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2008, 112, 16192–16195 Published on Web 09/27/2008
Streaming Current Measurements in a Glass Microchannel Array Ali Mansouri,† Larry W. Kostiuk, and Subir Bhattacharjee* Department of Mechanical Engineering, UniVersity of Alberta, Edmonton, AB, T6G 2G8, Canada ReceiVed: August 08, 2008; ReVised Manuscript ReceiVed: September 17, 2008
A glass microchannel array coated with a layer of gold on its end faces was used to perform a cyclic voltammetric study in a four-electrode cell. The cyclic voltammetry was conducted by altering the direction of the pressure driven flow across the channel and measuring the transient voltage and current response. The technique provides information regarding electrode polarization, and its influence on the measured currents and potentials. The key observations pertain to the uncertainties in streaming current measurements, and emphasizes the importance of electrode and reservoir impedance in these measurements. Streaming current (SC) and streaming potential (SP) for a finite length capillary transporting an electrolytic fluid due to an imposed pressure difference are defined, respectively, as the net rate of convective transport of charged species through the channel, and the open circuit transcapillary potential difference.1,2 These quantities, if measured appropriately and accurately, can be used to characterize interfacial properties (e.g., surface charge density or ζ-potential) of the capillary material by application of suitable models.3-5 SC and SP across a capillary microchannel are typically measured by placing an electrode in the inlet reservoir and another in the outlet reservoir.6,7 SP is measured by connecting the electrodes to a high impedance voltmeter, whereas SC is measured by connecting the electrodes to a low impedance ammeter. With very little modification, these concepts and approaches to measurement have routinely been employed to characterize the interfaces of porous media with unstructured pore geometry.8-12 Even for similar characteristic hydrodynamic dimensions, key differences exist between the single channel and porous media cases, including the effects of the unknown pore structure9,13-16 and vastly different electrical properties (impedance) in the channels relative to the reservoirs. The present study isolates these effects by using a highly structured porous medium consisting of a bundle of parallel circular microcapillaries to avoid the ambiguities introduced by the unstructured geometry. Focusing solely on the electrical characteristics, this letter describes an experimental study aimed toward bridging the gap between SC/SP measurements in single capillaries and a bundle of parallel capillaries, and by implication to the more general case of unstructured porous media. A 25 mm diameter disk-shaped glass microchannel array (GMA) consisting of 3 437 500 circular and parallel untreated lead silicate glass (45% SiO2, 55% PbO) microchannels, each of 10 µm diameter and 2 mm length, was employed in this study. Both faces of this GMA were coated with a 100 nm layer of gold (Au), which were supported by adhesion layers of Nichrome. The coated GMAs were supplied by Photonis USA * To whom correspondence should be addressed. E-mail: Subir.B@ ualberta.ca. Tel: (780) 492 6712. Fax: (780) 492 2200. † Present address: NOVA Chemicals Research & Technology Center, Calgary, AB, T2E 7K7, Canada.
10.1021/jp807114n CCC: $40.75
Inc. (formerly Burle Electro Optics Inc.), Sturbridge, MA, and used without further modification. The GMA was placed in a cell containing two additional platinized platinum (Pt) wire mesh electrodes in the two reservoirs located off its two ends. The Pt electrodes were prepared by electrodeposition at 50 mV from 2% chloroplatinic acid in 1 M HCl onto circular (25 mm diameter) pieces of pure platinum gauze (45 mesh woven from 0.198∼mm diameter wire, 99.9% Pt, Alfa Asesar, MA, USA). The position of the Pt electrodes in the reservoirs could be varied with a precision of (0.5 mm. Potential and current could be measured simultaneously through any combination of the Au and Pt electrodes, allowing four-electrode measurements. The scanning electron micrographs of the GMA and schematic representation of the cell are shown in Figure 1. Two electrometers (Keithley Inc., Model 6517A) controlled by a PC through a Labview program were used to measure either the potential or current between the electrodes at a frequency of 10 Hz to capture the transient response to changes in the flow. An aqueous electrolyte containing either 0.05, 0.1, or 0.5 mM KCl added to deionized water was pumped through the GMA in alternating flow directions employing a diaphragm pump (Shurflo Inc. USA) and two 3-way valves. The frequency of step changes in the flow direction was between 0.2-0.5 Hz. A flow rate of (0.72 L/min. was used. Conductivity and pH of the electrolyte were measured (Accumet AR50, Fisher Scientific, CA). All experiments were conducted by cyclically reversing the flow direction through the GMA. The first cycle (starting from a stationary state) was excluded in all analysis. Two modes of measurements were performed. First, a series of two-electrode measurements of potential or current were conducted employing either the Au or Pt electrode pairs. These measurements were used to determine the SP or the current (care being taken not to refer to this current as the SC). Second, experiments were performed in a cyclic voltammetric (CVA) mode simultaneously employing the four electrodes. In these experiments, the current and potential were measured using the Au and Pt electrodes, respectively. This hydrodynamically pulsed voltammetry method differs from standard CVA in that the potential difference is induced by the cyclically altering flow of electrolyte through the GMA and not by an external programmed potential source. 2008 American Chemical Society
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J. Phys. Chem. C, Vol. 112, No. 42, 2008 16193
Figure 3. Simultaneous measurement of current and voltage by Au and Pt electrodes, respectively, across the GMA. An aqueous 0.1 mM KCl solution was pumped through the GMA in alternating flow directions. Figure 1. (a) SEM image of a glass microchannel array (GMA) coated with a ∼100 nm layer of Au, showing an array of single, straight, and circular microchannels. (b) Longitudinal cross section of the microchannels, showing the Au penetration inside the capillaries. Bright regions indicate the Au deposited area. The GMA has a pore size of 10 µm with 55% porosity. SEM micrographs are courtesy of Photonis USA Inc. (formerly Burle Electro Optics Inc.) (c) Schematic of the experimental cell.
TABLE 1: Variation of Calculated SP, SC, Overall Resistance, R, and Capacitance, C, of the GMA, Along with the Estimated Values of the Resistance and Capacitance (Re and Ce, respectively) of the Gold Electrodes Based on an Equivalent Electrical Circuit Representation of the System electrolyte conc (mM KCl) SP (V) SC (µA) R (kΩ) C (nf) Re (kΩ) 0.05 0.1 0.5
Figure 2. External currents and streaming potentials measured with Au and Pt electrodes across the GMA and within the reservoirs for electrolyte solution of 0.1 mM KCl.
Figure 2 depicts the maximum potentials and the maximum external currents measured in the two-electrode configuration by placing the Pt electrodes at different distances from the GMA. The data corresponding to zero separation were obtained by the Au electrodes. In these experiments, the flow direction was altered every five seconds. The maximum current was recorded immediately after the flow reversal (the subsequent decay in current was attributed to the polarization of the electrodes), whereas the maximum potential was recorded immediately before changing the flow direction and had appeared to reach a steady state value. The potential difference recorded (circles) was 1.0 V, which we consider as a measure of the SP across the GMA, since no current was being drawn from the system. It is evident that SP measured by the Au electrodes was in agreement with those measured anywhere in the reservoir by the Pt electrodes. Furthermore, the measured SP compared extremely well with the independently calculated SP for the GMA in the Smoluchowski limit (Table 1). In contrast, external currents (squares) measured using different electrodes (Au or Pt) or different positions of the Pt electrodes in the reservoirs were different. Only the peak current measured by the Au
2.5 1.0 0.25
348 319 227
7.18 3.13 1.10
0.28 0.28 0.28
6.66 9.37 10.55
Ce (nf) 1.5 × 105 1.7 × 105 2.5 × 105
electrode was in reasonable agreement with the theoretically obtained SC for the GMA in the Smoluchowski limit (Table 1). It is discernible that the resistance of the electrolyte solution in the reservoirs attenuated external currents measured by the Pt electrodes. The transient current and voltage measured in the fourelectrode configuration with the Au and the Pt electrodes, respectively, while cyclically reversing the flow direction through the GMA, is depicted in Figure 3. Altering the flow direction results in the reversal of the current, with the maximum current ((319 µA) recorded almost instantaneously after changing the flow direction. Following this, the measured current diminishes in each cycle. The maximum potential difference measured across the GMA ((0.7V) is less than the SP (Figure 2), and corresponds to the minimum nonzero current. The ideal transient response of the current and the potential difference across a single capillary subjected to an applied pressure gradient was discussed in our earlier study.6 Briefly, the maximum current (SC) is attained almost instantaneously after imposition of a pressure gradient. The streaming current raises the transcapillary electric field, which, in turn, induces an oppositely directed conduction current. Over time, the total current (streaming + conduction) approaches zero, and the transcapillary potential difference increases to the SP as the system relaxes to a steady state steady flow (SSSF) condition. This led to the depiction of the equivalent circuit of a single microchannel as a current source in parallel with an RC element (cf. Figure 6 of Mansouri et al.6). This ideal transient response is also expected in the GMA as described in the Supporting Information. For N parallel capillaries in a GMA, the overall
16194 J. Phys. Chem. C, Vol. 112, No. 42, 2008 resistance will scale as Ri/N whereas the capacitance will be NCi, where Ri and Ci are resistance and capacitance of a single microchannel, respectively. The total current passing through the GMA will be the sum of currents through the individual capillaries. The experimental I-V response of the GMA in Figure 3 follows this trend, except that the current does not decay to zero at the end of each cycle, and the maximum potential difference is less than the SP. This is expected in the four-electrode configuration, since withdrawal of current through the Au electrodes lowers the potential difference across the Pt electrodes. Details of the equivalent electrical circuit of this system and the resulting I-V behaviors are also provided in Supporting Information. For aqueous KCl solutions, the overall resistance and capacitance of the structured GMA can be independently calculated. Using these values and fitting the transient I-V response to an equivalent electrical circuit of the system (described in Supporting Information), the impedance of the Au electrodes can be determined. Table 1 shows the resistance and capacitance of these electrodes under different electrolyte concentrations. It is evident that the capacitance of the electrodes, Ce, is much larger than that of the GMA. However, the resistance of the GMA and the electrodes are quite comparable. Figure 4 shows the voltammograms (open circles) extracted from transient response of current and voltage for three different ionic strengths. For comparison, the measured SP and the estimated SC based on Smoluchowski theory17 are shown in each case (filled symbols). The theoretical voltammogram based on a simple equivalent circuit representation of the porous medium is given by the polygon ABOCDOA in Figure 4a (calculations shown in Supporting Information). At low ionic strengths, (Figure 4a,b), the initial peak currents measured by the Au electrodes are in close proximity to the Smoluchowski estimates of SC. However, for 0.5 mM KCl (Figure 4c), the current measured using the four-electrode system deviates substantially from the SC. Furthermore, in the four electrode configuration, the SP can never be measured accurately as long as the Au electrodes are connected to an ammeter. Key observations from these plots are the following: (i) the steadystate currents or currents drawn after a finite time do not represent the SC, (ii) in case of dilute electrolyte solutions (0.05 and 0.1 mM KCl), the initial peak current closely approximates the SC, and (iii) in case of higher electrolyte concentrations, none of the transient measurements of current approach the SC. All the above observations stem from the fact that the overall resistance of the GMA is considerably less than that of a single capillary, in which case, the resistance of the electrodes (Table 1) or even the resistances associated with the ammeters may influence the current measurement. Current measurements for such systems can be seriously affected by the impedance of the electrode/electrolyte interfaces and external or reservoir fluid resistances. In this case, using an electrode pair coated directly on the GMA reduces the influence of the reservoir fluid resistances. Nevertheless, the timescales of the polarization of the GMA and the electrode/electrolyte interfaces come into bearing in such situations, and unless very rapid measurements are performed, even the initial peak current measured by the Au electrodes cannot be regarded as a measure of SC (Figure 4c). Under no circumstances do the current measurements using the Pt electrodes placed in the reservoirs provide a correct estimate of the SC through the GMA. Cyclic voltammetric studies as reported in this letter may be used to assess whether current measurements across a porous medium can be employed to estimate streaming currents.
Letters
Figure 4. Voltammograms from transient experiments in the four electrode system (open circles) for electrolyte solutions containing (a) 0.05, (b) 0.1, and (c) 0.5 mM KCl. The SP (filled circle) was measured independently using the Pt electrode pair. From this, the SC (filled square) was evaluated in the Smoluchowski limit. Each SP/SC pair for a given flow direction is joined by a solid line.
It is evident from the above results that direct meausrement of streaming current is nontrivial for porous media under most experimental conditions, particularly at higher solution conductivities. Typially, the polarizability of the current measuring electrodes adversely affect the measurements. In this context, the present cyclic voltammetry method, in conjuction with the GMA as a model porous structure, provides an accurate estimate of the electrode polarizability (more specifically, the electrode impedance). It is demonstrated here that coating the porous medium with electrodes eliminates the reservoir resistance, and reduce uncertainties in measurement of SC. In many modern microfluidic applications, it is virutally impossible to insert elaborate electrode arrangements for accurate electrical char-
Letters acterization. In particular, avoiding electrode polarization is becoming increasingly difficult since one needs to fabricate these miniature electrodes in microscopic volumes employing a limited set of materials (typically Au on glass). In such situations, collection of a large amount of data and analysis of these using transient response of an equivalent electrical circuit might be the only recourse for accurate electrical characterization of these systems. Acknowledgment. We are grateful to Photonis USA Inc. (formerly Burle Electro Optics Inc.) for custom fabricating the gold-coated glass microchannel arrays and for providing the SEM micrographs. Financial support from Canada Research Chair (CRC) Program, Canada Foundation for Innovation (CFI), and Natural Sciences and Engineering Research Council of Canada (NSERC) are gratefully acknowledged. We thank A. Mahmoodi for fruitful discussions on electrical analogy. Supporting Information Available: A brief description of the analysis of transient current measurements and the cyclic voltammetric experiments in the Smoluchowski limit is provided. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Quincke, G. Poggendorff’s Ann. Phys. 1859, 107, 1. (2) Sennett, P.; Olivier, J. P. Ind. Eng. Chem. 1965, 57, 32–50.
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