Streaming Current Measurements Research Articles

Streaming Current for Surfaces Covered by Square and Hexagonal Monolayers of Spherical Particles.

The interface and particle contributions to the streaming current of flat substrates covered with ordered square or hexagonal monolayers of spherical particles were theoretically evaluated for particle coverage up to close packing. The exact numerical results were approximated using fitting functions that contain exponential and linear terms to account for hydrodynamic screening and charge convection from the particle surfaces exposed to external flow. According to our calculations, the streaming currents for the ordered and random particle arrangements differ within a typical experimental error. Thus, streaming-current measurements, supplemented with our fitting functions, can be conveniently used to evaluate the particle coverage without detailed knowledge of the particle distribution. Our results for equal interface and particle ζ-potentials indicate that roughness can reduce the streaming current by more than 30%, even in the limit of the small size of spherical roughness asperities.

Surface Patterning of Closed Nanochannel Using VUV Light and Surface Evaluation by Streaming Current.

In nanofluidics, surface control is a critical technology because nanospaces are surface-governed spaces as a consequence of their extremely high surface-to-volume ratio. Various surface patterning methods have been developed, including patterning on an open substrate and patterning using a liquid modifier in microchannels. However, the surface patterning of a closed nanochannel is difficult. In addition, the surface evaluation of closed nanochannels is difficult because of a lack of appropriate experimental tools. In this study, we verified the surface patterning of a closed nanochannel by vacuum ultraviolet (VUV) light and evaluated the surface using streaming-current measurements. First, the C18 modification of closed nanochannels was confirmed by Laplace pressure measurements. In addition, no streaming-current signal was detected for the C18-modified surface, confirming the successful modification of the nanochannel surface with C18 groups. The C18 groups were subsequently decomposed by VUV light, and the nanochannel surface became hydrophilic because of the presence of silanol groups. In streaming-current measurements, the current signals increased in amplitude with increasing VUV light irradiation time, indicating the decomposition of the C18 groups on the closed nanochannel surfaces. Finally, hydrophilic/hydrophobic patterning by VUV light was performed in a nanochannel. Capillary filling experiments confirmed the presence of a hydrophilic/hydrophobic interface. Therefore, VUV patterning in a closed nanochannel was demonstrated, and the surface of a closed nanochannel was successfully evaluated using streaming-current measurements.

Evolution of calcite surfaces upon thermal decomposition, characterized by electrokinetics, in-situ XRD, and SEM

The present study analyses transformation pathways of pristine and thermally treated porous limestone and dense marble surfaces by means of time-resolved streaming current and potential measurements coupled with scanning electron microscopy and in-situ X-ray diffraction. The results reveal that under nonequilibrium conditions the zeta potential (ζ) of natural carbonates may exhibit positive and negative signs and ζ drifts in opposite directions. Sample surface roughness influences ζ because it contributes to dissolution, as observed particularly in the initial period of time-resolved measurements. Thermal treatment causes a temporary charge reversal from negative to positive. The reactivity of calcium hydroxide on calcite surfaces governs the net electrokinetic potential and isoelectric point (IEPpH), even at low surface coverage, as cross-validated by in-situ XRD. It was also found that pore conductivity may lead to ~90% underestimation of ζ when assessed by streaming potential. SEM studies revealed micro cracks inducement on marble after thermal treatment, which can result in underestimation of ζ up to the same extent as for the porous limestone. When an asymmetric cell configuration involving calcite and polypropylene surfaces is used, the fractional contribution of polypropylene to the IEPpH is 0.3 and to the overall determined ζ up to 0.5. Our findings contribute to the understanding of nonequilibrium and time-dependent electrokinetic potential modifications associated with the reactivity of porous surfaces. This study highlights the effectiveness of the streaming potential technique for monitoring such changes further supported by the use of ancillary techniques to analyze the extend of chemo-mineralogical and physical alterations.

The importance of specifically adsorbed ions for electrokinetic phenomena: Bridging the gap between experiments and MD simulations

Molecular Dynamics (MD) simulations are uniquely suitable for providing molecular-level insights into the Electric Double Layer (EDL) that forms when a charged surface is in contact with an aqueous solution. However, simulations are only as accurate in predicting EDL properties as permitted by the atomic interaction models. Experimental ζ-potential values and surface charges could provide a potentially suitable reference to validate and tune the interaction models, if not for the fact that they themselves are a product of imperfect models used to interpret the raw measurement data. Here, we present an approach to tune an interaction model by comparing Electro-Osmotic Flow (EOF) MD simulations against experimental Streaming Current (SC) measurements while minimizing potential modeling errors arising from both approaches. The point that is least susceptible to interpretation and modeling errors is argued to be at the concentration for which zero flow velocity is observed in EOF simulations and a net zero electric current is measured in SC experiments. At this concentration, the ζ-potential is also zero. We were able to match the experimental concentration at which ζ = 0 in MD simulations for a CaCl2 solution at pH 7.5 in contact with fused silica by tuning the ion-surface Lennard-Jones cross interactions. These interactions were found to greatly affect the ion distribution within the EDL and particularly the formation of inner-sphere surface-complexes, which, in turn, affects the electrokinetic flow. With the ion distribution determined explicitly, a series of properties can be calculated unambiguously, such as the capacitance needed for surface complexation models.

Interaction of Polyoxometalates and Nanoparticles with Collector Surfaces—Focus on the Use of Streaming Current Measurements at Flat Surfaces

Streaming current measurements were used to study the interaction of polyoxometalates (POMs) and nanoparticles (NPs) with flat surfaces as an alternative, innovative approach to infer POM and NP properties of potential sparse material in terms of charge and magnitude. With respect to POMs, the approach was able to reveal subtle details of charging properties of +7 vs. +8 charge at very low POM concentrations. For NPs, the sign of charge and even the zeta-potential curve was retrieved. Concerning NPs, mutual interaction between TiO2 and SiO2 surfaces was studied in some detail via macroscopic measurements. Post-mortem analysis of samples from electrokinetic studies and separate investigations via AFM and HRTEM verified the interactions between TiO2 NPs and SiO2 collector surfaces. The interactions in the SiO2/TiO2 system depend to some extent on NP morphology, but in all our systems, irreversible interactions were observed, which would make the studied types of NPs immobile in natural environments. Overall, we conclude that the measurement of streaming currents at flat surfaces is valuable (i) to study NP and POM collector surface interactions and (ii) to simultaneously collect NPs or POM (or other small mobile clusters) for further (structural, morphological or release) investigations.

Antimicrobial films of poly(2-aminoethyl methacrylate) and its copolymers doped with TiO2 and CaCO3

Polymeric coatings with positive surface charge offer potential antimicrobial activity, which they owe to a simple electrostatic attraction with negatively charged bacterial walls and membranes. We describe synthesis and characterization of poly(2-aminoethyl methacrylate) and its copolymers with methyl methacrylate and butyl acrylate, as potential binders for antimicrobial solvent-cast paints. TiO2 and CaCO3 mineral particles were employed as model pigments/fillers, as they are used in most real-life paint formulations. Electrokinetic (ζ) potential and antimicrobial activity of thin films made of the (co)polymers in the absence and presence of TiO2 and CaCO3 nanopowders were assessed using streaming current measurements and microbial growth inhibition tests, respectively. Independently of the structure of the monomers used for the synthesis, the films showed positive ζ-potential values (up to +95 mV) in the pH range 3.5-8.0. The presence of mineral particles at 50% dry weight of the films did not affect significantly the ζ(pH) curves. The films made of the mixed dispersions remained positively charged and inhibited growth of both Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria, as well as yeast (C. albicans). The mixed polymeric-mineral films described in this study seem to be promising potential candidates for designing antimicrobial coatings aimed to prevent spreading of bacterial infections.

Degradation of poly(ether sulfone)/polyvinylpyrrolidone membranes by sodium hypochlorite: insight from advanced electrokinetic characterizations.

Poly(ether sulfone) (PES)/polyvinylpyrrolidone (PVP) membranes are widely used in various industrial fields such as drinking water production and in the dairy industry. However, the use of oxidants to sanitize the processing equipment is known to impair the integrity and lifespan of polymer membranes. In this work we showed how thorough electrokinetic measurements can provide essential information regarding the mechanism of degradation of PES/PVP membranes by sodium hypochlorite. Tangential streaming current measurements were performed with ultrafiltration and nanofiltration PES/PVP membranes for various aging times. The electrokinetic characterization of membranes was complemented by FTIR-ATR spectroscopy. Results confirmed that sodium hypochlorite induces the degradation of both PES and PVP. This latter is easily oxidized by sodium hypochlorite, which leads to an increase in the negative charge density of the membrane due to the formation of carboxylic acid groups. The PVP was also found to be partly released from the membrane with aging time. Thanks to the advanced electrokinetic characterization implemented in this work it was possible for the first time to demonstrate that two different mechanisms are involved in the degradation of PES. Phenol groups were first formed as a result of the oxidation of PES aromatic rings by substitution of hydrogen by hydroxyl radicals. For more severe aging conditions, this membrane degradation mechanism was followed by the formation of sulfonic acid functions, thus indicating a second degradation process through scission of PES chains.

Transient streaming current measurements in nanochannels for molecular detection

Streaming currents were measured in silica slit-like nanochannels with integrated electrodes using a pulsed flow of deionized water. The analysis of the transient streaming current measurements in nanochannels shows that streaming currents are proportional to the surface charge density σ and that the effective reduction of nanochannel height by immobilized biomolecules needs to be taken into account. The σ of silica nanochannels was measured after performing sequential chemical surface modification steps. The nanochannel surface was first altered with positively charged polylysine and then with non-charged polyethylene glycol showing that nanochannels can sense the charge and size of surface-immobilized molecules.

Streaming currents in microfluidics with integrated polarizable electrodes

A surface in contact with an aqueous solution is electrically characterized by the zeta potential. One way of determining indirectly the zeta potential of a surface is by measuring the streaming currents generated by a Poiseuille flow through a capillary channel with charged walls. We report measurements of streaming current in individual rectangular glass/PDMS microchannels with integrated miniaturized electrodes. Experiments performed using solutions with different salt concentrations and different electrode materials showed that the measured electrical current depends on the electrode material and in general differs from the real value of the streaming current. To determine the streaming current from the experimental data, an equivalent circuit model is proposed. The extracted value of the streaming current is proportional to the flow rate of electrolyte and the calculated glass/PDMS zeta potential scales linearly with the logarithm of the salt concentration. This work offers a thorough analysis of the effects that come into play during streaming current measurements and, in particular, it describes potential sources of error that can affect the streaming current measurements and suggestions on how to correct the measured values.

Tangential streaming potential/current measurements for the characterization of composite membranes

The zeta-potential is an important and reliable indicator of the electrokinetic charge density. Streaming potential measurement is the most commonly used tool for determining the zeta-potential of membranes. Measurements of streaming potential can be performed in a transversal or tangential mode. Even if the tangential mode is used for some 15yrs (and also applied for a few years to tangential streaming current measurements), these measurements could have been misinterpreted for a number of studies due to the fact that the contribution of the membrane porous body to the streaming current was not considered. In this paper, this phenomenon is illustrated by performing streaming potential, streaming current and electric conductance measurements at various channel heights with three composite polymer membranes. It will be shown that: (i) the correct value of zeta-potential of external membrane surface can be determined either from coupled streaming potential coefficient (Δφs/ΔP) and electric conductance (Gt) measurements or directly from streaming current coefficient (Is/ΔP) measurements, both being necessarily performed at various channel heights, (ii) the measurement of the streaming potential coefficient alone is inefficient even if the channel height is varied, (iii) the combination of streaming potential coefficient measurements with electric conductance measurements or streaming current coefficient measurements for a single channel height is also inefficient due to the possible contribution of the membrane porous body to the streaming current (this procedure being only efficient if the membrane porous body makes a contribution only to the electric conductance), and (iv) the use of “conventional” methods to convert electrokinetic measurements into zeta-potential (by means of the Helmholtz–Smoluchowski relation, from the product (Δφs/ΔP)×Gt or Is/ΔP) can lead to an erroneous zeta-potential of external surface. In the present study, overestimated values were even found by using these classical methods, which has not been yet reported in the literature. Last, it will also be shown that the zeta-potential of pore surface of the membrane support can be determined from tangential electrokinetic measurements, which is not possible for the active layer pores.

Anion selectivity at the aqueous/polymeric membrane interface: A streaming current study of potentiometric Hofmeister effect

The anion-selectivity of model polymeric membranes as used in potentiometric sensors (ion-selective electrodes, ISE) was studied using ζ-potential measurements. For this purpose the streaming current measurements of plasticized poly(vinyl chloride) were performed by pushing an aqueous electrolyte through a thin channel with walls lined with the polymer. Despite the literature reports on accumulation of plasticizer on the PVC membrane surface, no significant effect of plasticizer on the ζ-potential vs pH curve was observed. A cationic surfactant, cetyltrimethylammonium bromide (CTAB) was chosen as a water-soluble analogue of tetraalkylammonium salts used in anion-selective ISE displaying a characteristic Hofmeister selectivity pattern. The effect of anions on ζ-potential was studied at fixed concentration of CTAB (10−4molL−1≈0.1cmc), by varying the concentration and nature of anion in the added 1:1 electrolytes (NaCl, NaBr, NaF, NaClO4, NaI and NaNO3). The anions chosen cover the whole Hofmeister series, and their influence on the measured ζ-potential parallels that of the Hofmeister effect in potentiometry, as well as in colloid/interface science. A significant deviation was observed only for fluoride ions. The effect of anion selectivity on ζ-potential is clearly notable, but rather weak, due to a large distance between the location of the Stern layer (where the ion-specific interactions take place), and that of the hydrodynamic slip plane (where the ζ-potential is actually measured).

Electrokinetics of a Poly(N-isopropylacrylamid-co-carboxyacrylamid) Soft Thin Film: Evidence of Diffuse Segment Distribution in the Swollen State

Streaming current measurements were performed on poly(N-isopropylacrylamid-co-carboxyacrylamid) (PNiPAAM-co-carboxyAAM) soft thin films over a broad range of pH and salt concentration (pH 2.5-10, 0.1-10 mM KCl) at a constant temperature of 22 °C. The films are negatively charged because of the ionization of the carboxylic acid groups in the repeat unit of the copolymer. For a given salt concentration, the absolute value of the streaming current exhibits an unconventional, nonmonotonous dependence on pH with the presence of a maximum at pH ∼6.4. This maximum is most pronounced at low electrolyte concentration and gradually disappears with increasing salinity. Complementary ellipsometry data further reveal that the average film thickness increases by a factor of ∼2.2 with increasing pH over the whole range of salt concentration examined. The larger the solution salt concentration, the lower the pH value where expansion of the hydrogel layer starts to take place. The dependence of film thickness on pH and electrolyte concentration remarkably follows that obtained for surface conductivity. The streaming current and surface conductivity results could be consistently interpreted on a quantitative basis using the theory we previously derived for the electrokinetics of charged diffuse (heterogeneous) soft thin films complemented here by the derivation of a general expression for the surface conductivity of such systems. In particular, the maximum in streaming current versus pH is unambiguously attributed to the presence of an interphasial gradient in polymer segment density following the heterogeneous expansion of the chains within the film upon swelling with increasing pH. A quantitative inspection of the data further suggests that pK values of ionogenic groups in the film as derived from the streaming current and surface conductivity data differ by ∼0.9 pH unit. Such a difference is attributed to the impact of position-dependent hydrophobicity across the film on the degree of ionization of carboxylic sites.

Electrokinetics of Diffuse Soft Interfaces. IV. Analysis of Streaming Current Measurements at Thermoresponsive Thin Films

Streaming current measurements were performed on poly(N-isopropylacrylamide)-co-N-(1-phenylethyl) acrylamide [P(NIPAAm-co-PEAAm)] thermoresponsive thin films above and below the transition temperature of the polymer (i.e., at 22 and 4 degrees C, respectively). Electrokinetic measurements (ionic strength 0.01-10 mM KCl, pH 2.5-9.5 in 1 mM KCl) revealed that the charging of the polymer/aqueous solution interface is determined by unsymmetrical adsorption of hydroxide and hydronium ions onto the Teflon AF substrate that supports the hydrogel film. The magnitude of the streaming current significantly decreased with decreasing temperature, that is, when the hydrogel was swelling. The pH- and ionic strength-dependent data for unswollen and swollen films were interpreted on the basis of the here-reported general theory for the electrokinetics of diffuse soft gel layers. The formalism based on the Debye-Brinkman equation for hydrodynamics and the nonlinear Poisson-Boltzmann equation for electrostatics extends previous theoretical studies by considering the most general situation of a charged gel layer supported by a charged rigid surface. Full analytical expression is provided for the streaming current in the limit of homogeneous distribution of segments under low potential conditions. Numerical analysis of the governing transport and electrostatic equations allows for the computation of streaming current for cases where analytical developments are not possible. The theory successfully reproduces the electrokinetic data for the P(NIPAAm-co-PEAAm) copolymer film at 22 and 4 degrees C over the whole range of pH and ionic strength examined. It is found that the 3-fold increase of the hydrogel film thickness with decreasing temperature from 22 to 4 degrees C (i.e., from 23 to 70 nm as measured by ellipsometry), is in line with homogeneous swelling and an increase of the hydrodynamic penetration length (1/lambdao) by a factor of approximately 1.6. Additionally, the hydrodynamic thicknesses (deltaH) of the swollen and unswollen hydrogels are evaluated in terms of their respective hydrodynamic penetration length and electrosurface characteristics of the supporting Teflon AF surface.

The effect of double layer overlap on measured streaming currents for toluene flowing through sandstone cores

Streaming current measurements have been made on toluene flowing through sandstone rock cores, whose pore size was estimated by mercury porosimetry. The streaming current method, though requiring pico–amp measurements, was preferred over streaming potential as the latter is subject to surface conductivity errors. Having estimated pore area/length ratios an (apparent) electrokinetic potential was obtained which, as expected, increased numerically with pore size as a result of double layer overlap effects. After a measurement using a 1 mm internal diameter silica capillary, to represent effective infinite pore size, the data were fitted to a theoretical expression for double layer overlap in slit-like pores.

Streaming potential and streaming current measurements at planar solid/liquid interfaces for simultaneous determination of zeta potential and surface conductivity

Strongly encouraged and supported by Stanislav S. Dukhin the authors of this article recently designed, built and tested a new device for the simultaneous determination of zeta potential and surface conductivity from streaming potential and streaming current measurements across rectangular slit channels formed between two planar samples. In this Microslit Electrokinetic Set-up (MES) the planar samples are adjusted in parallel to form a channel of variable height which can become as narrow as about 1 μm. Due to this key feature of the device electrokinetic measurements can be performed at conditions where surface conductivity can be neglected and at conditions where surface conductivity provides a substantial part of the total channel conductivity. Utilizing the novel set-up, zeta potential and surface conductivity data can be obtained for a wide variety of materials which can be prepared as thin films on top of planar, macroscopic glass carriers. In order to demonstrate the potentialities of the advanced experimental technique of electrokinetic surface characterization we discuss three examples reflecting different levels of complexity of the analysed solid/liquid interface: (1) The charge formation at unpolar polymers without dissociating surface functions is studied referring to an inert, plasma-deposited fluoropolymer layer (PDFP) in simple electrolyte solutions. An extended evaluation of the experimental data of zeta potential and surface conductivity is given; (2) Further, grafted polypetide chains bearing dissociating side groups (polyglutamic acid and polylysine) were characterized with regard to the pH-depended variation of zeta potential and surface conductivity to provide new insights into the interrelation of charge density and conformation; (3) Finally, adsorbed fibrinogen on top of plasma-deposited fluoropolymer was studied by zeta potential and surface conductivity measurements as an example for highly hydrated macromolecular adsorption layers.

Measurement of the electrokinetic potential at reservoir rock surfaces avoiding the effect of surface conductivity

The surface (zeta) potentials of sandstone rock cores in aqueous and non-aqueous systems were investigated under flow conditions. Rock surface potentials determined by streaming potential measurements can be in very considerable error as a result of surface conductivity, whereas streaming currents are free from this disadvantage. Streaming current measurements do however need a knowledge of the ratio area/length ( A/ L) of the effective capillaries. A new method was developed to determine the A/ L ratio for porous systems by combining streaming potential and current measurements so that the unambiguous streaming current method can be used. The impact of pore (fluid) pressure on streaming potential and current measurements was investigated. It was observed that the change in the behaviour of streaming potential and current measurements as a function of pore pressure is direction dependent, exhibiting a strong hysteresis. This phenomenon of hysteresis is important because of its direct link with the estimation of area to length ratio of the capillaries and the related rock surface (zeta) potential measurements.

Streaming potential and streaming current measurements to estimate surface conduction and electric double layer structure of biomaterials.

Streaming potential and streaming current measurements to estimate surface conduction and electric double layer structure of biomaterials.

Alternating streaming current measurements

The streaming currents which result when a sinusoidally alternating fluid flow is forced through a cylindrical capillary allow the rapid and reproducible measurement of electrokinetic phenomena. An apparatus for measuring the currents can be constructed easily and inexpensively. The zeta potential of any material which can be formed as a capillary or can be produced as a homogeneous coating on a supporting capillary can be measured by this technique. The use of this apparatus does require that the system be calibrated using a capillary whose zeta potential has been determined by an absolute method.

Electrostatic streaming current developed in the turbulent flow through a pipe

The results of measurement of electrostatic streaming currents in turbulent flow through round pipes have been correlated in terms of non-dimensional groups. A non-dimensional current group is found to vary with the same power of the Reynolds number as does the skin friction coefficient. A surface roughness that is too small to affect the skin friction coefficient is found to affect markedly the streaming current.

Electrostatic streaming current developed in the turbulent flow through a pipe

The results of measurement of electrostatic streaming currents in turbulent flow through round pipes have been correlated in terms of non-dimensional groups. A non-dimensional current group is found to vary with the same power of the Reynolds number as does the skin friction coefficient. A surface roughness that is too small to affect the skin friction coefficient is found to affect markedly the streaming current.