Single and ensemble response of colloidal ellipsoids to a nearby ac electrode

Yan, Jinchun; Rashidi, Aidin; Wirth, Christopher L.

doi:10.1016/j.colsurfa.2020.125384

Cited by 7 publications

(7 citation statements)

References 39 publications

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“…Micron-scale dielectric colloids in dilute aqueous electrolytes (10 –5 to 10 –2 M) are widely known to assemble into planar aggregates near charged electrodes in low-frequency (<1 kHz) oscillatory electric fields. − The formation of planar aggregates is attributed to attractive hydrodynamic drag forces created by electrohydrodynamic (EHD) fluid flow around each particle, which occurs due to perturbation of the otherwise spatially uniform electric field by the particles, creating a rectified electric field directed along the electrode surface that drives the EHD flow. , In basic (NaOH) or acidic (HCl) electrolytes that induce large ζ-potentials on particles (|ζ p | ∼ 100 mV), colloids do not aggregate and in some cases separate. ,− A large survey study found that the aggregation rate of colloids was correlated with colloid ζ-potential and electric field amplitude across 26 unique particle–electrolyte pairs . Recent theoretical and experimental work has shown that colloid ζ-potential, surface conductivity, , and cation/anion diffusivity mismatch , play roles in whether colloids aggregate or separate in a given electrolyte.…”

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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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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“…When the AC field is applied on a Janus ellipsoid with a metal tip (ϕ = 0), the ellipsoid’s longitudinal axis is aligned with the direction of applied field. − As a result, the ellipsoid propels linearly along the Z -direction (Figure a,f). Note that such linear motion along the Z -direction is observed in metallodielectric ellipsoids with Janus tip which is not the case with Janus spheres.…”

Section: Resultsmentioning

confidence: 99%

Fabrication and Electric Field-Driven Active Propulsion of Patchy Microellipsoids

et al. 2021

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Active colloids are a synthetic analogue of biological microorganisms that consume external energy to swim through viscous fluids. Such motion requires breaking the symmetry of the fluid flow in the vicinity of a particle; however, it is challenging to understand how surface and shape anisotropies of the colloid lead to a particular trajectory. Here, we attempt to deconvolute the effects of particle shape and surface anisotropy on the propulsion of model ellipsoids in alternating current (AC) electric fields. We first introduce a simple process for depositing metal patches of various shapes on the surfaces of ellipsoidal particles. We show that the shape of the metal patch is governed by the assembled structure of the ellipsoids on the substrate used for physical vapor deposition. Under high-frequency AC electric field, ellipsoids dispersed in water show linear, circular, and helical trajectories which depend on the shapes of the surface patches. We demonstrate that features of the helical trajectories such as the pitch and diameter can be tuned by varying the degree of patch asymmetry along the two primary axes of the ellipsoids, namely longitudinal and transverse. Our study reveals the role of patch shape on the trajectory of ellipsoidal particles propelled by induced charge electrophoresis. We develop heuristics based on patch asymmetries that can be used to design patchy particles with specified nonlinear trajectories.

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“…If the particle experiences a global field gradient, it will migrate toward or away from the gradient vector depending on its value of the Clausius–Mossotti function (eq ). Motion driven by a global electric and magnetic field gradient is referred to as electro- and magnetophoresis, respectively, and may be considered as a type of passive motion, similar to sedimentation occurring in the gravitational field. , This is in contrast with active motion, which refers to the propulsion of colloids that occurs from the coupling of fluid flows with local field gradients. , Such local gradients often result from asymmetry in the particle shape and/or surface properties. Field-induced phoresis is of interest in many applications ranging from lab-on-chip to the commercially available electronic inks made by e-ink (Figure a) .…”

Section: Self-propulsion Of Active Colloidsmentioning

confidence: 99%

“…This can couple with the shape anisotropy to give rise to a wide array of active particle motions. Polystyrene ellipsoids are readily obtained by stretching spherical microparticles to a specific aspect ratio. , Such anisotropic particles can then be rendered patchy using the same metal vapor deposition techniques used for microspheres but yielding more complex patch shapes. Lee et al reported a way to fabricate and characterize such particles based on the patch asymmetry, measured from the metal coverage of the transverse and longitudinal axis of the ellipsoid .…”

Section: Self-propulsion Of Active Colloidsmentioning

confidence: 99%

Field-Induced Assembly and Propulsion of Colloids

2022

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Electric and magnetic fields have enabled both technological applications and fundamental discoveries in the areas of bottom-up material synthesis, dynamic phase transitions, and biophysics of living matter. Electric and magnetic fields are versatile external sources of energy that power the assembly and self-propulsion of colloidal particles. In this Invited Feature Article, we classify the mechanisms by which external fields impact the structure and dynamics in colloidal dispersions and augment their nonequilibrium behavior. The paper is purposely intended to highlight the similarities between electrically and magnetically actuated phenomena, providing a brief treatment of the origin of the two fields to understand the intrinsic analogies and differences. We survey the progress made in the static and dynamic assembly of colloids and the self-propulsion of active particles. Recent reports of assembly-driven propulsion and propulsion-driven assembly have blurred the conceptual boundaries and suggest an evolution in the research of nonequilibrium colloidal materials. We highlight the emergence of colloids powered by external fields as model systems to understand living matter and provide a perspective on future challenges in the area of field-induced colloidal phenomena.

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Single and ensemble response of colloidal ellipsoids to a nearby ac electrode

Cited by 7 publications

References 39 publications

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

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

Fabrication and Electric Field-Driven Active Propulsion of Patchy Microellipsoids

Field-Induced Assembly and Propulsion of Colloids

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