Electroacoustic measurements at 1 MHz, using the Electro-Sonic Amplitude (ESA), on a kaolinite suspension provide a ready method for following the adsorption of hydrolyzable metal ions onto the clay surface. Co2+, Cd2+ and Cu2+ ions show similar behavior: The initially negative surface becomes less negative, approaches zero, and may become positive at pH values around neutral, depending on the initial metal concentration. At higher pH, electrokinetic potential goes through a maximum. If the surface has become positive, it becomes less so; and at still higher pH values it may become negative again, depending on the metal ion concentration. The behavior can be interpreted using the model proposed by James and Healy.

Adsorption of cetylpyridinium chloride (CPC) onto kaolinite can be followed using the electroacoustic effect. The dynamic mobility, measured at a frequency of 1 MHz, varies from about −2 to +1 × 10−8 m2 V−1 s−1 in a number of steps, reflecting the adsorption of two separate layers, with the bilayer being more obvious, especially at pH 5–8. The behaviour at different pHs reflects the different charge characteristics of the basal cleavage planes and the crystal edges. When the amount of added CPC is equal to the cation exchange capacity of the clay, the kinetic charge changes from negative to positive and there is a pronounced break in the conductivity curve. It is also possible to estimate the edge to face area from such measurements and so obtain a measure of the aspect ratio of the clay crystallites. The (minimum) value for this clay is about 5:1.

Recently, significant advances have been made in the theory and application of acoustic and electroacoustic spectroscopies for measuring the particle-size distribution (PSD) and zeta potential (ζ potential) of colloidal suspensions, respectively. These techniques extend or replace other techniques, such as light-scattering methods, particularly in concentrated suspensions. In this review, we summarize acoustic and electroacoustic theory and published results on clay mineral suspensions, detail theoretical constraints, and indicate potential applications for the study of environmentally significant clay mineral suspensions. Using commercially available instrumentation and suspension concentrations up to 45 vol.%, acoustic spectroscopy can characterize particle sizes from 10 nm to 10 µm, or greater. Electroacoustic spectroscopy can determine the ζ potential of a suspension with a precision and accuracy in the mV range. Despite the clear potential for their use in environmental settings, to date, acoustic methods have been used mainly on clay mineral colloids with industrial application, typically combined with similar measurements such as isoelectric point (IEP) determined from shear yield stress or ζ potential from electrophoretic mobility measurements. Potential applications in environmentally relevant suspension concentrations are significant, as PSD and ζ potential are important factors influencing the transport of mineral colloids and associated contaminants through porous media. Applications include determining the effects of suspension concentration, surfactants, electrolyte strength, pH and solution composition on soil clay properties and colloidal interactions, and determining changes in PSD, aggregation and ζ potential due to adsorption or variations in the clay mineralogy.

The aim of the aqueous electrokinetic experiments in the present study was to assess the relative contribution of the conducting polyindene (PIn) and inorganic colemanite components to the zeta (ζ) potentials of the composite particles, thus providing further insight into their surface composition in the dispersed state and establishing colloidally stable conditions for potential rheological, industrial applications. For this, PIn and a PIn/colemanite composite (containing 5 wt.% colemanite) were synthesized by chemical oxidative polymerization using FeCl3 as an oxidizing agent. Colemanite, PIn, and PIn/colemanite composite samples were characterized by Fourier-transform infrared spectroscopy, elemental analysis, conductivity, dielectric constant, magnetic susceptibility, density, particle-size measurements, thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction analysis, and scanning electron microscopy methods. The electrokinetic properties of colemanite and PIn/colemanite composite dispersions were determined by ζ-potential measurements in aqueous medium, taking into account the effects of time, pH, various electrolytes, surfactants, and temperature. The pH was observed to have a greater effect on the ζ potentials of colemanite in water but caused only slight changes in the presence of cationic (NaCl, BaCl2, AlCl3) and anionic (NaCl, Na2SO4) electrolytes. Increased pH values shifted the ζ potentials of PIn/colemanite composite dispersions to more negative values. The most effective surfactant acting on the ζ potentials of colemanite and PIn/colemanite composite dispersions was cetyltrimethylammonium bromide (CTAB), which shifted the ζ potentials to more positive regions. Elevated temperatures caused almost no change to the ζ potentials of either the colemanite or the PIn/colemanite composite dispersions.

The effects of surfactants on bentonites have been of great scientific interest for many years. Even though quaternary alkylammonium salts (QAS) have been studied, very few data are available on the comparative performance of different chain-length QAS for the modification of the surface properties and adsorption properties of bentonites. The objective of this study was to investigate the effect of chain length on the adsorption of cationic surfactants onto bentonite. The surface and adsorption properties of different chain-length QAS, i.e. hexadecyltrimethylammonium bromide (HTAB, C16), tetradecyltrimethylammonium bromide (TTAB, C14), and dodecyltrimethylammonium bromide (DTAB, C12), to produce organo- bentonites (OB) were studied. The concentrations of QAS were selected based on the cation exchange capacity (CEC) of the clay mineral. Zeta potential, swelling, and viscosity measurements and scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) analyses were used to explain the changes in surface properties. The results indicated that the best modification of bentonite was obtained using a 16-carbon chain length QAS (HTAB) in a 1:1 ratio of QAS to CEC. The basal spacing at this concentration was measured to be 22.19 Å, which also corresponded to the maximum adsorption density. The OB produced at this concentration showed the best hydrophobic character based on the swelling tests in toluene. The extent of hydrophobicity and adsorption density was correlated with the CEC and alkyl chain of the QAS. All these properties were used to elucidate the mechanism of modification governing the bentonite/QAS system.

Here we describe preliminary research into the in situ electrokinetic generation of continuous iron-rich precipitates to act as sub-surface barriers for the containment of contaminated sites. This is achieved using sacrificial iron electrodes emplaced either side of a soil/sediment mass to introduce iron into the system, and their dissolution and re-precipitation under the influence of an applied (DC) electric field. Continuous vertical and horizontal iron-rich bands (up to 2 cm thick) have been generated over a timescale of 300—500 h, at voltages of <5 V with an electrode separation of between 15 and 30 cm. The thickness of the iron-rich band increases as the applied voltage is increased. Geotechnical tests in sand indicate that the iron-rich band produced is practically impervious (coefficient of permeability of 10—9 ms—1 or less), and has significant mechanical strength (unconfined compressive strength of 10.8 N mm—2). By monitoring the current, the integrity of the iron-rich band may be assessed, and by continued application of current, the barrier may 'self heal'. The iron-rich barrier is composed of amorphous iron, goethite, lepidocrocite, maghemite and native iron.

A T-shaped microfluidic micro-mixer was designed to mix desired concentrations of two fluid streams and to prepare their homogenous mixture solution. A hydrostatic pressure gradient was induced in one of the branches of the system (mixing channel) by applying external electric field and generating electroosmotic flow in the two other branches of the system. The flow field and transferred mass into the mixing channel can be regulated by controlling the applied voltage of the system. In order to prepare more homogenous mixture solution, some obstacles were added to the mixing channel to induce perturbation in the flow field and enhance the mixing efficiency of the system. Numerical simulations were performed to show the correctness of the proposed mixing strategy and to investigate the influences of the applied voltage on the mixing efficiency and induced pressure flow in the mixing channel. A proposed design can be used as a guideline to control and enhance mixing efficiency, and consequently functionality, of different microfluidic devices.

The nonlinear dynamics of uncharged ideally polarizable spheres freely suspended in a viscous electrolyte in a uniform electric field are analysed using theory and numerical simulations. When a sphere polarizes under the action of the field, it acquires a non-uniform surface charge, which results in an electro-osmotic flow near its surface that scales quadratically with the applied field magnitude. While this so-called induced-charge electrophoresis yields no net motion in the case of a single sphere, it can drive relative motions by symmetry breaking when several particles are present. In addition, Maxwell stresses in the fluid also result in non-zero dielectrophoretic forces, which also cause particle motions. The combination of these two nonlinear electrokinetic effects, termed dipolophoresis, is analysed in detail by using numerical simulations. An efficient simulation method based on our previous analysis of pair interactions is presented and accounts for both far-field and near-field electric and hydrodynamic interactions in the thin-Debye-layer limit, as well as steric interactions using a novel contact algorithm. Simulation results in large-scale suspensions with periodic boundary conditions are presented. While the dynamics under dielectrophoresis alone are shown to be characterized by particle chaining along the field direction, in agreement with previous investigations, chaining is not found to occur under dipolophoresis, which instead causes transient particle pairings and results in a non-uniform microstructure with large number of density fluctuations, as we demonstrate by calculating pair distribution functions and particle occupancy statistics. Dipolophoresis is also found to result in significant hydrodynamic dispersion and velocity fluctuations, and the dependence of these two effects on suspension volume fraction is investigated.

There has been an enormous growth in the development of biotechnological applications, where advances in the techniques of microelectronic fabrication and the technologies of miniaturization and integration in semiconductor industries are being applied to the production of Laboratory-on-a-Chip devices. The aim of this development is to create devices that will perform the same processes that are currently carried out in the laboratory in reduced timescales, at a lower cost, requiring less reagents, and with a greater resolution of detection and specificity. The expectations of this Laboratory-on-a-Chip revolution is that this technology will facilitate rapid advances in gene discovery, genetic mapping and gene expression with broader applications ranging from infectious diseases and cancer diagnostics to food quality and environmental testing. A review of the current state of development in this field reveals the scale of the ongoing revolution and serves to highlight the advances that can be perceived in the development of Laboratory-on-a-Chip technologies. Since miniaturization can be applied to such a wide range of laboratory processes, some of the sub-units that can be used as building blocks in these devices are described, with a brief description of some of the fabrication processes that can be used to create them.