Article pubs.acs.org/Langmuir
Observing Electrokinetic Janus Particle−Channel Wall Interaction Using Microparticle Image Velocimetry Alicia Boymelgreen and Gilad Yossifon* Faculty of Mechanical Engineering, Micro- and Nanofluidics Laboratory, Technion - Israel Institute of Technology - Technion City 32000, Israel S Supporting Information *
ABSTRACT: Three-dimensional/two-component microparticle image velocimetry is used to examine the hydrodynamic flow patterns around metallodielectric Janus particles 15 μm in diameter adjacent to insulating and conducting walls. Far from the walls, the observed flow patterns are in good qualitative agreement with previous experimental and analytical models. However, close to the conducting wall, strong electrohydrodynamic flows are observed at low frequencies, which result in fluid being injected toward the particle. The proximity of the metallic hemisphere to the conducting wall is also shown to produce a localized field gradient, which results in dielectrophoretic trapping of 300 nm polystyrene particles across a broad range of frequencies.
1. INTRODUCTION
UICEP =
Nonlinear electrokinetic motion of polarizable particles was initially observed by Gamayunov and Murtsovkin1 and Murtsovkin and Mantrov,2 who noted that asymmetric quartz particles translated under AC electric fields. The translation of metallic spheres and cylinders, partially coated with dielectric material due to a term they coined ‘induced-charge electrophoresis’ (ICEP) was predicted by Squires and Bazant in 20043 and demonstrated experimentally by Gangwal et al.4 The basic underlying principle is that any asymmetry, whether geometric, electrostatic, or even surface chemistry,5 will distort the quadrupolar induced-charge electroosmotic flow (ICEO), which occurs around symmetric polarizable spheres when subjected to an electric field, to produce a net electrophoretic velocity. Specifically, in terms of the partially coated particles, the difference in polarizability between the two hemispheres results in a stronger induced-charge electroosmotic flow around the more polarizable hemisphere, which then drives the particles in the direction of the dielectric end. For a spherical “metallodielectric” Janus particle (JP), comprised of one conducting and one insulating hemisphere, the electroosmotic flow in combination with the induced-dipole ensure that the particle will always align the interface between the metallic and dielectric hemispheres with the applied field, and travel perpendicular thereto so that the particles have a single stable state.6,7 For the simplified case of a thin EDL and ideal dielectric and polarizable hemispheres, the velocity can be written as6 © 2015 American Chemical Society
9 ⎛ εRE2 ⎞ ⎟ ⎜ 64 ⎝ η ⎠
(1)
where E denotes the magnitude of the applied electric field, η is the dynamic viscosity of the solute, and ε is its permittivity, while R is the particle radius. Since the electroosmotic velocity arises from the interaction of the electric field with an EDL is itself inducedas opposed to the EDL, which shields a natively charged surfacethe effect is nonlinear, and net motion is observed even under AC fields, scaling with the magnitude of the field squared. This is in contrast to linear electrokinetic effects, e.g., linear electro-osmotic flow and electro-phoresis, which have a vanishing time averaged effect with increasing frequencies. Experimentally, ICEP has been demonstrated for single Janus spheres4,8 and doublets,9 while ICEO over metallodielectric surfaces has also been used for pumping.10 Recently, the hydrodynamic flow field at the midplane of a stagnant JP adjacent to an insulating wall has been examined using microparticle image velocimetry (μPIV) by Peng et al.,11 extending previous works that show such flow around homogeneous spheres8 and rods.12−14 Additionally, the dielectrophoretic (DEP) response of JPs under AC fields has also been well studied15,16 and most recently combined with Received: April 1, 2015 Revised: June 28, 2015 Published: July 6, 2015 8243
DOI: 10.1021/acs.langmuir.5b01199 Langmuir 2015, 31, 8243−8250
Article
Langmuir
Figure 1. Schematic of the (a,b) insulating and (c,d) conducting substrate experimental setups. Parts b and d portray the view from the microscope (cross-section A-A).
self-propelling catalytic micromotors to control the particle pathline.17 In general, experiments on ICEO,18 including those on ICEP of JP4,9 have been plagued by quantitative discrepancies between the theory and experiments, most often in terms of the frequency dispersion and magnitude of the induced electrophoretic velocities. A number of explanations have been suggested to close the gap between theory and experiment, including nonlinear capacitance and steric effects,18 surface conductance,19,20 Faradaic reactions,21 and electrode contamination.22 One phenomena that has been noted in multiple experiments is the tendency of the JPs to be attracted to both conducting and insulating channel walls, traveling parallel thereto.4,9 The interaction of the JP with an insulating wall has been modeled theoretically and numerically,23,24 but the case of a conducting wall remains unexamined. The latter case is significantly more complicated, as it is necessary to also account for electrohydrodynamic flow (EHD) at the electrode surface.25−28 Specifically, the presence of a particle adjacent to an electrode has been demonstrated theoretically and experimentally to induce flow in its proximity, the direction of which depends on parameters including the electrolyte, surface charge distribution of the particle, and frequency and strength of the applied field.29 This flow is presumed to be responsible for the aggregation or spreading of particles on the substrate and in some cases can actually elevate the particles from the surface.29,30 A number of underlying physical models have been proposed, all of which are predicated on the idea that the presence of the particle disturbs the otherwise uniform field, which acts on the electric double layers in the system (particle and/or electrode) to drive fluid motion. Much of the debate centers around whether the double layers are native to the system or induced by the applied field (see ref 31 for a detailed summary of the relevant mechanisms). To date, research has focused primarily on insulating homogeneous particles, and the introduction here of metallic, let alone inhomogeneous particles, may further enhance understanding of these mechanisms and their exploitation for practical purposes.
In this work, the use of three-dimensional/two-component (3D-2C) μPIV to examine the hydrodynamic flow around a stagnant JP simultaneously reveals the frequency-dependent presence of electrohydrodynamic flow and dielectrophoretic trapping at the substrate induced by the presence of the JP at the electrode surface. Although the resultant entrainment and trapping signify the divergence of the particles from acting as simple passive tracers to exhibiting their own dynamics, we are nevertheless able to reveal qualitative information on the competition between the various EHD, ICEO, and DEP effects, which previously were perhaps hinted to in the discrepancies between the theory and experiments tracking the mobility of freely suspended particles.
2. EXPERIMENTAL MATERIALS AND METHODS The JPs were manufactured following the methodology in ref 9. Briefly, 15 μm polysterene (PS) particles (Fluoro-max) were half coated with a thickness of 10 nm chrome for adhesion followed by 20 nm of gold. PS tracer particles (Fluoro-max), 300 nm in size, were suspended in 10−5 M KCl solution (measured conductivity 3 μS/cm) at approximately 0.01% w/v. Upon placement in the experimental cell, the JPs sank to the bottom substrate. For applied fields on the order of 40 kV/m, a large number of particles were observed to remain stagnant, and it was around these that measurements were taken. We consider two distinct experimental setups, both of which provide a uniform electric field. The first, henceforth referred to as “insulating substrate” consists of one standard glass slide and one coverslip separated by a silicon reservoir (Grace Bio), 120 μm in height and 2 mm in diameter. Electrodes were deposited on the glass slide using standard photolithography, with a thickness of 30 nm Cr followed by 200 nm Au, 1 mm apart (See Figure 1). In the second case, the “conducting substrate”, an indium tin oxide (ITO)-coated glass slide (Delta Technologies) and coverslip (SPI Systems) serve as the electrodes, separated by the same reservoir. The electrodes were connected to a signal generator (Agilent 33250A), and the signal was monitored by an oscilloscope (Tektronix TPS-2024). Both chips were mounted onto a Nikon Eclipse Ti-E inverted microscope equipped with Yokagawa CSU-X1 spinning disk confocal scanner and Andor iXon-897 EMCCD camera, coverslip side down, and images were magnified with ×60 oil immersion lens. The variation of the velocity vector field around the circumference of the particle (in the x−y plane) and along its height (z-direction) was determined taking 200 planar images at 35 ms intervals, every 1 μm 8244
DOI: 10.1021/acs.langmuir.5b01199 Langmuir 2015, 31, 8243−8250
Article
Langmuir along the height of the particle beginning approximately 0.5 μm from the floor of the channel wall. For the microscope setup described above, the optical slice thickness can be calculated as 1.4 μm.32 For the PIV analysis, images were adjusted so that the origin is located at the center of the particle, i.e., (x, y, z) = (r, θ, z) = (0, 0, 0) and the metallic hemisphere lies in the positive x-direction (Figure 1). The orientation of the interface was qualitatively determined from the zscan images (see Figure S1 in the Supporting Information). Measuring the particle radius at each height also yields an estimation of the particle diameter as 14 and 14.5 μm for the insulating and conducting walls, respectively, which is close to the specification of the manufacturer (15 μm ± 10%). The μPIV analysis was performed using commercial Insight 4G software. The processing interrogation region is 32 × 32 pixels on a standard Nyquist grid, with the results presented as a sum of correlations of minimum 200 image pairs. It is noted that for the maximum recorded velocities, which are on the order of 20 μm/S, the maximum displacement corresponds to a displacement of 3.5 pix/frame. The larger grid and high number of correlation pairs reduce the error of Brownian motion to at least an order of magnitude below the minimum recorded velocities according to33 ε = (1/N)((2kBT/(3πηdΔt))1/2 ∼ 0.3 μm/s), where Δt = 35 × 10−3 s is the time between successive frames, kB is the Boltzmann constant, T ∼ 300 K is the temperature, μ ∼ 10−3 Ns/m2 is the viscosity, d = 300 nm is the diameter of the tracer particle, and N is the number of image pairs (200) multiplied by the number of particles per interrogation region in each pair (∼5).
3. RESULTS AND DISCUSSION Three-dimensional hydrodynamic fields. In the following two figures, we analyze the flow patterns around a sample JP adjacent to both insulating and conducting substrates, where the interface between the dielectric and metallic hemispheres are aligned effectively parallel to the electric field (see Figure S1 in the Supporting Information), corresponding with the stable orientation of a mobile JP.7 It is noted that qualitatively similar fields were observed around multiple similarly oriented particles in over eight experimental cells. In Figure 2, we illustrate the changes of the velocity distribution over the height of the particle. We complement the vector fields with a plot of the variation of Ur as a function of height in the z-direction. To minimize noise, we have chosen to position the interrogation window at the region of maximum ejection (−π/16 < θ < π/ 16), beginning 1 μm from the surface of the particle and extending 7.5 μm in the radial direction. The interrogation window was adjusted to track the circumference of the particle, so that for −8 μm < z < 8 μm, the minimum radius signifying the start of the interrogation window was calculated according to r = ((R + 1)2 − z2)1/2 μm (shaded region in Figure 2c). Above the particle, the origin of the interrogation window was kept constant at r = 3 μm. Error bars were used to indicate the standard deviation of the variation of the velocity within the interrogation window. It is noted that increasing the size of the interrogation window decreased the velocity magnitude and increased the standard deviation, but qualitative trends remained the same. We observe that for the insulating substrate, the ejection is relatively evenly distributed along the height of the particle, as opposed to the conducting case where it is concentrated in the top half of the upper hemisphere. In an unbounded medium, we would expect both curves in Figure 2c to be symmetric around the center of the particle (z = 0). In this case, however, the proximity of the wall introduces a broken symmetry, which also impacts the ICEO around the JP,34 especially in the case of the particle adjacent to the conducting wall where the injection occurs along the z-axis. In both cases, as expected, the velocities
Figure 2. Vector fields taken at multiple z positions for (a) insulating and (b) conducting walls. (c) Distribution of radial velocity with respect to the height of the particle. The particle lies in the shaded area. Measurements for both particles were taken at 1 kHz. The applied fields were 33.2 kV/m and 42.5 kV/m for the insulating and conducting walls, respectively. Error bars indicate the standard deviation of the velocity within the interrogation window.
decrease as we move away from the particle toward the fluid bulk. One of the most interesting aspects of Figure 2 is the significantly large negative radial velocities, i.e., injections that are observed adjacent to the conducting wall. Initially, it seems reasonable to expect that due to the proximity of the particle to the wall, conservation of mass will dictate that some fluid be convected along the wall to supply the injection toward the particle center. However, examination of this injection at the substrate across a broad range of frequencies indicates a strong frequency dependence that deviates from the behavior of the ICEO at the centerplane, indicating the presence of additional physical mechanisms, which will be discussed in more detail below. To obtain further understanding of the hydrodynamic flow patterns, the vector fields at the plane of maximum velocity of 8245
DOI: 10.1021/acs.langmuir.5b01199 Langmuir 2015, 31, 8243−8250
Article
Langmuir
Figure 3. (a,b) Velocity vector field at plane of maximum velocity (z = 0 and z = 4 μm for the insulating and conducting cases, respectively) for JP adjacent to (a) insulating and (b) conducting walls. Comparison of (c) Ur and (d) Uθ velocity components close to the particle surface for both cases. Part e illustrates the decay of Ur far from the particle. Error bars indicate standard deviation of the velocity within the window of interrogation.
Figure 4. Microscope images at the base of the JP adjacent to the conducting wall after 16 s at applied fields of (a) 200 Hz, (b) 1 kHz, and (c) 500 kHz, along with the corresponding PIV vector fields for (d) 200 Hz, (e) 1 kHz, and (f) 500 kHz. (g) Plot of radial velocity as a function of azimuthal angle at a distance of r = 8 μm. (h) Image intensity as a function of azimuthal angle, θ. Interrogation window as illustrated in part a.
Information. To complement these figures, we compare the distribution of the radial (Ur) (Figure 3c) and tangential (Uθ) (Figure 3d) velocity components close to the particle surface (r = R) as a function of the azimuthal direction, θ, using a
the particle (z ≈ 0 and z ≈ 4 μm for the insulating and conducting cases, respectively) are illustrated in Figure 3a,b. Qualitatively, similar plots, taken at the centerplane of each particle (z = 0) are presented in Figure S2 of the Supporting 8246
DOI: 10.1021/acs.langmuir.5b01199 Langmuir 2015, 31, 8243−8250
Article
Langmuir
Figure 5. Plot of radial velocity at the JP midplane and channel wall along with the change of intensity as a function of frequency for (a) Au hemisphere and (b) Ps hemisphere. Error bars indicate the standard deviation of the velocity within the interrogation window.
one would expect to see for a slice taken in the x−z plane of the insulating case at y = 0. Conversely, Figure 3a corresponds to the conducting setup of Figure 3b in the x−z plane at y = 0. Near Wall Effect. Immediately adjacent to the conducting wall, in contrast to the velocity profiles taken close to the insulating wall, and at the midplane of both JPs, distinct motion of the tracer particles was still seen at high frequencies, well beyond the charge relaxation time, τ = λR/D, which for a double layer, λ ∼ 100 nm (10−5 M KCl), R = 7.5 μm, and D ∼ 2 × 10−9 m2/s corresponds to an applied frequency of approximately f = 1/2πτ ∼ 420 Hz. Three distinct frequency regimes were observed: At very high frequencies (20 kHz to 1 MHz) isolated tracer particles translate toward the JP and are trapped at the metallic hemisphere (Figure 4c,f) where in the latter case, trapping around the metallic hemisphere is reflected as an increase in intensity around θ = 0. At low frequencies (100−400 Hz), particles are mass convected at high velocity toward the JP, accumulating around the entire circumference (Figure 4a,d) although a greater percentage of particles appear to be entrained at the metallic hemisphere |θ| < π/2, while velocities are higher at the dielectric hemisphere |θ| > π/2 Maximum velocity and intensity are recorded at 200 Hz. In fact, in this frequency range, the EHD is so strong that in some experiments it successfully lifted the particle above the substrate. Figure 4a illustrates one such example, wherein the lifting of the particle is evidenced by the presence of only a point sized area below the particle, which is depleted of tracers, as opposed to the ∼3 μm seen adjacent to stuck particles (see Figure S5 in the Supporting Information). Levitation of 6 μm particles on the order of hundreds of nanometers have been previously observed by the group of Prieve,29,37 while Woehl et al. have recently demonstrated a bifurcating lift force that can result in elevation on the order of microns.30 Interestingly, Woehl et al. specifically did not observe levitation in KCl. However, we note that their work concerns millimolar concentrations, while here we are using 10−5 M KCl. Finally, in the midrange (400 Hz to 20 kHz), particles are convected toward the JP (Figure 4b,e) but very little trapping is observed. Sample videos taken at the substrate at frequencies of 200 Hz, 1 kHz, and 100 kHz have been included in the Supporting Information. It is necessary to note at this point that, throughout multiple experiments, the overall trends described in this section and the next were consistently observed. Slight variations in the quantities of tracers trapped and the transition frequencies were recorded (see Figure S3 in
cylindrical coordinate system. The values were taken by averaging the velocities within an interrogation window, centered at (r = 11 μm) and extending 5 μm in the radial direction while spanning π/10 in the azimuthal direction (i.e., R + 1 μm < r < R + 6 μm, θi < θi+1 < θi + π/10, z = zmax), so that the average position of the profile is approximately (r = 11 μm). Error bars were used to represent the standard deviation of velocity within the interrogation window. As above, use of larger interrogation windows was shown to yield similar trends, but with smaller mean velocities and larger standard deviations. Since the full analytical solution of the velocity field for a fixed value of r is expected to be governed by an infinite series of Legendre polynomials and their derivatives,35 we have fitted the results here with a Fourier series,11 ∞
Ur = A 0 +
∑ A n cos(nθ), n=1
∞
Uθ = B0 +
∑ Bn sin(nθ) n=1
(2)
where the coefficients An and Bn are given in the Supporting Information. In part e of Figure 3, we plot the decay of the radial ejection velocity between −π/16 < θ < π/16 as a function of r. Here the experimental data was fitted with a curve Ur ∝ (9/16)(R2((R2 − r2)/r4)), which corresponds to the expected decay of ICEO around a sphere.36 The flow field around the JP adjacent to an insulating wall (Figure 3a) corresponds well with the recent μPIV results of Peng et al.11 Here, both injection and ejection occur in the plane of view (at the poles and equator respectively) and are thus clearly visible both qualitatively (Figure 3a) and quantitatively (Figure 3c,d). Specifically, the regions of injection and ejection are characterized by high negative and positive radial velocities, respectively, while the transition is marked by large tangential velocities. On the other hand, in the case of the previously unexamined conducting wall, the injection occurs out of plane, along the z-axis so that using 2D PIV, only ejection may be measured directly. Thus, in Figure 3c, all radial velocities are positive while in Figure 3d the tangential components are significantly smaller than the case of the insulating wall. It is worth noting that the velocity vector fields of each of the examined cases, i.e., conducting and insulating walls, complement each other to give a qualitative description of the missing third, out of plane, velocity vector field in the absence of nearwall effects. Specifically, the velocity field in Figure 3b is what 8247
DOI: 10.1021/acs.langmuir.5b01199 Langmuir 2015, 31, 8243−8250
Article
Langmuir
Supporting Information), where the EHD is reduced and ICEO dominates, uniform injection is observed around the metallic hemisphere, while, interestingly, the PS exhibits injection far from the particle but ejection very close to the particle edge (Figure S4b). The above observations require further study since the presence of accumulated particles and out of plane vortices significantly inhibits μPIV analysis. A comprehensive study of the difference in EHD around polarizable and dielectric particles close to the wall, using different size tracers and bulk solutions with different electrolytes at varying concentrations, is left for future work. Of particular interest is measurement of the EHD during the transient stage that occurs after the application of the field but before the tracers accumulate, particularly in the case of the metallic particle as well as the incidence of particle levitation. Further support of the idea that electrohydrodynamics play an important role close to the electrode surface is obtained from the frequency dispersion wherein a distinct maxima is observed for both the trapping and injection velocities and it is noted that this maxima occurs earlier than that at the midplane (Figure 5). This distinct shift suggests that the presence of the channel wall creates additional physical mechanisms, which are at playin contrast to the midplanefar from the wall, where the ICEO is presumed to be more isolated. Indeed, based on the 120 μm height of the reservoir (H), the injection and intensity maxima are shown to occur closer to the charge relaxation time of the ITO electrodes, τ = λH/D ∼ [(100 × 10−9) × (120 × 10−6)]/(2 × 10−9), which corresponds to an applied frequency f = 1/2πτ ∼ 30 Hz. It is noted that, although the maxima appears closer to 100−200 Hz, it may in fact be lower since the use of ensemble μPIV with the current time step of 35 ms can only give us a very broad qualitative understanding of the hydrodynamics in this region due to the ongoing accumulation of particles at the surface, which shield the field disturbances, resulting in decreased velocities over the full recording. Thus, the faster the accumulation, the lower the recorded velocity over the recorded time period. A better quantitative understanding may be obtained by taking faster measurements of the flow field in the transient stage, during the formation of this accumulation, but this is left for future work. As the frequency increases, and the flow is dominated by ICEO, the injection significantly reduces in intensity and occurs most strongly at the dielectric hemisphere, which is in qualitative agreement with the numerical simulations (see Supporting Information). Note that this stronger injection at the dielectric hemisphere also appears to be responsible for the seeming reversal of the flow velocity from injection to ejection at the Au hemisphere between 3 and 10 kHz (Figure 5a). This simply appears to be overshooting of particles convected from the dielectric hemisphere toward the gold hemisphere, which were not then convected to the midplane via ICEO. At the same time, in this mid-frequency range of 400−8000 Hz, the amount of tracers trapped at the metallic hemisphere decreases significantly, decaying almost to zero as the EHD ceases to entrain tracers, and the ICEO is sufficiently strong to overcome the DEP trapping, convecting the particles from the bottom of the metallic hemisphere toward the midplane. This dominance of ICEO is reflected by the plateau in intensity in Figure 5 between 400 Hz and 8 kHz, which corresponds directly with the maxima of ICEO and diminishing of EHD flow. As the ICEO decays beyond its charge relaxation time, an increase in intensity is again observed, indicating dielectrophoretic trapping
the Supporting Information), and it is presumed that these changes may stem from small differences in the positioning of the JP ,which may affect the size of the localized field gradient and EHD velocities. Frequency Dispersion. In order to better understand these phenomena, we plot in Figure 5 the radial velocity as a function of frequency at both the midplane (z = 0 μm) and on the channel floor (z = −8 μm), separating the metallic and dielectric hemispheres into two separate graphs. On a secondary axis we also plot the ratio If/I0 change in intensity between t = 0 (I0) and t = 16 s (If), signifying the number of particles accumulated over time. An additional set of data has been supplied in the Supporting Information. Here we can clearly see that in the high frequency domain (>20 kHz), the velocity at the midplane and channel wall decay toward zero for both metallic and dielectric hemispheres. At such high frequencies, the induced EDL has no time to charge, and thus the associated nonlinear electrokinetic effects must decay to zero. However, an observed increase in intensity of the metallic hemisphere (If/I0 > 1) indicates trapping of isolated particles, which is not seen at the dielectric hemisphere. Similar trapping is observed around the entire circumference of the contact point of JPs where the metallic hemisphere is facing down (i.e., the interface is parallel to the wall). Thus, rather than being an effect arising from the Janus asymmetry, such trapping appears to be the result of a net dielectrophoretic force that arises from a field gradient created due to the jump in potential between the metallic hemisphere, which acts as a floating electrode, and the solid surface acting as an active electrode; both of which as conducting surfaces must have a uniform potential (see numerical simulations in Figure S8 of the Supporting Information). Since the crossover frequency for the polystyrene tracers is on the order of MHz,38 within the range of frequencies illustrated above, the tracers are expected to undergo positive dielectrophoresis (pDEP), in which case they accumulate around the gold hemisphere, resulting in an increased image intensity. The presence of DEP was further verified by recording the shift from pDEP to nDEP at around 3 MHz wherein repulsion of particles was observed (Supporting Video). What is interesting here is that, as opposed to traditional DEP, where the applied field is either nonuniform or there is geometric nonuniformity built into the channel, here the field gradient stems from the proximity of the JP, specifically its metallic hemisphere to the wall. At the low frequency end of the spectrum, significant particle trapping was also evident up until around 400 Hz, after which it decays almost to zero until around 8 kHz (Figure 4a,h). However, in contrast to the high frequency domain described above, here some trapping is evident at the dielectric hemisphere as well (|θ| > π/2). It has previously been demonstrated both theoretically and experimentally that the proximity of a homogeneous insulating silica particle to an active ITO electrode can induce a uniform injection around the base of the particle.25,28,31 Examination of injection around a homogeneous (uncoated) PS sphere and a JP with the metallic hemisphere facing downward (so that with respect to the wall, the particle mimics a uniform metallic sphere), using the conducting experimental setup, are provided in the Supporting Information. Briefly, at low frequencies, uniform injection was observed by the authors around an uncoated homogeneous 15 μm particle (Figure S4a) in agreement with ref.25 The metallic hemisphere on the other hand, appears to show a mixture of injection and ejection. At higher frequencies (Figure S4d in the 8248
DOI: 10.1021/acs.langmuir.5b01199 Langmuir 2015, 31, 8243−8250
Langmuir
■
although occurring at a slower rate due to the absence of hydrodynamic convection.
ASSOCIATED CONTENT
* Supporting Information S
Addtional details, figures, and videos as described in the text. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01199.
■
REFERENCES
(1) Gamayunov, N.; Murtsovkin, V. A. Motion of Disperse Particles in a Uniform Alternating Electric Field. Colloid J. USSR 1987, 49 (3), 543. (2) Murtsovkin, V. A.; Mantrov, G. I. Study of the Motion of Anisometric Particles in a Uniform Variable Electric Field. Colloid J. USSR 1990, 52 (6), 933−936. (3) Bazant, M. Z.; Squires, T. M. Induced-Charge Electrokinetic Phenomena: Theory and Microfluidic Applications. Phys. Rev. Lett. 2004, 92 (6), 066101. (4) Gangwal, S.; Cayre, O. J.; Bazant, M. Z.; Velev, O. D. InducedCharge Electrophoresis of Metallodielectric Particles. Phys. Rev. Lett. 2008, 100 (5), 058302. (5) Boymelgreen, A. M.; Miloh, T. A Theoretical Study of InducedCharge Dipolophoresis of Ideally Polarizable Asymmetrically Slipping Janus Particles. Phys. Fluids 2011, 23, 072007. (6) Squires, T. M.; Bazant, M. Z. Breaking Symmetries in InducedCharge Electro-Osmosis and Electrophoresis. J. Fluid Mech. 2006, 560, 65−101. (7) Boymelgreen, A. M.; Miloh, T. Induced-Charge Electrophoresis of Uncharged Dielectric Spherical Janus Particles. Electrophoresis 2012, 33 (5), 870−879. (8) Daghighi, Y.; Sinn, I.; Kopelman, R.; Li, D. Experimental Validation of Induced-Charge Electrokinetic Motion of Electrically Conducting Particles. Electrochim. Acta 2013, 87, 270−276. (9) Boymelgreen, A.; Yossifon, G.; Park, S.; Miloh, T. Spinning Janus Doublets Driven in Uniform Ac Electric Fields. Phys. Rev. E 2014, 89 (1), 011003. (10) Paustian, J. S.; Pascall, A. J.; Wilson, N. M.; Squires, T. M. Induced Charge Electroosmosis Micropumps Using Arrays of Janus Micropillars. Lab Chip 2014, 14 (17), 3300−3312. (11) Peng, C.; Lazo, I.; Shiyanovskii, S. V.; Lavrentovich, O. D. Induced-Charge Electro-Osmosis around Metal and Janus Spheres in Water: Patterns of Flow and Breaking Symmetries. Phys. Rev. E 2014, 90 (5), 051002. (12) Canpolat, C.; Qian, S.; Beskok, A. Micro-PIV Measurements of Induced-Charge Electro-Osmosis around a Metal Rod. Microfluid. Nanofluid. 2013, 14 (1−2), 153−162. (13) Canpolat, C.; Zhang, M.; Rosen, W.; Qian, S.; Beskok, A. Induced-Charge Electroosmosis Around Touching Metal Rods. J. Fluids Eng. 2013, 135 (2), 021103. (14) Levitan, J. A.; Devasenathipathy, S.; Studer, V.; Ben, Y.; Thorsen, T.; Squires, T. M.; Bazant, M. Z. Experimental Observation of Induced-Charge Electro-Osmosis around a Metal Wire in a Microchannel. Colloids Surf., A 2005, 267 (1−3), 122−132. (15) Zhang, L.; Zhu, Y. Dielectrophoresis of Janus Particles under High Frequency Ac-Electric Fields. Appl. Phys. Lett. 2010, 96 (14), 141902. (16) Honegger, T.; Lecarme, O.; Berton, K.; Peyrade, D. 4-D Dielectrophoretic Handling of Janus Particles in a Microfluidic Chip. Microelectron. Eng. 2010, 87 (5−8), 756−759. (17) Chen, J.; Zhang, H.; Zheng, X.; Cui, H. Janus Particle Microshuttle: 1D Directional Self-Propulsion Modulated by AC Electrical Field. AIP Adv. 2014, 4 (3), 031325. (18) Bazant, M. Z.; Kilic, M. S.; Storey, B. D.; Ajdari, A. Towards an Understanding of Induced-Charge Electrokinetics at Large Applied Voltages in Concentrated Solutions. Adv. Colloid Interface Sci. 2009, 152 (1−2), 48−88. (19) Soni, G.; Squires, T. M.; Meinhart, C. D. Nonlinear Phenomena in Induced Charge Electroosmosis. In ASME Proceedings: Micro and Nano Systems, ASME 2007 International Mechanical Engineering Congress and Exposition, Seattle, Washington, Nov 11−15, 2007; ASME: New York, 2007; pp 761−771. (20) Schnitzer, O.; Yariv, E. Induced-Charge Electro-Osmosis beyond Weak Fields. Phys. Rev. E 2012, 86 (6), 061506. (21) González, A.; Ramos, A.; García-Sánchez, P.; Castellanos, A. Effect of the Combined Action of Faradaic Currents and Mobility Differences in Ac Electro-Osmosis. Phys. Rev. E 2010, 81 (1), 016320.
4. CONCLUSIONS The measurement of microscale hydrodynamic flows using μPIV is complicated by the difficulty in obtaining “ideal tracers”, which are unaffected by competing forces within the system. Within this work, we use 3D-2C PIV to examine the hydrodynamic flow patterns over the height of 15 μm JPs adjacent to both insulating and conducting walls using 300 nm fluorescent tracers. In both cases, far from the wall, the tracers behave ideally, tracking the flow and revealing flow patterns that correspond well with previous experimental and analytical results. However, in the case of the conducting wall, the proximity of the metallic hemisphere of the JP to the wall results in localized field gradients that act on the tracer particles inducing dielectrophoretic trapping. As well as limiting the ability of the μPIV to obtain quantitative measurements near the wall, the resultant aggregation impedes visualization of hydrodynamic flow close to the surface (particularly the metallic hemisphere). Nevertheless, we were able to successfully observe the presence of strong injection, commensurate with EHD flow induced by the proximity of a particle to an electrode25 which competes with the ICEO around the JP. Further study of these rich EHD phenomena, by varying tracer size and electrolyte composition and concentration to measure flow and DEP trapping around metallic, dielectric and JP particles, is intended for future work. It is expected that such flow, as well as the localized gradients generated by the proximity of the metallic hemisphere to the wall would have a significant effect on the mobility of freely suspended JPs translating near a conducting wall and may be responsible for some of the quantitative discrepancy between previous experiments9 and theory.
■
Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors would like to acknowledge US−Israel Binational Science Foundation Grant 2009371 and the RBNI and Gutwirth graduate fellowships. AB acknowledges partial support from ISF 1945/14. The JP preparation was possible through the financial and technical support of the Technion RBNI (Russell Berrie Nanotechnology Institute) and MNFU (Micro Nano Fabrication Unit). We also thank Dr. Sinwook Park and Matan Zehavi for technical assistance and Prof. Touvia Miloh for helpful discussions. 8249
DOI: 10.1021/acs.langmuir.5b01199 Langmuir 2015, 31, 8243−8250
Article
Langmuir (22) Pascall, A. J.; Squires, T. M. Induced Charge Electro-Osmosis over Controllably Contaminated Electrodes. Phys. Rev. Lett. 2010, 104 (8), 088301. (23) Miloh, T. Dipolophoresis of Janus Nanoparticles in a Microchannel. Electrophoresis 2013, 34 (13), 1939−1949. (24) Kilic, M. S.; Bazant, M. Z. Induced-Charge Electrophoresis near a Wall. Electrophoresis 2011, 32 (5), 614−628. (25) Ristenpart, W. D.; Aksay, I. A.; Saville, D. A. Electrohydrodynamic Flow around a Colloidal Particle near an Electrode with an Oscillating Potential. J. Fluid Mech. 2007, 575, 83−109. (26) Ristenpart, W. D.; Jiang, P.; Slowik, M. A.; Punckt, C.; Saville, D. A.; Aksay, I. A. Electrohydrodynamic Flow and Colloidal Patterning near Inhomogeneities on Electrodes. Langmuir 2008, 24 (21), 12172− 12180. (27) Yariv, E.; Schnitzer, O. Electrokinetic Particle-Electrode Interactions at High Frequencies. Phys. Rev. E 2013, 87 (1), 012310. (28) Yariv, E. Electro-Hydrodynamic Particle Levitation on Electrodes. J. Fluid Mech. 2010, 645, 187−210. (29) Fagan, J. A.; Sides, P. J.; Prieve, D. C. Mechanism of Rectified Lateral Motion of Particles near Electrodes in Alternating Electric Fields below 1 kHz. Langmuir 2006, 22 (24), 9846−9852. (30) 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 (1), 011023. (31) Prieve, D. C.; Sides, P. J.; Wirth, C. L. 2-D Assembly of Colloidal Particles on a Planar Electrode. Curr. Opin. Colloid Interface Sci. 2010, 15 (3), 160−174. (32) Park, J. S.; Choi, C. K.; Kihm, K. D. Optically Sliced Micro-PIV Using Confocal Laser Scanning Microscopy (CLSM). Exp. Fluids 2004, 37 (1), 105−119. (33) Wereley, S. T.; Meinhart, C. D. Recent Advances in MicroParticle Image Velocimetry. Annu. Rev. Fluid Mech. 2010, 42 (1), 557− 576. (34) Zhao, H.; Bau, H. H. On the Effect of Induced Electro-Osmosis on a Cylindrical Particle Next to a Surface. Langmuir 2007, 23 (7), 4053−4063. (35) Happel, J.; Brenner, H. Low Reynolds Number Hydrodynamics: With Special Applications to Particulate Media, 1983 ed.; Kluwer: The Hague/Boston/Hingham, MA, 1983. (36) Squires, T. M.; Bazant, M. Z. Induced-Charge Electro-Osmosis. J. Fluid Mech. 2004, 509, 217−252. (37) Fagan, J. A.; Sides, P. J.; Prieve, D. C. Evidence of Multiple Electrohydrodynamic Forces Acting on a Colloidal Particle near an Electrode due to an Alternating Current Electric Field. Langmuir 2005, 21 (5), 1784−1794. (38) Basuray, S.; Chang, H.-C. Induced Dipoles and Dielectrophoresis of Nanocolloids in Electrolytes. Phys. Rev. E 2007, 75 (6), 060501.
8250
DOI: 10.1021/acs.langmuir.5b01199 Langmuir 2015, 31, 8243−8250
