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Highlights: [•] Theoretical modelling of concentrated “realistic salt-free suspensions”. [•] Influence of CO2 and H2O derived ionic species. [•] Added non-equilibrium dissociation–association kinetic equations. [•] Augmented electroviscous coefficient and DC electrophoretic mobility. [ABSTRACT FROM AUTHOR]

We have studied the rheological behavior of dilute suspensions of electrostatically dispersed Na-montmorillonite to elucidate the importance of the electroviscous effect under the condition of zero stress limits. The viscosities of the suspensions were measured using an original spiral-type viscometer that consisted of two measuring cylinders connected with a one-meter-long capillary tube. Extremely dilute suspensions, whose volume fractions ranged from 2.0×10−4 to 2.0×10−3, were used for the experiments. The ionic strength of the suspensions was controlled to be less than 0.001M. Thus, a fully developed electrical double layer was formed. It was confirmed that the measured viscosities of the suspensions increased with a decrease in their ionic strength, in accordance with a manifestation of the electroviscous effect. The viscosity increased markedly owing to the secondary electroviscous effect under the limit of the salt-free condition. To analyze this effect, we focused on the effective radii of the suspended montmorillonite particles. We estimated their effective radii by fitting the Dougherty–Krieger equation to the viscosity data. We found that the obtained effective radii depended significantly on the ionic strength in the low Peclet number regime, which was similar to the theoretical dependence of the effective radii determined from the interaction balance between the particles governed by hydrodynamic, electrostatic, and diffusive energies. Thus, the increase in the effective radius with a decrease in the ionic strength gives rise to the expansion of the electric repulsive force, resulting in an increase of the viscosity of this clay suspension. In addition, the magnitudes of the effective radii were greater than the Debye length. This indicates that even when the distance between the montmorillonite particles is greater than the Debye length, the electrical repulsive force affects the interaction between the particles. [ABSTRACT FROM AUTHOR]

Abstract: Several authors observed in the past a larger than twofold increase in viscosity of organic liquids under the influence of an electric field of the order of 106 V/m. This was called electro viscous effect (EVE). Significantly higher electric fields, of up to 108–109 V/m, arise in the electric double layer in solutions close to an electrode. Therefore, the viscosity can be expected to increase at strongly charged liquid–solid interfaces. In more recent years, it was also observed that even in the absence of an externally controlled electric field the viscosity of water can be up to 107 times higher close to a hydrophilic surface than in the bulk (“hydrophilic forces”). Here, we present electrochemical atomic force microscopy (EC-AFM) measurements by which we can overcome the critical threshold of the electric field H =106 V/m by the control of the potentials applied to both a conducting sample and a conducting tip immersed in solution. Using the EC-AFM, we have investigated for the first time the EVE in an aqueous electrolyte. We can show that by controlling the applied potential, we can control the viscosity and the thickness of the super viscous liquid layer close to the solid interface. Using this technique, we are further able to separate effects on viscosity induced by the hydrophilicity of the surfaces, by the strong nanoconfinement of the liquid between tip and surface, and by the applied electric field. [Copyright &y& Elsevier]

Abstract: The electroviscous effect of realistic salt-free colloidal suspensions is analyzed theoretically. We study the influence on the electroviscous coefficient of the surface charge density and the particle volume fraction. By realistic salt-free colloidal suspensions we mean aqueous suspensions which have been deionized as far as possible without any electrolyte added during the preparation, in which the only ions present can be (i) the so-called added counterions, coming from the ionization of surface groups and thus counterbalancing the surface charge, (ii) the H+ and OH− ions from water dissociation, and (iii) the ions produced by the atmospheric CO2 contamination. Our model is elaborated in the framework of a classical mean-field theory, using the spherical cell model approach and the appropriate local equilibrium reactions. It is valid for arbitrary surface charge density and particle concentrations. We have also made a new interpretation of the electroviscous coefficient: the electroviscous coefficient p of the suspension is the ratio between the electrohydrodynamic and the pure hydrodynamic contributions to the specific viscosity of the suspension. The numerical results show that it is necessary to consider the water dissociation influence for volume fractions lower than approximately 10−3, whereas the atmospheric contamination, if the suspensions are open to the atmosphere, is important in the region of volume fractions ϕ <0.03. [Copyright &y& Elsevier]

Abstract: Understanding non-Newtonian flow in microchannels is of both fundamental and practical significance for various microfluidic devices. A numerical study of non-Newtonian flow in microchannels combined with electroviscous effect has been conducted. The electric potential in the electroviscous force term is calculated by solving a lattice Boltzmann equation. And another lattice Boltzmann equation without derivations of the velocity when calculating the shear is employed to obtain flow field. The simulation of commonly used power-law non-Newtonian flow shows that the electroviscous effect on the flow depends significantly on the fluid rheological behavior. For the shear thinning fluid of the power-law exponent n <1, the fluid viscosity near the wall is smaller and the electroviscous effect plays a more important role. And its effect on the flow increases as the ratio of the Debye length to the channel height increases and the exponent n decreases. While the shear thickening fluid of n >1 is less affected by the electroviscous force, it can be neglected in practical applications. [Copyright &y& Elsevier]

Abstract: The aim of this investigation was to develop a basic understanding of the emulsification process by considering simple systems such as n-hexane, n-heptane, n-decane, and kerosene oil in water. The technique employed for the purpose was ultrasonification. The effect of ultrasonification time, chain length, viscosity, surface tension, oil content, and ionic strength of the media on the quality of emulsion has been studied. The emulsions were viewed through microscope to measure the number, size, and size distribution of droplets. Quantification of turbidity and viscosity was also used to characterize the emulsions. It has been found that the number and size of the droplets vary with the time of ultrasonification, contents of oils, molecular mass of the oils, and ionic strength of the media, and hence the quality of the emulsion is influenced by these parameters. The droplet size decreases, whereas the number of drops increases with the time of emulsification, approaching an optimum distribution at about 15 min of ultrasonification. Further, the increase in the molecular mass of the oil increases the size of the droplets and hence decreases the stability of the emulsion. The addition of electrolytes encourages coalescence and enhances the instability in the system. The results are in accord with the equations proposed by us. [Copyright &y& Elsevier]

Streaming potential, electroviscous effect, pore conductivity and membrane potential were measured for a ceramic ultrafiltration membrane at various KCl concentrations. A space charge model was used to calculate the surface potentials from the experimental data. Surface potentials determined from the four experimental methods are in relatively good agreement although some discrepancies occur at low ionic concentrations. Pore conductivity and membrane potential methods lead to similar surface potentials on the whole range of concentrations studied but these latter are smaller than those obtained for both streaming potential and electroviscous effect measurements. [Copyright &y& Elsevier]

• Autonomous filling of viscoelastic fluid in a capillary. • sPTT model is used to describe the rheology of viscoelastic fluid. • Effect of streaming potential on the filling dynamics is investigated. • Oscillations are seen in the temporal evolution of the filling dynamics. • A unique scale (x ∼ t) at a later stage of filling in the capillary is established. We study the streaming potential modulated autonomous filling dynamics of a viscoelastic fluid in a microfluidic channel. To describe the rheology of viscoelastic fluid, we consider a simplified Phan-Thien-Tanner (sPTT) model. Considering the electroviscous effect, along with the effects of surface tension force and viscous resistance, we derive a reduced-order model for obtaining the variations in the filling dynamics for different physical parameters of the system. We observe that a complex competition between the viscoelasticity and the electroviscous effect leads to the non-trivial behaviour in the temporal evolution of the filling dynamics. Moreover, our analysis unveils that an enhancement in the electroviscous effect due to increasing viscoelasticity of the fluid leads to a non-linear reduction in the filling length of the fluid in the channel. Consequently, the notion of having a higher filling length for more elastic nature of the fluid is contradicted here due to the presence of the electroviscous effect. Finally, we establish a regime (x ∼ t) at the later stage of filling in the channel. We show that this regime is unique to the surface tension driven filling of viscoelastic fluids in a charged fluidic pathways under the influence of electroviscous effect. Image, graphical abstract [ABSTRACT FROM AUTHOR]