Electrophoretic Velocity Research Articles

Nonlinear Electrophoresis of Colloids Controlled by Anisotropic Conductivity and Permittivity of Liquid-Crystalline Electrolyte

When a standard, isotropic electrolyte is replaced with a liquid crystal, a nonlinear mechanism of electrophoresis is introduced, and one unresolved issue is how the electrophoretic velocity depends on the crucial properties of the liquid crystal: the anisotropy of its dielectric permittivity and conductivity. The authors establish these dependencies with experiments and simulations, and demonstrate how to use them for controlled $r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ of electrokinetics, by changing either temperature or composition. This work embraces a wide range of problems, from electrokinetic phenomena and microfluidics to soft matter physics.

Faradaic Ion Concentration Polarization on a Paper Fluidic Platform.

We describe the design and characteristics of a paper-based analytical device for analyte concentration enrichment. The device, called a hybrid paper-based analytical device (hyPAD), uses faradaic electrochemistry to create an ion depletion zone (IDZ), and hence a local electric field, within a nitrocellulose flow channel. Charged analytes are concentrated near the IDZ when their electrophoretic and electroosmotic velocities balance. This process is called faradaic ion concentration polarization. The hyPAD is simple to construct and uses only low-cost materials. The hyPAD can be tuned for optimal performance by adjusting the applied voltage or changing the electrode design. Moreover, the throughput of hyPAD is 2 orders of magnitude higher than that of conventional, micron-scale microfluidic devices. The hyPAD is able to concentrate a range of analytes, including small molecules, DNA, proteins, and nanoparticles, in the range of 200-500-fold within 5 min.

Electrophoresis and diffusiophoresis of a colloidal sphere with double-layer polarization in a concentric charged cavity

The electrophoretic and diffusiophoretic motions of a dielectric spherical particle situated at the center of a spherical cavity filled with an electrolyte solution are studied analytically. The applied electric field and electrolyte concentration gradient are uniform; the electric double layers at the particle surface and cavity wall are thin relative to the radius of the particle and distance between the solid surfaces, but the diffuse-layer polarization effect over the particle surface is considered. After solving the equations of conservation governing the systems, explicit formulas for the electrophoretic and diffusiophoretic velocities of the confined particle are obtained and their results relative to those of a particle under identical conditions in an unbounded solution are presented for various values of the radius ratio and zeta potential ratio between the particle and the cavity and of other parameters in the systems. The contributions from the electroosmotic and diffusioosmotic flows occurring along the cavity wall and from the wall-corrected electrophoretic and diffusiophoretic driving forces to the particle velocities are equivalently important and can be superimposed due to the linearity of the problems. The normalized migration velocities of the particle in general decrease with an increase in the particle-to-cavity radius ratio and increase with an increase in the cavity-to-particle zeta potential ratio. The effects of the charged cavity wall on the electrokinetic migrations of the particle are significant and can reverse their directions.

One-step selective electrokinetic removal of inorganic anions from small volumes and its application as sample clean-up for mass spectrometric techniques

The presence of inorganic anions in a sample interferes with mass spectrometric (MS) analysis. Here, a simple method to remove these ions from a liquid sample in one-step is described. The inorganic anions present in a 50μL sample were extracted into a low pH solution inside a 200μm i.d.×33cm long capillary by the use of an electric field. The selective removal of unwanted anions and retention of target analytes was accomplished by control of the apparent electrophoretic velocities of anions and analytes at a boundary that separated the sample and extraction solution. No physical barrier (e.g., membrane) was required and with the boundary situated at the tip of the capillary, efficient removal of inorganic anions (e.g., >80% removal) and good recovery of target analytes (e.g., >80% recovery) were achieved. The time required for removal of the inorganic anions was found to depend on their initial concentrations. The removal process was investigated using different concentrations of bromide and nitrate (as potassium salts) and negatively chargeable drugs as target analytes. This micro-sample clean-up technique used no organic solvents and little consumables and was studied to the determination of 0.6μg/L arsenic and 8.3μg/L vanadium in 500mg/L sodium chloride using inductively coupled plasma MS and 50μM angiotensin I in 1000mg/L sodium chloride using electrospray ionisation MS. Micro-sample clean-up was performed for 45min at 3kV in both demonstrations. The calculated recoveries for the metals at trace levels were 110–130%, and for the peptide was 103.8%.

Broadening of analyte streams due to a transverse pressure gradient in free-flow isoelectric focusing

Pressure-driven cross-flows can arise in free-flow isoelectric focusing systems (FFIEF) due to a non-uniform electroosmotic flow velocity along the channel width induced by the pH gradient in this direction. In addition, variations in the channel cross-section as well as unwanted differences in hydrostatic heads at the buffer/sample inlet ports can also lead to such pressure-gradients which besides altering the equilibrium position of the sample zones have a tendency to substantially broaden their widths deteriorating the separations. In this situation, a thorough assessment of stream broadening due to transverse pressure-gradients in FFIEF devices is necessary in order to establish accurate design rules for the assay. The present article describes a mathematical framework to estimate the noted zone dispersion in FFIEF separations based on the method-of-moments approach under laminar flow conditions. A closed-form expression has been derived for the spatial variance of the analyte streams at their equilibrium positions as a function of the various operating parameters governing the assay performance. This expression predicts the normalized stream variance under the chosen conditions to be determined by two dimensionless Péclet numbers evaluated based on the transverse pressure-driven and electrophoretic solute velocities in the separation chamber, respectively. Moreover, the analysis shows that while the stream width can be expected to increase with an increase in the value of the first Péclet number, the opposite trend will be followed with respect to the latter. The noted results have been validated using Monte Carlo simulations that also establish a time/length scale over which the predicted equilibrium stream width is attained in the system.

The fabrication and characterization of a PLGA nanoparticle–Pheroid® combined drug delivery system

The combination of polymeric nanoparticles (NPs) as a core and lipid vesicles as a shell has emerged to be a robust and promising drug delivery strategy. This study explores the development of a novel combined delivery system where poly d,l, lactic-co-glycolic acid (PLGA) NPs are entrapped within Pheroid® drug delivery system. The solid NPs were combined with the Pheroid® vesicles using two different methods: pre-mix and post-mix. The surface properties of the PLGA NPs were altered through the inclusion (pos-NPs) and exclusion (neg-NPs) of chitosan (CT) and polyethylene glycol (PEG), to evaluate their interaction with the Pheroid® Vesicles. The average particle size of the novel NP–Pheroid® combined system ranged from approximately 1990–2450 nm while the zeta potential (ZP) ranged from −18 to −30 mV, measured using dynamic light scattering (DLS) and electrophoretic velocity techniques, respectively. The NP/Pheroid® mixing ratio experiment indicated that a maximum of 2.5% (w/v) NPs can be optimally added to the Pheroid® vesicles without compromising the structure and the stability of the NP–Pheroid® combined system. Visual analysis of this system was done through transmission electron microscopy (TEM), cryogenic (cryo) TEM and confocal laser scanning microscopy (CLSM) techniques to obtain adequate information of this novel combined drug delivery system which includes the localization of the PLGA NPs with the Pheroid ® vesicles.

Electric-field-assisted stagnation-swirl-flame synthesis of porous nanostructured titanium-dioxide films

Mesoporous nanostructured titanium-dioxide (TiO2) films, with high uniformity and low packing density, are synthesized in a stagnation swirl-flame setup under an applied electric field, with voltage bias on the substrate and the burner grounded. The effects of uniform external electric-field strength and polarity are studied for different substrate temperatures and precursor-loading concentrations. The results manifest considerable differences in film characteristics, for differing electric fields, with more columnar structures and higher porosities produced under low-magnitude substrate voltages up to |±400|V. Interestingly, films have higher packing density at higher magnitude voltages of |±800|V. At low substrate temperatures, the morphology and structure are more prominent owing to less on-substrate sintering of the nanoparticles. At low-magnitude substrate voltages, oppositely-charged particles are attracted to the substrate, increasing their electrophoretic velocity but decreasing their in-flame growth and agglomeration, resulting in smaller particle size. Along with van der Waals forces and induced dipole–dipole interactions, more columnar-structured growth with lower packing density results. At high-magnitude voltages, the particles seem to acquire the same charge polarity as the voltage bias applied, being repelled and residing in the flame longer, thus increasing in-flame agglomeration and particle size. More branched-structured film growth with high packing density develops.

Enhancement of surface adsorption-desorption rates in microarrays invoking surface charge heterogeneity

This investigation is devoted to the influences of non-uniform wall characteristics on the surface adsorption-desorption rates in an electrokinetic microarray. Utilizing already explored electroosmotic and electrophoretic velocities, the species transport equations are solved by a finite-volume-based numerical approach. Uniform, sinusoidal, and pulse-like distributions of the zeta potential are considered in the analysis. The developed model is validated by comparing the results with those of two analytical solutions that are derived for limiting conditions. The results reveal that, in some cases, the surface charge heterogeneity can reduce the saturation time by more than 60%. The efficacy of the surface charge heterogeneity is limited to high Damkohler numbers and its effects are negligible for small values of this parameter. Whereas the impact of non-uniform surface characteristics is amplified by increasing the Peclet number, it is reduced by increasing either of the channel length to radius ratio or the electrophoretic mobility of the analytes.

Nonlinear effects on electrophoresis of a charged dielectric nanoparticle in a charged hydrogel medium

The impact of the solid polarization of a charged dielectric particle in gel electrophoresis is studied without imposing a weak-field or a thin Debye length assumption. The electric polarization of a dielectric particle due to an external electric field creates a non-uniform surface charge density, which in turn creates a non-uniform Debye layer at the solid-gel interface. The solid polarization of the particle, the polarization of the double layer, and the electro-osmosis of mobile ions within the hydrogel medium create a nonlinear effect on the electrophoresis. We have incorporated those nonlinear effects by considering the electrokinetics governed by the Stokes-Brinkman-Nernst-Planck-Poisson equations. We have computed the governing nonlinear coupled set of equations numerically by adopting a finite volume based iterative algorithm. Our numerical method is tested for accuracy by comparing with several existing results on free-solution electrophoresis as well as results based on the Debye-Hückel approximation. Our computed result shows that the electrophoretic velocity decreases with the rise of the particle dielectric permittivity constant and attains a saturation limit at large values of permittivity. A significant impact of the solid polarization is found in gel electrophoresis compared to the free-solution electrophoresis.

Importance of core electrostatic properties on the electrophoresis of a soft particle

The impact of the volumetric charged density of the dielectric rigid core on the electrophoresis of a soft particle is analyzed numerically. The volume charge density of the inner core of a soft particle can arise for a dendrimer structure or bacteriophage MS2. We consider the electrokinetic model based on the conservation principles, thus no conditions for Debye length or applied electric field is imposed. The fluid flow equations are coupled with the ion transport equations and the equation for the electric field. The occurrence of the induced nonuniform surface charge density on the outer surface of the inner core leads to a situation different from the existing analysis of a soft particle electrophoresis. The impact of this induced surface charge density together with the double-layer polarization and relaxation due to ion convection and electromigration is analyzed. The dielectric permittivity and the charge density of the core have a significant impact on the particle electrophoresis when the Debye length is in the order of the particle size. We find that by varying the ionic concentration of the electrolyte, the particle can exhibit reversal in its electrophoretic velocity. The role of the polymer layer softness parameter is addressed in the present analysis.

Determination of chloramphenicol in biological matrices by solid-phase membrane micro-tip extraction and capillary electrophoresis.

Solid-phase membrane micro-tip extraction (SPMMTE) and capillary electrophoresis (CE) methods were developed and validated for analysis of chloramphenicol in human plasma and urine samples. Iron composite nanoparticles were prepared using green technology. CE was carried out using a silica capillary (60 cm × 50 μm i.d.), phosphate buffer (50 mm, 8.0 pH)-acetonitrile (95:5, v/v) as the background electrolyte, 10 kV voltage, 280 nm detection, 20 s injection time and 27 ± 1°C temperature. Frusemide was used as an internal standard. The values of migration time, electrophoretic mobility, electrophoretic velocity and theoretical plates of chloramphenicol were 12.254 min, 4.44 × 10, 7.41 × 10 and 11,227. The limits of detection and quantitation of chloramphenicol were 0.1 and 1.0 μg/mL. Recovery of chloramphenicol in the standard solution was 95%. Solid-phase membrane micro-tip extraction and capillary electrophoresis methods may be used to analyze chloramphenicol in human plasma and urine samples of any patient.

Thin films of copper indium selenide fabricated with high atom economy by electrophoretic deposition of nanocrystals under flow

Electrophoretic deposition (EPD) of colloidal nanocrystals (NCs) under flow is explored as a general method for the fabrication of semiconducting thin films. For photovoltaic applications, a low process voltage is highly desirable to avoid damaging the accreting semiconductor. Here we report a continuous flow reactor design that can operate at reduced voltage compared to a traditional batch reactor while preserving the electrophoretic velocity of the NCs by utilizing narrow electrode spacing. In a batch reactor, the low ratio of reactor volume to electrode surface area dictated by such a narrow spacing of the electrodes would impose a limit on the mass of nanocrystals that are resident in the reactor and therefore the thickness of the films that can be deposited. By continuously flowing the colloidal dispersion of NCs this limitation is obviated and thick films can be deposited. Through modeling and experiment we demonstrate the process parameters necessary to completely utilize the NCs in the feed solution, thereby achieving nearly 100% atom economy in the deposition process. The reactor design is compatible with large area substrates and is specifically designed to enable continuous, high-rate fabrication of the active layer of photovoltaic cells.

Separating nanoparticles by surface charge group using pH-controlled passivated gel electrophoresis

ABSTRACTBecause ionically stabilized colloids in aqueous dispersions have net surface charges that depend on pH, it is potentially possible to separate mixtures of nanospheres having identical radii, yet different types of stabilizing surface charge groups, efficiently using passivated gel electrophoresis (gel-EP). To demonstrate this, we separate a binary dispersion of polystyrene nanospheres that have nearly identical radii and surface group densities, yet different types of anionic stabilizing surface charge groups: sulfate and carboxylate. We achieve an efficient separation by adjusting the pH of the running buffer to lie between the pKa values of these charge groups, resulting in significantly different protonation and, consequently, different electrophoretic propagation velocities of the nanospheres. The measured steady-state propagation velocities of both types of anionic nanoparticles as a function of pH can be fit well by an equilibrium model of pH-dependent protonation of anionic surface charge groups. Thus, pH-controlled passivated gel-EP opens a route for separating similarly sized charged colloidal objects that are stabilized by a variety of different surface charge groups.

Electrophoretic Deposition of Nanoparticles into Three-Dimensional Felt As an Electrocataylst for the Solar Sulfur Ammonia Thermochemical Cycle

The penetration and coating of nanoparticles by electrophoretic deposition (EPD) into three-dimensional substrates was studied. Specifically, EPD of cobalt ferrite nanoparticles (20 nm) was investigated as a possible electrocatalyst to improve the kinetically slow anodic reaction of the oxidation of ammonium sulfite to ammonium sulfate in the hydrogen production sub-cycle of the solar sulfur-ammonia thermochemical water-splitting process. The primary goals were to uniformly penetrate and coat nanoparticles into 3D substrates by electrophoretic deposition and then to test their electrocatalytic activity by linear sweep voltammetry in 2 M ammonium sulfite. Electrophoretic deposition of cobalt ferrite nanoparticles from five bath compositions was performed on the following substrates: aluminum foil, graphite paper, 3 mm carbon felt, and 6 mm graphite felt. The bath compositions were 2 g/L of cobalt ferrite nanoparticles in 90/10 vol. % water/isopropanol with 1 mM (CHigh) or 0.05 mM (CLow) of hexadecyltrimethlyammonium bromide (CTAB), in 100% ethanol (E), in 100% acetylacetone (AA), and 100% acetylacetone with 0.2 wt. % polyethylenimine (AA-PEI). For electrophoretic deposition, a constant voltage was applied from 20 V to 128 V for a deposition time from 1 to 13 minutes. The zeta potential was measured to be 23 1 mV at pH of 6 for CHigh bath and 13 2 mV at pH 5 for E bath. Dry and wet adhesion of EPD deposits on aluminum was performed to determine adherence of nanoparticles to the substrate both after deposition and during the electrochemical process. Nanoparticles had good adherence to the substrate. To determine if nanoparticles could penetrate into 3D felt substrates by EPD, the ratio of Peclet number to Damköhler number (ratio of electrophoretic velocity to local deposition rate) was calculated to be greater than 1000, which indicated full penetration. Scanning electron microscopy (SEM) of the top and middle of the EPD deposits was used to examine the nanoparticle penetration and deposit morphology. It was confirmed by SEM images that there was full penetration and coating of nanoparticles into the 3 mm carbon felt and the 6 mm graphite felt from the 100% ethanol bath. The electrocatalytic activity of the EPD deposits on aluminum foil, graphite paper, 3 mm carbon felt, and 6 mm graphite felt substrates were analyzed by linear sweep voltammetry at 50 mV/s for the oxidation of 2 M ammonium sulfite. For the 3 mm carbon felt substrates, the highest electrochemical activity occurred on EPD deposits from the AA-PEI bath and then E bath. The highest electrochemical activity of the EPD deposits on 6 mm graphite felt occurred from the bath chemistries in decreasing order: CLow, AA-PEI, and E. The EPD deposit on 6 mm graphite felt from CLow bath obtained the highest electrochemical activity of all the deposits. For the 3 mm carbon felt, the ratio of current density of deposit compared to blank substrate ranged from 4 to 8 from E bath. The ratio of current density of EPD deposit on graphite paper and 6 mm graphite felt from E bath compared to blank substrate was about 1.6. The 3 mm carbon felt EPD deposits from AA-PEI bath ranged from 9 to 14, for the ratio of current density of the deposit compared to the blank substrate. EPD deposits from AA-PEI bath had a ratio of current density of deposit to blank substrate of 1.9 for deposits on aluminum foil, graphite paper, and 6 mm graphite felt. Overall, EPD was successful as a method to penetrate and coat cobalt ferrite nanoparticles up to 6 mm thick 3D felt. EPD deposits from AA-PEI bath improved the electrochemical activity compared to the blank substrate more than EPD deposits from E bath. But the EPD deposits from E bath were thin and uniform as well as deposited at much lower voltages compared to deposits from AA-PEI bath. EPD deposits from 3 mm carbon felt resulted in the largest improvement in electrochemical activity compared to the blank substrate from both E and AA-PEI bath. However, EPD deposits into 6 mm graphite felt had the highest electrochemical activity from all of the deposits. The EPD deposits with the highest electrochemical activity into 6 mm graphite felt in decreasing order were CLow, AA-PEI and E. The EPD conditions for these deposits were 40 V for 1 min from CLow resulting in current density of 75 mA/cm2, 127 V for 4 min from AA-PEI resulting in current density of 60 mA/cm2, and 40 V for 1 min from E resulting in current density of 38 mA/cm2.

Ultrafast electrokinetics.

The influence of a high electric field applied on both fluid flow and particle velocities is quantified at large Peclet numbers. The experiments involved simultaneous particle image velocimetry and flow rate measurements. These are conducted in polydimethylsiloxane channels with spherical nonconducting polystyrene particles and DI water as the background flow. The high electric field tests produced up to three orders of magnitude higher electrokinetic velocities than any previous reports. The maximum electroosmotic velocity and electrophoretic velocity measured were 3.55 and 2.3 m/s. Electrophoretic velocities are measured over the range of 100 V/cm < E < 250000 V/cm. The results are separated according to the different nonlinear theoretical models, including low and high Peclet numbers, and weak and strong concentration polarization. They show good agreement with the models. Such fast velocities could be used for flow separation, mixing, transport, control, and manipulation of suspended particles as well as microthrust generation among other applications.

Influence of rigid core permittivity and double layer polarization on the electrophoresis of a soft particle: A numerical study

The nonlinear electrophoresis of a soft particle with a polarizable uncharged rigid core coated with a polyelectrolyte layer is studied. Due to the coupled nature of the governing electrokinetic equations, a numerical approach is adopted. Our numerical solutions are in good agreement with the existing experimental and theoretical results for a particle with a non-polarizable core when the impacts of the nonlinear effects are low. The induced surface potential of the dielectric rigid core has an impact on the soft particle electrophoresis. The combined effects of the solid polarization of the core and double layer polarization have not been addressed previously in the context of soft particle electrophoresis. We have found that both these effects create retardation on the electrophoresis and are significant when the applied electric field is not weak. The double layer polarization is significant when the Debye length is in the order of the particle size. The range of the applied electric field for which the electrophoretic velocity of a soft particle with a non-polarizable core varies linearly with the applied electric field may create a nonlinear variation in electrophoretic velocity when the core is considered to be polarizable.

Gel electrophoresis and size selectivity of charged colloidal particles in a charged hydrogel medium

The electrophoresis of a charged colloid particle embedded in a charged hydrogel medium is studied based on the numerical computation of Stokes–Nernst–Planck–Poisson equations. In this study, no prior assumption on surface charge density of the particle, Debye length and imposed external electric field are made. We have compared our computed results for a lower range of surface charge density with the existing analytical solution based on the weakly charged particle and found them in good agreement when the fixed charge density of the hydrogel is low. Nonlinearity effects in gel electrophoresis is pronounced for a thick Debye length and higher values of particle ζ-potential. Even in the absence of a gel medium (free-solution), the numerical procedure used in this work yields mobilities different from the previous theoretical analysis based on the Debye–Hückel approximation when the scaled ζ-potential exceeds 2 and the Debye length is in the order of the particle size. The strong background electroosmotic flow (EOF) for a high fixed charge density of the polyelectrolyte hydrogel with a mesh size comparable to the particle radius drags the particle along the direction of the EOF. In this case, the electrophoretic velocity of the particle varies with its size. The particle electrophoretic velocity and forces are determined for a wide range of intrinsic parameters values.

A simple method for assessment and minimization of errors in determination of electrophoretic or electroosmotic mobilities and velocities associated with the axial electric field distortion.

It is commonly accepted that the modern CE instruments equipped with efficient cooling system enable accurate determination of electrophoretic or electroosmotic mobilities. It is also often assumed that velocity of migration in a given buffer is constant throughout the capillary length. It is simultaneously neglected that the noncooled parts of capillary produce extensive Joule heating leading to an axial electric field distortion, which contributes to a difference between the effective and nominal electric field potentials and between velocities in the cooled and noncooled parts of capillary. This simplification introduces systematic errors, which so far were however not investigated experimentally. There was also no method proposed for their elimination. We show a simple and fast method allowing for estimation and elimination of these errors that is based on combination of a long-end and short-end injections. We use it to study the effects caused by variation of temperature, electric field, capillary length, and pH.

Numerical study of the influence of solid polarization on electrophoresis at finite Debye thickness.

The influence of solid polarization on the electrophoresis of a uniformly charged dielectric particle for finite values of the particle-to-fluid dielectric permittivity ratio is analyzed quantitatively without imposing the thin Debye length or weak-field assumption. Present analysis is based on the computation of the coupled Poisson-Nernst-Planck and Stokes equations in the fluid domain along with the Laplace equation within the solid. The electrophoretic velocity is determined through the balance of forces acting on the particle. The solid polarization of the charged particle produces a reduction on its electrophoretic velocity compared to a nonpolarizable particle of the same surface charge density. In accordance with the existing thin-layer analysis, our computed results for thin Debye layer shows that the solid polarization is important only when the applied electric field is strong. When the Debye length is in the order of the particle size, the electrophoretic velocity decreases with the rise of the particle permittivity and attains a saturation limit at large values of the permittivity. Our computed solution for electrophoretic velocity is in agreement with the existing asymptotic analyses based on a thin Debye layer for limiting cases.

Surface Charge Measurement of SonoVue, Definity and Optison: A Comparison of Laser Doppler Electrophoresis and Micro-Electrophoresis

Microbubble (MB) contrast-enhanced ultrasonography is a promising tool for targeted molecular imaging. It is important to determine the MB surface charge accurately as it affects the MB interactions with cell membranes. In this article, we report the surface charge measurement of SonoVue, Definity and Optison. We compare the performance of the widely used laser Doppler electrophoresis with an in-house micro-electrophoresis system. By optically tracking MB electrophoretic velocity in a microchannel, we determined the zeta potentials of MB samples. Using micro-electrophoresis, we obtained zeta potential values for SonoVue, Definity and Optison of −28.3, −4.2 and −9.5 mV, with relative standard deviations of 5%, 48% and 8%, respectively. In comparison, laser Doppler electrophoresis gave −8.7, +0.7 and +15.8 mV with relative standard deviations of 330%, 29,000% and 130%, respectively. We found that the reliability of laser Doppler electrophoresis is compromised by MB buoyancy. Micro-electrophoresis determined zeta potential values with a 10-fold improvement in relative standard deviation.