strengths due to increased particle height over the electrode and the existence of a deep secondary minimum in the particle pair interaction potential at higher ionic strength that traps particles in close proximity to one another. The results suggest that electrically induced crystallization will readily occur only over a narrow range of ionic strengths.

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Section: ■ Experimental Methodsmentioning

confidence: 99%

Influence of Electrolyte Concentration on the Aggregation of Colloidal Particles near Electrodes in Oscillatory Fields

2016

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“…25 In these experiments, vertical motion of the particle was effected by faradaic current density passing through the electrochemical cell. 25,26 Colloids in electrolytes where the cations and anions have different mobilities, e.g., NaOH, have been shown to separate and levitate tens of microns above the electrode surface in low-frequency oscillatory fields. 15,18,23 Colloid levitation was explained by the recently identified asymmetric rectified electric field (AREF), which is induced by the action of an oscillatory potential on an electrolyte with different cation and anion diffusivities.…”

Section: ■ Introductionmentioning

confidence: 99%

“…Separation and aggregation of colloids in various electrolytes has previously been explained by competition between multiple extensile and contractile electrokinetic flows. , Previous work has shown that steady electric fields induce particle aggregation or separation due to electroosmotic (EO) flow along the particle surface, which creates either extensile or contractile flow around each particle depending on the sign of the steady potential and particle ζ-potential. ,,, Motion of particles normal to the electrode surface also plays an important role in determining whether colloids aggregate or separate in oscillatory electric fields. , The phase angle between the oscillatory electrophoretic motion of colloids and the electric field magnitude correlates with whether particles will aggregate or separate. ,,,, Prior work by the Prieve group has demonstrated changes in particle height upon application of electrochemical potentials that drive water electrolysis . In these experiments, vertical motion of the particle was effected by faradaic current density passing through the electrochemical cell. , Colloids in electrolytes where the cations and anions have different mobilities, e.g ., NaOH, have been shown to separate and levitate tens of microns above the electrode surface in low-frequency oscillatory fields. ,, Colloid levitation was explained by the recently identified asymmetric rectified electric field (AREF), which is induced by the action of an oscillatory potential on an electrolyte with different cation and anion diffusivities . The disparate electrophoretic motion of ions creates free charge density outside the diffuse layer near the electrode, which partially rectifies the oscillatory current to create the AREF .…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2021

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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.

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“…This additional flow along the electrode causes a difference in the height dependence of the force in the two limits. This force was used as a probe for the high throughput evaluation of electrocatalysts [3,4,39].…”

Section: A the Response Of An Isotropic Particle To A Nearby Poweredmentioning

confidence: 99%

“…The directed assembly of micrometer-scale colloidal particles with an external electric field has potential application in a range of industries, from advanced materials to sensing and sorting in chemical and biological environments [1][2][3][4][5][6]. Consequently, work over the past three decades in this area has sought to understand both the fundamental aspects and technological obstacles in the response of colloidal particles to external dc and ac electric fields .…”

Section: Introductionmentioning

confidence: 99%

Response of a doublet to a nearby dc electrode of uniform potential

Wirth

Nuthalapati

2016

Phys. Rev. E

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The electric-field-assisted directed assembly of spherical colloidal particles near an electrode has been studied for nearly three decades. Recently, focus has shifted to the electric-field-assisted assembly and propulsion of nonspherical (i.e., anisotropic) particles. This paper describes calculations and results for a doublet of asymmetric ζ potential and size responding to a dc electric field. The doublet experienced a net vertical force that depended on both the asymmetry in ζ potential and lobe size. In addition, the doublet experienced a net lateral force perpendicular to the applied electric field. The lateral force depended on the difference in ζ potential of the two lobes, the lobe size asymmetry, and also the angle of inclination of the doublet. The net force was used to predict an apparent lateral velocity, which was found to be perpendicular to the applied electric field. In addition, the particle experienced rotation from a net torque that depended on the lobe size asymmetry and also the angle of inclination of the doublet. The magnitude of the predicted velocity was of the same order of magnitude as has been observed for particles responding to ac electric fields in experiments. These results demonstrate that lobe ζ potential, lobe size, and orientation of a colloidal doublet can be tuned to achieve propulsion.

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Ensemble average TIRM for imaging amperometry

Cited by 5 publications

References 28 publications

Influence of Electrolyte Concentration on the Aggregation of Colloidal Particles near Electrodes in Oscillatory Fields

Influence of Electrolyte Concentration on the Aggregation of Colloidal Particles near Electrodes in Oscillatory Fields

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

Response of a doublet to a nearby dc electrode of uniform potential

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