Controlled Levitation of Colloids through Direct Current Electric Fields

Jun 27, 2017 - We report the controlled levitation of surface-modified colloids in direct current (dc) electric fields at distances as far as 75 μm f...
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Article Cite This: Langmuir 2017, 33, 10861-10867

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Controlled Levitation of Colloids through Direct Current Electric Fields Carlos A. Silvera Batista,* Hossein Rezvantalab, Ronald G. Larson, and Michael J. Solomon Department of Chemical Engineering and Biointerfaces Institute, University of Michigan, Ann Arbor, Michigan 48109, United States S Supporting Information *

ABSTRACT: We report the controlled levitation of surface-modified colloids in direct current (dc) electric fields at distances as far as 75 μm from an electrode surface. Instead of experiencing electrophoretic deposition, colloids modified through metallic deposition or the covalent bonding of poly(ethylene glycol) (PEG) undergo migration and focusing that results in levitation at these large distances. The levitation is a sensitive function of the surface chemistry and magnitude of the field, thus providing the means to achieve control over the levitation height. Experiments with particles of different surface charge show that levitation occurs only when the absolute zeta potential is below a threshold value. An electrodiffusiophoretic mechanism is proposed to explain the observed large-scale levitation.

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INTRODUCTION Electric fields are a principal way to control the motion and position of colloids. A large parameter space, including field strength, frequency, phase, waveform, and electrode configuration, allows the surface and body forces that colloids experience to be controlled in space and time. For example, electric fields induce anisotropic pair potentials (e.g., dipoles) that enable colloids to assemble into diverse structures.1 The ability to rapidly and easily change the frequency or field strength facilitates the dynamic reconfiguration of colloids that can result in actuation.2 Furthermore, electric fields provide a connection to self-assembly by enabling the rapid concentration of colloids; the resulting change in number density can induce fluid−crystal phase transitions.3 Electric fields have been combined with anisotropic particle shape and surface chemistry to produce artificial swimmers and propulsion.4,5 Levitation, such that the gravitational body force on the colloid is balanced by an external electrokinetic force, is an unusual response of colloids in electric fields that can be useful for the characterization, separation, manipulation, and assembly of colloids.6 Among the variety of electrokinetic phenomena that colloids experience, including electrophoresis (EP),7 diffusiophoresis,8 dielectrophoresis (DEP),6 contact-charge electrophoresis (CCEP),9 induced-charge electrophoresis (ICEP),10 and electrohydrodynamic (EHD) flows,11,12 levitation has been generated by means of DEP and EHD flows. However, the levitation of colloids through such electrokinetic phenomena has been limited to either methods that involve complex electrode design and control or to limited distances, ranging from hundreds of nanometers to a maximum levitation of a few micrometers. Levitation through electrostatic fluid traps encounters similar limitations.13 Large-scale, controllable © 2017 American Chemical Society

levitation in devices of simple design has therefore not been reported by any method. In this article, we show that the application of direct current (dc) electric fields to particles with low zeta potentials in dimethyl sulfoxide (DMSO) can lead to the controlled levitation of colloids up to very large (many tens of micrometers) distances from the electrodes. This mode of transport emerges when the surface of polystyrene (PS) particles is modified through metal deposition or through coupling reactions of PEG. Instead of experiencing electrophoretic deposition, these surface-modified colloids undergo migration and focusing that results in levitation far from the electrode surfaces. Electrodiffusiophoretic levitation is proposed as the mechanism.

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EXPERIMENTAL METHODS

Comprehensive details on the synthesis of particles, their characterization, and electric field experiments are provided in the Supporting Information. Briefly, the model systems used here to establish the large-scale levitation are fluorescent carboxylate-modified PS particles (CB-PS) with diameters of 1.0 and 0.5 μm. The particles are modified by cross-linking PEG chains to the carboxyl groups on the particle surface (PEG-modified particles, PEG-PS) or by metal deposition on one hemisphere (Janus particles, JP-PS). All particles are dispersed in neat DMSO with a volume fraction of approximately 1 × 10−3 %. The volume fraction of the suspensions is measured using disposable hemocytometers. The CB-PS particles are negatively charged, with an average zeta potential (ζ) of −37 ± 2 mV. The surface modifications result in particles with less-negative ζ; the values of ζ for PEG-PS and JP-PS particles are −19 ± 1 and −23 ± 4 mV, respectively. For the Received: March 12, 2017 Revised: May 19, 2017 Published: June 27, 2017 10861

DOI: 10.1021/acs.langmuir.7b00835 Langmuir 2017, 33, 10861−10867

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Figure 1. (A) Schematic of the device (electrochemical cell) for the field-induced levitation experiments. (B−D) 3D confocal images of uniform 1 μm, 10 kDa PEG-PS particles under dc electric fields. Image (B) is before a field is applied. (C and D) 60 s after applying 0.82 kV/m (id = 0.16 A/ m2) in the positive and negative z directions, respectively. The images report volumes of cross-sectional area 106 × 106 μm2 and electrode gaps of 116 (B−C) and 124 μm (D). CB-PS and PEG-PS particles, ζ is measured through electrophoretic light scattering; for JP-PS particles, ζ is measured through microelectrophoresis. The conductivity (λc) of the suspensions is bounded in the range of 0.1−0.7 mS/m. The Debye length in our samples is ∼50 nm, as computed from an estimated ion concentration of ∼0.02 mM determined from the solvent conductivity measurements and the assumption of an average ion diffusion coefficient equal to 2.5 × 10−9 m2/s. An advantage of the DMSO solvent is that it enables direct visualization of particles throughout the full ∼150 μm electrode gap by means of confocal microscopy. Figure 1A shows a schematic of the electrode device. The behavior of particles is studied by confining ∼20 μL of suspension between two ITO-coated glass slides that are separated by a dielectric spacer with nominal thickness ∼120 μm. The ITO coatings are positioned so as to be in contact with the solvent. The slides are precleaned by sequentially sonicating in acetone, isopropanol, and DI water for 10 min in each solvent. Then, right before the assembly of the devices, the slides are exposed to a UV− ozone treatment for 20 min. The electric fields are applied by connecting the device to a potentiostat operated in galvanostatic mode. Current densities range from 0.02 to 0.5 A/m2. An approximate value of the applied electric field magnitude (|E|) is determined from the ratio of the established current density (id) and the conductivity of the medium, id/λc. Particles and their motion were imaged using a Nikon A1R confocal laser scanning microscope (CLSM) with 100× and 60×, 1.4 NA oil-immersion objective lenses. Multichannel detection enables the simultaneous imaging of the particle fluorescence, the reflection from the gold cap of JP-PS particles, and the reflection from the bottom and top electrodes (Figure S1).

(Figure 1C). A similar migration is observed when the field direction is inverted by applying a negative potential at the bottom electrode. In this case, particles reach a levitation height of 48 μm above the bottom electrode (Figure 1D). When the field is turned off, particles begin to diffuse away from the levitation position, indicating the reversibility of the phenomenon. JP-PS particles in the absence of an applied field sediment rapidly and undergo Brownian motion in the vicinity of the bottom electrode (Figure S5A). This sedimentation is consistent with the high density (1770 kg/m3) of JP-PS particles. However, immediately after the bottom electrode is positively polarized (Figure S5B) and a field of 2.1 kV/m is established, most negatively charged JP-PS particles move upward with velocity 5.3 ± 0.2 μm/s (a few particles remain at the bottom electrode because they are irreversibly adsorbed or become detached and move upward moments later). Similar to PEG-PS particles, the JP-PS particles move upward until they reach a steady vertical position of 78 μm, which they maintain while the field is on. The orientation of JP-PS particles is observed to be random during levitation. When the direction of the field is inverted, JP-PS particles move even faster (∼9.0 ± 0.4 μm/s) toward a steady-state position of 74 μm above the bottom electrode (Figure S5C). CB-PS particles, in contrast, deposit on the bottom or top electrode when either is positively polarized (Figure S5D-F). The levitation of PEG-PS and JP-PS particles under a dc electric field is surprising. First, the particles move upward (away from the positive electrode) despite experiencing both gravitational and electrophoretic forces that pull them toward the bottom electrode. Second, the modified particles do not deposit on the top electrode but instead levitate at a stable position away from both electrodes. In contrast, CB-PS particles, which are negatively charged, just like the PEG-PS and JP-PS particles, move with velocity ∼5.2 ± 0.3 μm/s toward the bottom, positive electrode, consistent with electrophoresis. The phenomenon is reproducible; the levitation was observed in samples prepared from different batches of particles. Although a detailed description is beyond the scope of this article, we remark that at higher field strengths and

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RESULTS AND DISCUSSION The 3D images in Figure 1B−D showing the space between the top and bottom electrodes (green slabs) reveal the response of PEG-PS particles under applied dc fields. Before applying a current (Figure 1B), the PEG-PS particles are homogeneously dispersed between the bottom and top electrodes. Creaming in this case is slow because the particle density (ρ = 1050 kg/m3) is slightly lower than the density of DMSO (ρ = 1100 kg/m3). However, when a field of magnitude 0.82 kV/m is applied, surprisingly, these negatively charged particles rapidly migrate away from both electrodes toward the interior of the cell until they reach a steady position 68 μm above the bottom electrode. Particles maintain this vertical position while the field is on 10862

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Langmuir volume fractions we observe the particles laterally interact, yielding particle rafts that levitate in the field. We image the levitation by acquiring volumes of the region between the electrodes in ∼4 s by means of the fast scanning capabilities of the confocal microscope. We resolve the motion of the particles toward their steady-state height (Hs). JP-PS particles focus into a band whose spread and position can be tracked by measuring the cross-sectional averaged intensity distribution of the particles along the z axis of the electrochemical cell (Figure 2A,B); see SI for details on data analysis. For each 3D volume image (Figure 2A), the intensity at each z position is averaged to yield a 1D intensity profile (Figure 2B) along the electrode gap. In Figure 2B, the most probable position of the particles is indicated by the peak intensity. The distribution, when fitted to a Gaussian, yields the mean position of the levitated particles (the maximum of the distribution) and their dispersion (from the full-width at half-maximum of the peak, fwhm). The intensity distribution is used to track the position of particles. For example, Figure 2C plots the trajectory of JP-PS under a field of magnitude 1 kV/m. The motion of particles is uniform. All particles concentrate in a narrow band, ∼2 μm in width, and maintain a steady-state position while the field is on; in this case, they remain at ∼90 μm for the full 150 s of the experiment. The levitation behavior is a sensitive function of |E| and surface chemistry, thus providing the means to achieve control over the levitation height. Figure 2D plots the levitation height, Hs, of JP-PS particles as a function of |E|. The error bars in Figure 2D are from three replicates at the same value of |E| for the two field directions and using the same sample. Figure S6 shows a replicate data set covering a similar range in |E|, which further demonstrates the reproducibility of the phenomenon. At lower values of |E| for a field that points upward, particles hover near the electrode, but the levitation height increases rapidly with |E| until it plateaus near the center of the cell. As the value of |E| decreases, so does the proportion of particles that levitate. The curves provide two important insights. First, the curves for the two field directions (pointing upward/ downward) are symmetric with respect to field direction. Second, JP-PS particles levitate only above a threshold value of | E| (0.25 kV/m). Below this threshold, particles stick to the ITO surface and become immobile, suggesting that electrophoresis dominates at low |E|. The charges on the PS particles stem from the deprotonation of surface carboxyl groups. The coupling of PEG chains through amide cross-linking reduces the surface charge on PS particles while still providing a steric repulsion barrier. The PEG chains potentially bury adjacent charge groups once they are anchored onto the particle surface. As a result, PS particles functionalized with PEG chains of different molecular weights display varying zeta potential. (See Table S1 for a list of PEGPS particles and corresponding zeta potentials.) Figure 3 maps the levitation and deposition behavior of PEG-PS particles as a function of zeta potential and field strength. PEG-PS particles display levitation (L) and deposition (D). In Figure 3, there is an apparent transition from levitation (|ζ| ≤ 26 mV) to deposition (|ζ| ≥ 29 mV). These results indicate that levitation for PEG-PS particles is observed when their absolute ζpotential, |ζ|, is below a critical value. Particles with |ζ| ≤ 26 mV do not show deposition at all on the electrodes for any value of |E|, whereas particles with |ζ| = 40 mV show complete deposition. The differences in behavior between PEG-PS and JP-PS particles is an interesting observation that presumably

Figure 2. (A, B) Vertical positions (H) of 1 μm JP-PS particles 115 s after a field of value of −1.0 kV/m is turned on. For each 3D image such as in (A), the average intensity profile along the z axis of the device exhibits a sharp maximum (B). (C) Trajectory of JP-PS particles for experiment shown in (A). The trajectory is represented by the maximum in the intensity profile at each time point. The error bars are given every five points and represent the full width at halfmaximum of the intensity profile. The inset indicates the polarity of the field in the device. (D) Behavior of the steady-state levitation height (Hs) as a function of field magnitude, |E|. In these experiments, the field points upward (+ bottom) or downward (− bottom).

reflects the different surface properties of the particles. Under the levitation force, particles migrate large distancestens of micrometers awayfrom the electrodes; in fact, when using a 10863

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in our experiments. First, levitation occurs by means of applied dc fields for particles dispersed in DMSO without any added electrolyte. Previous levitation experiments have applied ac fields (to avoid or minimize Faradaic reactions) to particles suspended in ∼1 mM electrolyte solutions that result in a conductivity of ∼20 mS/m.11,12,14 Second, in our experiments, the combination of surface modification and suspension in DMSO results in particles with lower absolute ζ than for particles without any surface modification or that are suspended in water. By comparison, previous experiments have used particles with large |ζ| (∼90 mV), especially in water. The experiments for JP-PS (Figure 2D) and PEG-PS (Figure 3) particles show that levitation occurs when |E| is above a critical threshold and |ζ| is below 29 mV, at which points the electrophoretic mechanism provides the dominant force. We hypothesize that the levitation of Janus and PEG-PS colloids is driven by the combination of electrophoresis and diffusiophoresis (electrodiffusiophoresis). A common assumption in the modeling of electrophoresis is that the voltage applied across the electrochemical cell results in uniform electric fields and that the concentration of ions remains constant.15 However, the Faradaic reactions that sustain the constant current across the electrochemical cell can also be accompanied by gradients of ionic species and local variations in the magnitude of the electric field. These gradients can be significant and spatially varying, especially in thin cells16−18 and in systems in which multiple ionic species are present.19,20 These gradients of ionic species can drive diffusiophoresis7,21 and affect the magnitude of the electric field. The theory of electrodiffusiophoresis (EDP) can predict levitation.15,22,23 Specifically, the theory predicts a sign reversal in particle velocity across an electrochemical cell that depends on the relative magnitudes of the purely electrophoretic and diffusiophoretic contributions (the latter itself being the result of electrophoretic and chemiphoretic contributions). The purely electrophoretic contribution is a linear function of current density and particle surface charge (or zeta potential), whereas the diffusiophoretic contribution depends on the diffusion coefficients (D) of ions and the ion concentration gradient. For the case of a binary electrolyte with ions of different D and for particles with fixed surface charge, according to the theory of Bazant and Rica,22 the velocity of the particle is described by

Figure 3. Phase diagram representing the behavior of PEG-PS particles as a function of the magnitude of the zeta potential and electric field. Particles display levitation (L) and deposition (D). The zeta potential of particles is modulated through functionalization with PEG polymers of different molecular weights (Table S1). The values of |E| correspond to current densities equal to 0.02, 0.07, and 0.2 A/m2.

thicker cell (1 mm), particles move beyond the field of view of the microscope (>180 μm). To levitate to these distances, particles are acquiring gravitational potential energies in the range of 10−100 kBT. Also, as a consequence of the levitation effect, particles move with significant speeds, on the order of 6 μm/s. This convective velocity can be used to estimate a Peclet number of the levitation effect. Here, the levitation Peclet number (PeL) is defined as PeL = dV/D, where d, V, and D are the particle diameter, velocity, and diffusivity. The motion toward the levitation position is characterized by PeL that ranges between approximately 5 and 80, therefore indicating the dominance of the convective motion over Brownian motion. The magnitude of the levitation height is significantly larger than that reported in previous experiments. In previous reports, levitation has been limited to 0.5−7 μm, an order of magnitude less than observed here.11,12,14 For example, Ristenpart and collaborators reported that as the ac frequency of the applied electric field decreases as particles move further away from the electrodes.11,12 The levitation heights were measured for 2−6 μm sulfonated or amine-functionalized PS particles with |ζ| ≈ 90 mV in 0.1−1 mM electrolyte water solutions (i.e., NaCl and NaOH) by applying fields of ∼2.5 kV with frequencies in the range of 50−500 Hz. In these experiments, the larger levitation heights (∼5 μm) were explained through a model that combines the effect of EHD flows with the electrostatic and van der Waals interactions the colloids experience near the electrodes.12 For the present findings, we rule out EP, CCEP,9 EHD flow,11,12 ICEP,10 and DEP6 as possible mechanisms for our observed large-scale, controllable behavior of PEG-PS and JPPS particles. These mechanisms cannot explain the following observations: (i) the levitation extends many tens of micrometers away from the electrodes; (ii) particles move along field lines; (iii) the fields are geometrically uniform; and (iv) dielectric particles (PEG-PS) show similar (although not the same) behavior as JP-PS particles. Specifically, (i) is not consistent with EP, CCEP, and EHD flows; (ii) and (iv) are not consistent with ICEP; and (iii) is not consistent with DEP. The unusual levitation response is observed in our experiments and not in previous ones presumably because of different material properties and electric field parameters used

UEDP

( ) ⎞⎟⎟

⎛ −1 2 ⎡ ⎛ ζ ̃ ⎞⎤⎜ 12cg̃ + (9cg̃ − 16)tanh ̃ ⎢ = U (α + β)ζ + 4 ln cosh⎜ ⎟⎥⎜ ⎝ 4 ⎠⎦⎜ 9cg̃ 2 ⎣ ⎝

3cg̃ 4

⎟ ⎠

(1)

cg̃ =

2ag∞ c0

U=

,β=

(1 − β 2)j∞ D+ − D− α , , = D+ + D− 2g∞Deff

2 εrε0 ⎛ kBT ⎞ ⎜ ⎟ aη ⎝ ze ⎠

(2)

where εrε0, η, kBT, z, and e are the permittivity, viscosity, thermal energy, valence of ionic species, and electron charge; a is the particle radius; D+ and D− are the diffusivities of the positive and negative ions in the solution; and Deff is the effective diffusivity defined as Deff = 2D+D−/(D+ + D−). ze ζ̃ = ζ is the zeta potential scaled to the thermal voltage,

( ) kBT

j∞ is the current density (flux of ionic species), g∞ is the ion 10864

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Figure 4. Summary of the suggested electrodiffusiophoretic levitation mechanism. (Left) Scaled concentration profile of ionic species and velocity of particles calculated according to the theory of Bazant and Rica for particles with zeta potentials of −9, −25, and −40 mV. The anode (+V) and cathode (−V) are located at positions 0 and 120 μm, respectively. Positive velocity indicates motion from the anode to cathode. The background electrolyte concentration and applied current density are 0.12 mM and 0.46 A/m2, and β is set to 0.27 on the basis of the diffusivities of H+ and OH− ions. For −9 mV, the levitation height (point of zero velocity) occurs at 75 μm. (Right) Cyclic voltammograms of DMSO in the electric field device of Figure 1A. The figure shows voltammograms of DMSO after 1 and 10 cycles at a scanning rate of 1 V/s. The differences between the 1st and 10th cycles suggest that multiple reactions are taking place in DMSO and that intermediate compounds are forming.

reactions taking place in DMSO. In the first cycle, significant current in the anodic region is observed only at potentials higher than 2.5 V. However, in the 10th cycle there is significant flow of current below 2.5 V as well. Therefore, in the 10th scan, the onset of 3 reaction waves appears at approximately 0.13, 0.8, and 3 V. The increasing flow of current at potentials below 3 V with the increasing number of scans indicates that multiple electrochemical processes are taking place. Figure S2 shows chronopotentiograms measured during the course of a levitation experiment. When 30 μA (0.23 A/m2 and equivalent to |E| ≈ 1.2 kV in Figure 3) is applied, the curves show four distinct regions. Initially, the voltage changes rapidly up to 2.5 V within the first 5 s, whereas during the next 20 s the voltage changes slowly from 2.5 to 2.8 V. Subsequently, for the next 15 s the voltage increases rapidly to 3.15 V where it remains for the last 90 s of the experiments. The electrochemical processes revealed by the voltammograms are mirrored, as a function of time, in the chronopotentiograms (Figure S2), in which the steep changes in potential occur at the onset of the reaction waves described in Figure 4B. Therefore, Figure S2 suggests that during the course of an experiment the system favors the reaction taking place at 3.15 V once the reaction at lower potential reaches its end point. The reaction at higher voltage is likely the electrolysis of DMSO because the electrochemical window of DMSO is from −2.9 to 1.5 V (vs Fc+/Fc).24 It is also possible that electrochemical processes related to impurities such as O2, CO2, and H2O are taking place as well.25,26 DMSO is hygroscopic; therefore, water constitutes an important impurity unless DMSO is treated (dried). Also, given that in the current experiments no supporting electrolyte is used, we conjecture that the ions carrying the charges are the products of the autoionization of DMSO as well as dissolved water. To summarize, future work to evaluate the electrodiffusiophoretic levitation hypothesis could involve direct measurement of ion concentration gradients in the cell, tuning the difference in ion diffusivities in the cell, and the incorporation of an induced charge contribution to the zeta potential. Similarly, a more definitive description of the reactions producing the current flow will facilitate the modeling of the levitation phenomena by providing quantitative information about ion concentration profiles in the device.

concentration gradient, and c0 is the background concentration of ionic species. The first term within brackets in eq 1 accounts for electrophoresis, whereas the second term is due to chemiphoresis. Preliminary calculations (Figure 4) using eqs 1 and 2 and a parsimoniously selected concentration profile of a linear gradient across the electrochemical cell (Figure 4A) suggest that the EDP theory for particles of fixed surface charge can account for two key experimental observations: (a) that JP-PS and PEG-PS particles levitate and (b) that a transition from levitation to deposition occurs with zeta potential. Figure 4B shows that for particles with ζ = −9 mV the theoretical UEDP is positive near the bottom half of the cell (movement toward the cathode) and negative in the upper region (movement toward the anode). This velocity sequence yields a position of zero velocity near the center of the cell, to which the particles would levitate and accumulate. The transition between levitation and deposition regimes observed in the experiments is accounted for by the theory through the following: increasing the particle zeta potential primarily affects the purely electrophoretic contribution of the velocity. This term dominates the chemiporetic term for the particles with ζ-potential values of −25 and −40 mV, resulting in deposition in these cases. Therefore, the balancing of the different terms in eq 1 can result in the observed experimental behavior with respect to current density and zeta potential. Equation 1 also indicates that the electrophoretic and chemiphoretic terms become comparable when g∞ ≈ 1 M/m for the range of parameters used in our experiments. The proposed mechanism suggests several avenues for additional future investigation. For example, although the background concentration c0 used in the above profile is close to the 0.1 mM value estimated from our electrical conductivity measurements, there is a discrepancy in the magnitude of the measured and calculated velocities. (The calculated velocities are at least an order of magnitude lower than the measured velocities under the same experimental conditions.) Possible sources of this discrepancy are in the parsimoniously selected ion concentration profile (Figure 4A) as well as the assumption of a binary electrolyte. Both sources motivate future work to better understand the electrochemistry that supports the large-scale levitation mechanisms. The cyclic voltammogram reported in Figure 4B provides an initial survey of the 10865

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Figure 5 indicates that the controllability and height of the levitation could find applications in separations, such as in lab-

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Carlos A. Silvera Batista: 0000-0002-3509-601X Hossein Rezvantalab: 0000-0002-1794-9402 Ronald G. Larson: 0000-0001-7465-1963 Michael J. Solomon: 0000-0001-8312-257X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work with PEG-PS particles was supported by the U.S. Army Research Office under grant award no. W911NF-10-10518; the work with JP-PS particles was supported by the Department of Energy, Basic Energy Sciences under grant DESC0013562. R.G.L. acknowledges funding from the NSF under grant CBET 1602183. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation (NSF). C.A.S.B. acknowledges a President’s Postdoctoral Fellowship from the University of Michigan.

Figure 5. Three-dimensional confocal images of mixtures of uniform PS (blue) and 30 kDa PEG-PS (red) 0.5 μm particles: (A) initial dispersion and (B) 35 s after 1 kV/m is applied. The uniform PS blue particles with ζ = −44.5 ± 0.1 mV deposit at the bottom electrode, and the 30 kDa PEG-PS red particles with ζ = −8.9 ± 1.0 mV levitate.

on-a-chip technologies. Particles of different ζ and surface chemistry respond to the field by focusing on a very narrow range of positions. Figure 5 shows a mixture of 0.5 μm particles with sulfate surface groups and ζ = −44.5 ± 0.1 mV (blue) as well as PEG-PS (red) with ζ = −8.9 ± 1.0 mV. After an electric field of 1 kV/m is applied, the blue particles migrate toward the bottom electrode while the red particles with less-negative ζ values levitate. This mode of transport could be used to complement or multiplex other separation modalities used in microfluidics, such as dielectrophoretic and hydrodynamic focusing.

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(1) Bharti, B.; Velev, O. D. Assembly of Reconfigurable Colloidal Structures by Multidirectional Field-Induced Interactions. Langmuir 2015, 31, 7897−7908. (2) Shah, A. A.; Schultz, B.; Zhang, W.; Glotzer, S. C.; Solomon, M. J. Actuation of Shape-Memory Colloidal Fibres of Janus Ellipsoids. Nat. Mater. 2015, 14, 117−124. (3) Ferrar, J. A.; Solomon, M. J. Kinetics of Colloidal Deposition, Assembly, and Crystallization in Steady Electric Fields. Soft Matter 2015, 11, 3599−3611. (4) Howse, J. R.; Jones, R. A. L.; Ryan, A. J.; Gough, T.; Vafabakhsh, R.; Golestanian, R. Self-Motile Colloidal Particles: From Directed Propulsion to Random Walk. Phys. Rev. Lett. 2007, 99, 048102. (5) Ma, F.; Yang, X.; Zhao, H.; Wu, N. Inducing Propulsion of Colloidal Dimers by Breaking the Symmetry in Electrohydrodynamic Flow. Phys. Rev. Lett. 2015, 115, 208302. (6) Morgan, H.; Green, N. G. AC Electrokinetics; Research Studies Press: Philadelphia, PA, 2003. (7) Anderson, J. L. Colloid Transport by Interfacial Forces. Annu. Rev. Fluid Mech. 1989, 21, 61−99. (8) Velegol, D.; Garg, A.; Guha, R.; Kar, A.; Kumar, M. Origins of Concentration Gradients for Diffusiophoresis. Soft Matter 2016, 12, 4686−4703. (9) Drews, A. M.; Cartier, C. A.; Bishop, K. J. M. Contact Charge Electrophoresis: Experiment and Theory. Langmuir 2015, 31, 3808− 3814. (10) Gangwal, S.; Cayre, O. J.; Bazant, M. Z.; Velev, O. D. InducedCharge Electrophoresis of Metallodielectric Particles. Phys. Rev. Lett. 2008, 100, 058302. (11) Bukosky, S. C.; Ristenpart, W. D. Simultaneous Aggregation and Height Bifurcation of Colloidal Particles Near Electrodes in Oscillatory Electric Fields. Langmuir 2015, 31, 9742−9747. (12) Woehl, T. J.; Chen, B. J.; Heatley, K. L.; Talken, N. H.; Bukosky, S. C.; Dutcher, C. S.; Ristenpart, W. D. Bifurcation in the Steady-State Height of Colloidal Particles Near an Electrode in Oscillatory Electric Fields: Evidence for a Tertiary Potential Minimum. Phys. Rev. X 2015, 5, 011023. (13) Krishnan, M.; Mojarad, N.; Kukura, P.; Sandoghdar, V. Geometry-Induced Electrostatic Trapping of Nanometric Objects in a Fluid. Nature 2010, 467, 692−695.

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SUMMARY We have shown that modifying the surface properties of PS particles either through the covalent coupling of PEG or through metal deposition causes them to respond to dc fields in pristine DMSO by migrating and levitating to positions that span the complete electrode gap; the specific levitation position can be controlled by adjusting the field strength (current density) and surface chemistry. Future work can focus on evaluating the hypothesized electrodiffusiophoretic levitation mechanism as well as applying the method to control and manipulate colloids for separations and lab-on-a-chip applications. The experimental methods and systems we describe throughout the article also suggest further fundamental studies of colloidal electrokinetics. For example, we highlighted that at higher field strengths and volume fractions, particles show interesting rafting behavior. Another possible path of inquiry is the detailed quantitative description of the movement of particles as they levitate.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00835. Experimental details. Synthesis and characterization of materials as well as experimental procedures. (PDF) 10866

DOI: 10.1021/acs.langmuir.7b00835 Langmuir 2017, 33, 10861−10867

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DOI: 10.1021/acs.langmuir.7b00835 Langmuir 2017, 33, 10861−10867