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Journal ArticleDOI

AC electrokinetics of conducting microparticles: A review*

TL;DR: In this article, the authors reviewed both theory and experimental observation of the AC electrokinetic properties of conducting microparticles suspended in an aqueous electrolyte and highlighted the importance of the RC time for charging the double layer.
Abstract: This paper reviews both theory and experimental observation of the AC electrokinetic properties of conducting microparticles suspended in an aqueous electrolyte. Applied AC electric fields interact with the induced charge in the electrical double layer at the metal particle–electrolyte interface. In general, particle motion is governed by both the electric field interacting with the induced dipole on the particle and also the induced-charge electro-osmotic (ICEO) flow around the particle. The importance of the RC time for charging the double layer is highlighted. Experimental measurements of the AC electrokinetic behaviour of conducting particles (dielectrophoresis, electro-rotation and electro-orientation) are compared with theory, providing a comprehensive review of the relative importance of particle motion due to forces on the induced dipole compared with motion arising from induced-charge electro-osmotic flow. In addition, the electric-field driven assembly of conducting particles is reviewed in relation to their AC electrokinetic properties and behaviour.

Summary (4 min read)

1. Introduction

  • The use of AC electric fields for manipulating and characterising small particles in suspension is well-known [1, 2] and many applications have appeared over the last decades in fields such as colloidal science and biotechnology.
  • The research community is also exploring the use of nanowires and nanotubes as biosensors [7], as building blocks for novel nanocircuits [8], and/or elements of dense arrays for light absorption in solar cells [9].
  • Reversible assembly of patterns of metal nanowires has also been demonstrated using AC fields [14].
  • Importantly, the authors show that the charging of the electrical double layer (EDL) is the interfacial polarization mechanism responsible for particle behaviour.
  • 2 AC C EP TE D M AN U SC R IP T ACCEPTED MANUSCRIPT Furthermore, the inclusion of the EDL charging implies that the motion of the particles is not only generated by the electrical forces on the induced dipole (or multipoles), but electroosmotic slip velocities on the particle surface play an important role.

1.1. Polarization of a conducting particle

  • When an uncharged conducting particle is suddenly exposed to an electric field, electrons in the conductor move almost instantaneously so as to make the electric field inside equal to zero; electric field lines intersect the particle surface perpendicularly (see fig. 1a).
  • In the following analysis, the authors assume that the induced voltage across the double layer ∆φ ∼ Eℓ (where E is applied electric field and ℓ is typical particle size) is below the threshold voltage for these Faradaic reactions Vredox so that no direct current passes through the EDL, i.e. the electrolyte/conductor interface is perfectly polarizable.
  • The typical time for charging the electrolyte/metal interface is tRC = εℓ/λDσ [22, 23], where ε and σ are, respectively, the electrical permittivity and conductivity of the electrolyte, and λD is the Debye length.
  • When an AC field is applied, then for frequencies of applied signal much higher than the reciprocal of the charging time (f ≫ fRC ≡ 2π/tRC), the induced EDL charge is negligible, field lines intersect the particle surface perpendicularly, and the situation is equivalent to a conductor in a dielectric (see fig. 1a).
  • In other words, the particle has a low Dukhin number [24], where the Dukhin number is given by σs/σℓ, with σs the particle surface conductivity.

1.2. Induced motion of a conducting particle

  • The electrically induced motion of an uncharged conducting particle in an electrolyte arises from the interaction of the electric field with the induced dipole of the particle plus its double layer, together with the viscous stresses of the induced electrokinetic flow around the particle, a combination of effects called dipolophoresis by Shilov and co-workers [25, 26].
  • Early observations of ICEO flow around a metal sphere were reported by Gamayunov et al [32].
  • The experiments are quantitatively compared with the theoretical predictions of the previous section.
  • Finally, the authors include a section on the assembly of particles on the surface of microelectrodes or between them, and self-assembly induced by electric fields.

2. Mathematical formulation for small voltage and thin double layer

  • The authors assume that the typical induced particle velocity is small enough so that convection of charge is negligible.
  • Here the authors have assumed that the signal frequency is much smaller than the charge relaxation frequency ω ≪ σ/ε in order that the diffuse layer can be considered to be in quasi-equilibrium [51].
  • Experimental observations show that the slip velocity is usually smaller than the ideal case (Λ < 1) and sometimes very much smaller [53].
  • The translational and rotational velocities are determined by imposing equilibrium of forces and torques.
  • The authors mainly deal with particles with sufficient symmetry (orthotropic bodies) so that the coupling tensor is zero, C = 0.

3. Induced translational motion

  • In electrophoresis charged particles move in a fluid due to the action of a spatially uniform electric field [63].
  • In general, using AC electric fields, particles 8 AC C EP TE D M AN U SC R IP T ACCEPTED MANUSCRIPT undergo translational motion if the electric field is inhomogeneous.
  • If the electric field is homogeneous, particles can move by ICEP if they are asymmetric, but only using low frequency AC fields.
  • For high-frequency fields (ω ≫ ωRC) the translational motion is only due to the force on the dipole, (p · ∇)E, and this force is zero in homogeneous fields.

3.1. Uniform field

  • At low frequencies, including DC, the induced flow around an asymmetric particle can produce particle motion [42].
  • Breaking the geometric symmetry of the ICEO flow around a conducting object generally leads to net pumping of fluid past the object, which for a suspended microparticle leads to translation.
  • This idea was experimentally demonstrated using Janus spherical particles [45].
  • The particle then moves perpendicularly to the electric field lines, advancing in the direction of the non-conducting dielectric end.
  • For saline (NaCl) concentrations greater than approximately 10 mM, no particle motion was observed.

3.2. Non-uniform field

  • The authors begin by analysing the case of a conducting sphere subjected to a nonuniform electric field.
  • The Au-coated microparticles were slightly heavier than water and sedimented so that the observed motion was for particles confined to the bottom wall of the sample cell.
  • For a polarizable rod, the induced dipole at low frequencies is very small since the electric field lines surround the rod almost without deviation (see fig.

4. Induced rotational motion

  • In this section the authors describe results where rotation of a conducting particle occurs upon application of an AC field.
  • Alignment of a non-spherical particle in an AC field with fixed direction, called electroorientation; and the continuous rotation of a particle subjected to a rotating electric field, called electro-rotation, also known as Two cases are distinguished.
  • In both cases, the particle angular velocity is usually much smaller than the frequency of the AC field.
  • Electro-orientation (electro-rotation) results from the interaction of the applied field with the inphase (out-of-phase) induced charge, which means that the frequency dependence of EOr (ROT) mirrors the real part of the particle polarizability.
  • In both cases, the authors analyse rotation due to the electrical torque on the induced dipole and rotation due to the ICEO flow at the metal-electrolyte interface.

4.1. Unidirectional field

  • When exposed to an applied field, slender metal particles (e.g. rods) tend to align with the applied electric field for all frequencies.
  • The expression for the ICEP rotational velocity for slender particles at low frequencies is ΩICEP = (ε/2η)ΛE 2 0 cos θ sin θ.
  • The ICEP orientation of a nanowire can be qualitatively understood according to the schematic diagrams in Figure 4a which shows a metal nanowire subjected to an electric field of arbitrary direction [20].
  • This is particularly useful when the goal is to compare experiments with electrokinetic theories, which are mainly developed for small voltage drops across the EDL (∆φ < kBT/e ≈ 25 mV).
  • It was shown that an assembly of three Janus particles will rotate as a consequence of the individual ICEP motion of each Janus particle.

4.2. Rotating field

  • It was observed that for spheres with rough surface the capacitance was greater than that predicted by the Debye-Huckel theory ε/λD [65], but for Ti spheres with smooth surfaces the capacitance was approximately close (CDL ∼ ε/λD) [79].
  • According to theory [20, 71], when subjected to a rotating field, slender conducting particles rotate due to ICEO flow around the particle and to the torque on the induced dipole.

5. Electric-field induced assembly of metal particles

  • In previous sections the authors focused on the electrical manipulation of a single metal particle immersed in a electrolyte.
  • Several publications also describe the behaviour of a collection of metal particles subjected to AC fields.
  • Both electrical and hydrodynamic particle-particle interactions occur upon application of an electric field.
  • These interactions usually lead to reconfigurable particle patterns with potential applications in microelectronics and sensing.
  • In the following section the authors review experiments on the assembly of metal particles.

5.1. Metal particle assembly on microelectrode structures

  • Electric fields in microdevices are usually generated using planar microelectrode structures fabricated on glass or other insulating substrates.
  • Several publications describe experiments where metal nanocolloids dispersed in an electrolyte move towards electrodes and thereby accumulate.
  • This motion of metal particles could be due to positive DEP at high frequencies, as described in section 3.2, and/or by fluid flow induced on the electrodes at low frequencies, such as AC electroosmosis [22].
  • Since then, several works have extensively reported the use of metal nanoparticles for bridging electrodes [81–88].
  • AC electric fields are used to bring and place a nanowire between microelectrodes [10, 89], see figure 5c.

5.2. Chaining and self assembly of metal particles

  • As previously mentioned, particle-particle interactions occur upon application of an electric field to an aqueous dispersion of metal particles.
  • As schematically depicted in figure 6a, if the particles are oriented with their line of centers parallel to the applied field, the interaction between induced dipoles is attractive irrespective of the signal frequency.
  • For low frequencies, assembly into a different pattern was ob- 19 AC C EP TE D M AN U SC R IP T ACCEPTED MANUSCRIPT served (see fig. 6c).
  • As observed in ref. [74], when two nanowires are close they first align end-to-end, then slide next to one another, and finally move apart from their centers.
  • The formation of bands takes longer for increasing conductivity of the electrolyte [14].

6. Conclusions and outlook

  • Electrical forces on conducting particles can lead to linear motion, rotation and orientation, which can be used for particle alignment, separation and assembly.
  • Particle dynamics can be explained from the action of the applied field on the charges induced within the EDL.
  • This influence could not be observed in experiments, which showed that ROT of nanowires could be explained by the torque on the induced dipole.
  • ICEP velocities in experiments are often one order of magnitude smaller than predicted by the simplest theory.
  • Exploiting electric and hydrodynamic particle-particle interactions could be used for programmable assembly or synthesis of large nano-scale structures.

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 
AC Electrokinetics of conducting microparticles: A review
Antonio Ramos, Pablo García -S´anchez, Hywel Morgan
PII:
S1359-0294(16)30076-0
DOI: doi: 10.1016/j.cocis.2016.06.018
Reference: COCIS 1056
To appear in: Current Opinion in Colloid & Interface Science
Received date: 6 May 2016
Revised date: 27 June 2016
Accepted date: 29 June 2016
Please cite this article as: Ramos Antonio, a-S´anchez Pablo Garc, Morgan Hywel, AC
Electrokinetics of conducting microparticles: A review, Current Opinion in Colloid & In-
terface Science (2016), doi: 10.1016/j.cocis.2016.06.018
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
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apply to the journal pertain.

ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
AC Electrokinetics of conducting microparticles: a
review
1
Antonio Ramos, Pablo Garc´ıa-S´anchez
University of Seville, Spain
Hywel Morgan
University of Southampton, UK
Abstract
This paper reviews both theory and experimental observation of the AC elec-
trokinetic properties of conducting microparticles suspended in an aqueous elec-
trolyte. Applied AC electric fields interact with the induced charge in the elec-
trical double layer at the metal particle-electrolyte interface. In general, particle
motion is governed by both the electric field interacting with the induced dipole
on the particle and also the induced-charge electro-osmotic (ICEO) flow around
the particle. The importance of the RC time for charging the double layer is
highlighted. Experimental measurements of the AC electrokinetic behaviour of
conducting particles (dielectrophoresis, electro-rotation and electro-orientation)
are compared with theory, providing a comprehensive review of the relative
importance of particle motion due to forces on the induced dipole compared
with motion arising from induced-charge electro-osmotic flow. In addition, the
electric-field driven assembly of conducting particles is reviewed in relation to
their AC electrokinetic properties and behaviour.
Keywords: AC electrokinetics, induced charge electrophoresis,
dielectrophoresis, metallic
microparticles
1
Dedicated to the memory of Prof. Antonio Castellanos (March 1947-Jan 2016), who in-
spired so many of us.
Preprint submitted to Journal of L
A
T
E
X Templates July 8, 2016

ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
1. Introduction
The use of AC electric fields for manipulating and characterising small par-
ticles in suspension is well-known [1, 2] and many applications have appeared
over the last decades in fields such as colloidal science and biotechnology. More
recently, the manipulation of metal and semiconducting particles has been ex-
plored in the context of microfluidics and nanoelectronics. The use of AC elec-
trokinetic methods for the rapid and precise control of particle assembly has been
used for bottom-up fabrication of engineered microstructures. One example in-
cludes the formation of microwires between electrodes by assembling metallic
nanoparticles from suspension [3, 4]. The chaining of metallodielectric Janus
spheres has also been demonstrated [5, 6]. The research community is also ex-
ploring the use of nanowires and nanotubes as biosensors [7], as building blocks
for novel nanocircuits [8], and/or elements of dense arrays for light absorption
in solar cells [9]. Experimental work has proven the ability of AC electric fields
to position and rotate nanowires [10–12]. Krupke et al [13] demonstrated the
separation of semiconducting from metal carbon nanotubes. Reversible assem-
bly of patterns of metal nanowires has also been demonstrated using AC fields
[14].
In this review article, we focus on the effects of AC electric fields on con-
ducting microparticles suspended in aqueous electrolytes. We summarise the
more relevant experimental papers and clarify the underlying mechanisms that
give rise to the observed motion of conducting microparticles. Importantly,
we show that the charging of the electrical double layer (EDL) is the inter-
facial polarization mechanism responsible for particle behaviour. This is in
contrast with early publications where conducting particles in electrolytes have
often been incorrectly characterized by a finite conductivity and permittivity,
ignoring the double-layer polarization at the metal-electrolyte interface [15–18].
This simplistic model predicts positive polarizabilities for all frequencies in the
quasi-electrostatic regime and cannot explain the experimental observations of
electrorotation and electroorientation spectra of metallic particles [11, 19, 20].
2

ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
Furthermore, the inclusion of the EDL charging implies that the motion of
the particles is not only generated by the electrical forces on the induced dipole
(or multipoles), but electroosmotic slip velocities on the particle surface play an
important role. The AC electrokinetic behavior of conducting particles turns to
be a very interesting problem from a fundamental point of view. We will show
situations where the particle velocity is measured for a known EDL potential.
This implies that the induced zeta potential can be determined from the velocity
measurement, while the voltage drop across the double layer is known, allowing
for a direct comparison with standard electrokinetic theory. The paper begins
with a qualitative analysis of the electrical response of a conducting particle in
an electrolyte followed by a description of the induced motion on this particle.
1.1. Polarization of a conducting particle
When an uncharged conducting particle is suddenly exposed to an electric
field, electrons in the conductor move almost instantaneously so as to make
the electric field inside equal to zero; electric field lines intersect the particle
surface perpendicularly (see fig. 1a). Since the particle is suspended in an
electrolyte, ions in solution are driven by the field and accumulate at the elec-
trolyte/particle interface, inducing an EDL. Diffusion opposes this counter-ion
attraction to the interface, and the characteristic charge cloud at which the two
forces balance defines the diffuse layer. If the electric field is high enough, elec-
trons can jump from the conducting particle to the molecules in the liquid or
vice versa leading to redox reactions. In the following analysis, we assume that
the induced voltage across the double layer φ E (where E is applied elec-
tric field and is typical particle size) is below the threshold voltage for these
Faradaic reactions V
redox
so that no direct current passes through the EDL, i.e.
the electrolyte/conductor interface is perfectly polarizable. When steady state
is reached, the EDL induced charge is maximum, there are no normal currents
at the interface and the field lines surround the particle (see fig. 1b). Typical
values for a threshold voltage for Faradaic reactions V
redox
can be around 1
3

ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
volt. For example, mercury in contact with a deaerated KCl solution behaves
as a perfectly polarizable interface over a potential range of about 2 volts [21].
The typical time for charging the electrolyte/metal interface is t
RC
= εℓ/λ
D
σ
[22, 23], where ε and σ are, respectively, the electrical permittivity and conduc-
tivity of the electrolyte, and λ
D
is the Debye length. This time can be viewed
as the RC time for charging the EDL capacitance through the resistance of the
bulk electrolyte t
RC
= C
DL
ℓ/σ with C
DL
= ε/λ
D
the EDL capacitance (per
unit area) using the Debye-Huckel approximation. When an AC field is applied,
then for frequencies of applied signal much higher than the reciprocal of the
charging time (f f
RC
2π/t
RC
), the induced EDL charge is negligible, field
lines intersect the particle surface perpendicularly, and the situation is equiva-
lent to a conductor in a dielectric (see fig. 1a). For applied frequencies much
lower than 2π/t
RC
, there is enough time for the EDL to fully charge and the
electric field lines surround the particle. From the perspective of an observer,
the situation is equivalent to that of an insulating particle (see fig. 1b).
In figure 1 we have assumed that particle surface conduction is negligible
compared to electrolyte bulk conduction. In other words, the particle has a
low Dukhin number [24], where the Dukhin number is given by σ
s
, with σ
s
the particle surface conductivity. This number can be high for metal nanocol-
loids ( 1 µm), which are usually highly charged in order to form stable
suspensions. In this case, the native double layer may contribute to surface
conductance implying a high Dukhin number, which can lead to concentration
polarization phenomena [24]. In this paper, we will only deal with small Dukhin
numbers and/or negligible intrinsic charge.
1.2. Induced motion of a conducting particle
The electrically induced motion of an uncharged conducting particle in an
electrolyte arises from the interaction of the electric field with the induced dipole
of the particle plus its double layer, together with the viscous stresses of the in-
duced electrokinetic flow around the particle, a combination of effects called
dipolophoresis by Shilov and co-workers [25, 26]. Typical particle motions aris-
4

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Abstract: Building on our recent work on induced-charge electro-osmosis (ICEO) and electrophoresis (ICEP), as well as the Russian literature on spherical metal colloids, we examine the rich consequences of broken geometric and field symmetries upon the ICEO flow around conducting bodies. Through a variety of paradigmatic examples involving ideally polarizable (e.g. metal) bodies with thin double layers in weak fields, we demonstrate that spatial asymmetry generally leads to a net pumping of fluid past the body by ICEO, or, in the case of a freely suspended colloidal particle, translation and/or rotation by ICEP. We have chosen model systems that are simple enough to admit analysis, yet which contain the most important broken symmetries. Specifically, we consider (i) symmetrically shaped bodies with inhomogeneous surface properties, (ii) ‘nearly symmetric’ shapes (using a boundary perturbation scheme), (iii) highly asymmetric bodies composed of two symmetric bodies tethered together, (iv) symmetric conductors in electric-field gradients, and (v) arbitrarily shaped conductors in general non-uniform fields in two dimensions (using complex analysis). In non-uniform fields, ICEO flow and ICEP motion exist in addition to the more familiar dielectrophoretic forces and torques on the bodies (which also vary with the square of the electric field). We treat all of these problems in two and three dimensions, so our study has relevence for both colloids and microfluidics. In the colloidal context, we describe principles to ‘design’ polarizable particles which rotate to orient themselves and translate steadily in a desired direction in a DC or AC electric field. We also describe ‘ICEO spinners’ that rotate continuously in AC fields of arbitrary direction, although we show that ‘near spheres’ with small helical perturbations do not rotate, to leading order in the shape perturbation. In the microfluidic context, strong and steady flows can be driven by small AC potentials applied to systems containing asymmetric structures, which holds promise for portable or implantable self-powered devices. These results build upon and generalize recent studies in AC electro-osmosis (ACEO). Unlike ACEO, however, the inducing surfaces in ICEO can be physically distinct from the driving electrodes, increasing the frequency range and geometries available.

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Journal ArticleDOI
TL;DR: This work introduces a version of the dye-sensitized cell in which the traditional nanoparticle film is replaced by a dense array of oriented, crystalline ZnO nanowires, which features a surface area up to one-fifth as large as a nanoparticle cell.
Abstract: Excitonic solar cells1—including organic, hybrid organic–inorganic and dye-sensitized cells (DSCs)—are promising devices for inexpensive, large-scale solar energy conversion. The DSC is currently the most efficient2 and stable3 excitonic photocell. Central to this device is a thick nanoparticle film that provides a large surface area for the adsorption of light-harvesting molecules. However, nanoparticle DSCs rely on trap-limited diffusion for electron transport, a slow mechanism that can limit device efficiency, especially at longer wavelengths. Here we introduce a version of the dye-sensitized cell in which the traditional nanoparticle film is replaced by a dense array of oriented, crystalline ZnO nanowires. The nanowire anode is synthesized by mild aqueous chemistry and features a surface area up to one-fifth as large as a nanoparticle cell. The direct electrical pathways provided by the nanowires ensure the rapid collection of carriers generated throughout the device, and a full Sun efficiency of 1.5% is demonstrated, limited primarily by the surface area of the nanowire array.

5,308 citations


"AC electrokinetics of conducting mi..." refers background in this paper

  • ...The research community is also exploring the use of nanowires and nanotubes as biosensors [7], as building blocks for novel nanocircuits [8], and/or elements of dense arrays for light absorption in solar cells [9]....

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Book
01 Jan 1965
TL;DR: Low Reynolds number flow theory finds wide application in such diverse fields as sedimentation, fluidization, particle-size classification, dust and mist collection, filtration, centrifugation, polymer and suspension rheology, and a host of other disciplines.
Abstract: Low Reynolds number flow theory finds wide application in such diverse fields as sedimentation, fluidization, particle-size classification, dust and mist collection, filtration, centrifugation, polymer and suspension rheology, flow through porous media, colloid science, aerosol and hydrosal technology, lubrication theory, blood flow, Brownian motion, geophysics, meteorology, and a host of other disciplines. This text provides a comprehensive and detailed account of the physical and mathematical principles underlying such phenomena, heretofore available only in the original literature.

4,648 citations


"AC electrokinetics of conducting mi..." refers background in this paper

  • ...Here M, N and C are, respectively the translation, rotation and coupling tensors [61]....

    [...]

Frequently Asked Questions (18)
Q1. What have the authors contributed in "Ac electrokinetics of conducting microparticles: a review" ?

This paper reviews both theory and experimental observation of the AC electrokinetic properties of conducting microparticles suspended in an aqueous electrolyte. Applied AC electric fields interact with the induced charge in the electrical double layer at the metal particle-electrolyte interface. In general, particle motion is governed by both the electric field interacting with the induced dipole on the particle and also the induced-charge electro-osmotic ( ICEO ) flow around the particle. Experimental measurements of the AC electrokinetic behaviour of conducting particles ( dielectrophoresis, electro-rotation and electro-orientation ) are compared with theory, providing a comprehensive review of the relative importance of particle motion due to forces on the induced dipole compared with motion arising from induced-charge electro-osmotic flow. In addition, the electric-field driven assembly of conducting particles is reviewed in relation to their AC electrokinetic properties and behaviour. 

Future work could focus on an improved analysis of the effect of the wall on both the electrical and hydrodynamic problems. Future work could lead to bottom-up methods for assembling a diverse range of nano-systems from simple building blocks. From a fundamental perspective, the AC electrokinetics of conducting particles is a problem where the particle velocity is measured for a known EDL potential. This implies that the zeta potential can be measured for a controlled voltage drop across the double layer. 

The MW frequency for a leaky dielectric slender spheroid in a leaky dielectric liquid is approximately ωMW ≈ (σf +σpL)/(εf + εpL), where subscripts f and p refer to the fluid and particle, respectively, and L is the depolarization factor along the particle axis [1]. 

The typical time for charging the electrolyte/metal interface is tRC = εℓ/λDσ [22, 23], where ε and σ are, respectively, the electrical permittivity and conductivity of the electrolyte, and λD is the Debye length. 

The use of AC electric fields for manipulating and characterising small particles in suspension is well-known [1, 2] and many applications have appeared over the last decades in fields such as colloidal science and biotechnology. 

Electrokinetic fabrication of vertically aligned carbonnanotube/polymer composites has been reported using electro-orientation at low frequencies so that the ICEO flow favored individually aligned CNTs rather than tip-to-tip chaining [72]. 

Electric fields in microdevices are usually generated using planar microelectrode structures fabricated on glass or other insulating substrates. 

In this case, the native double layer may contribute to surface conductance implying a high Dukhin number, which can lead to concentration polarization phenomena [24]. 

the theory is for uncharged particles and gold nanoparticles are highly charged in order that they form stable suspensions. 

According to theory, the ROT of spheres is only affected by the electrical torque on the dipole, and experiments are in close agreement with theory. 

When an uncharged conducting particle is suddenly exposed to an electric field, electrons in the conductor move almost instantaneously so as to make the electric field inside equal to zero; electric field lines intersect the particle surface perpendicularly (see fig. 1a). 

For a polarizable rod, the induced dipole at low frequencies is very small since the electric field lines surround the rod almost without deviation (see fig. 

The observation that metal nanowires align with its main axis parallel to the applied electric field direction has been extensively reported [12, 17, 68], idem for carbon nanotubes [72]. 

In the following analysis, the authors assume that the induced voltage across the double layer ∆φ ∼ Eℓ (where E is applied electric field and ℓ is typical particle size) is below the threshold voltage for these Faradaic reactions Vredox so that no direct current passes through the EDL, i.e. the electrolyte/conductor interface is perfectly polarizable. 

In these experiments, the composite particles rotated and translated on a substrate and the unidirectional field was applied perpendicularly to the substrate. 

Bridging of18AC CEP TED MAN USC RIP TACCEPTED MANUSCRIPTlarger gaps between electrodes has also been reported by the dielectrophoretic assembly of several metallic nanowires [69] or carbon nanotubes [90], see figure 5d. 

The main experimental observation concerning the translational motion of metal rods is that they move to high field regions [16, 68, 69]. 

The translational motion of Janus spheres in uniform fields is only possible due to ICEP, and observed particle velocities are smaller than for the ideal case.