Ionic Fluids Research Articles

Ionic magnetic fluids in polar solvents with tuned counter-ions

The aim of the present study is to propose a new reproducible method for preparing colloidal dispersions of electrostatically charged nanoparticles (NPs) in polar solvents with different kinds of counter-ions. Maghemite NPs are here dispersed in solvents of different dielectric constant, namely water, dimethylsulfoxide (DMSO) and an ionic liquid, ethylammonium nitrate (EAN). If the existence of a NP superficial charge happens to be necessary for the colloidal stability of the dispersions in these three solvents, the standard DLVO theory cannot be used any more to describe the colloidal stability in EAN. The structure of the dispersions and the strength of the interparticle repulsion are investigated by small angle X-ray scattering measurements, in association with Ludwig–Soret coefficient determinations. Specificities, associated to the nature of the counter-ions are identified in this work on the colloidal stability, on the interparticle repulsion and on the Ludwig–Soret coefficient.

Corresponding-states behavior of an ionic model fluid with variable dispersion interactions.

Guggenheim's corresponding-states approach for simple fluids leads to a remarkably universal representation of their thermophysical properties. For more complex fluids, such as polar or ionic ones, deviations from this type of behavior are to be expected, thereby supplying us with valuable information about the thermodynamic consequences of the interaction details in fluids. Here, the gradual transition of a simple fluid to an ionic one is studied by varying the relative strength of the dispersion interactions compared to the electrostatic interactions among the charged particles. In addition to the effects on the reduced surface tension that were reported earlier [F. Leroy and V. C. Weiss, J. Chem. Phys. 134, 094703 (2011)], we address the shape of the coexistence curve and focus on properties that are related to and derived from the vapor pressure. These quantities include the enthalpy and entropy of vaporization, the boiling point, and the critical compressibility factor Zc. For all of these properties, the crossover from simple to characteristically ionic fluid is seen once the dispersive attraction drops below 20%-40% of the electrostatic attraction (as measured for two particles at contact). Below this threshold, ionic fluids display characteristically low values of Zc as well as large Guggenheim and Guldberg ratios for the reduced enthalpy of vaporization and the reduced boiling point, respectively. The coexistence curves are wider and more skewed than those for simple fluids. The results for the ionic model fluid with variable dispersion interactions improve our understanding of the behavior of real ionic fluids, such as inorganic molten salts and room temperature ionic liquids, by gauging the importance of different types of interactions for thermodynamic properties.

Long-Range Ordering of Ionic Liquid Fluid Films.

We report the transformation of ionic liquid films from isotropic bulk to a fluid-ordered state over micrometer length scales. Data from infrared and nonlinear spectroscopy measurements show clear transitions that, for varying ionic liquids, occur over time frames of 10 min to 2 h. These maturation times depend linearly on the chosen ionic liquids' bulk viscosities. Interestingly, the ionic liquids do not form solids upon ordering but do exhibit strong preferential alignments of molecules that persist throughout the fluid films' thicknesses. Our measurements characterize this ordering process and show that it is largely insensitive to substrate surface chemistry or small amounts of absorbed water. Additional experiments show the transition is observed across several of the most common ionic liquid cations and that the process is completely reversible. The driving force for this organization is attributed to electrostatic and steric forces combined with a slow shearing of the viscous ionic liquid. These interactions work together to slowly bring the molecules within the film to a preferred, global orientation. The physical length and time scales of this transformation are unexpected and intriguing and invite additional studies to develop an understanding and control of ionic liquid materials' behavior, particularly near surfaces, to benefit their uses in lubrication, capacitive energy storage, and heterogeneous catalysis.

Ionic Vapor Composition in Critical and Supercritical States of Strongly Interacting Ionic Compounds.

The critical point, CP (T, P), of the phase diagram quantifies the minimum amount of kinetic energy needed to prevent a substance from existing in a condensed phase. Therefore, the CP is closely related to the properties of the fluid far below the critical temperature. Approaches designed to predict thermophysical properties of a system necessarily aim to provide reliable estimates of the CP. Vice versa, CP estimation is impossible without knowledge of the vapor phase behavior. We report ab initio Born-Oppenheimer molecular dynamics (BOMD) simulations of sodium and potassium chlorides, NaCl and KCl, at and above their expected CPs. We advance the present knowledge regarding the existence of ionic species in the vapor phase by establishing significant percentages of atomic clusters: 29-30% in NaCl and 34-38% in KCl. A neutral pair of counterions is the most abundant cluster in the ionic vapors (ca. 35% of all vaporized ions exist in this form). Unexpectedly, an appreciable fraction of clusters is charged. The ionic vapor composition is determined by the vapor density, rather than the nature of the alkali ion. The previously suggested CPs of NaCl and KCl appear overestimated, based on the present simulations. The reported results offer essential insights into the ionic fluid properties and assist in development of thermodynamic theories. The ab initio BOMD method has been applied to investigate the vapor phase composition of an ionic fluid for the first time.

Dense ionic fluids confined in planar capacitors: in- and out-of-plane structure from classical density functional theory

The ongoing scientific interest in the properties and structure of electric double layers (EDLs) stems from their pivotal role in (super)capacitive energy storage, energy harvesting, and water treatment technologies. Classical density functional theory (DFT) is a promising framework for the study of the in- and out-of-plane structural properties of double layers. Supported by molecular dynamics simulations, we demonstrate the adequate performance of DFT for analyzing charge layering in the EDL perpendicular to the electrodes. We discuss charge storage and capacitance of the EDL and the impact of screening due to dielectric solvents. We further calculate, for the first time, the in-plane structure of the EDL within the framework of DFT. While our out-of-plane results already hint at structural in-plane transitions inside the EDL, which have been observed recently in simulations and experiments, our DFT approach performs poorly in predicting in-plane structure in comparison to simulations. However, our findings isolate fundamental issues in the theoretical description of the EDL within the primitive model and point towards limitations in the performance of DFT in describing the out-of-plane structure of the EDL at high concentrations and potentials.

A generalized hydrodynamic model for acoustic mode stability in viscoelastic plasma fluid

In this paper a generalized hydrodynamic (GH) model to investigate acoustic-mode excitation and stability in simplified strongly coupled bi-component plasma is proposed. The goal is centered in seeing the viscoelasticity-influences on the instability properties. The dispersive and nondispersive features are methodologically explored followed by numerical illustrations. It is seen that, unlike usual plasma acoustic mode, here the mode stability is drastically modified due to the considered viscoelastic effects contributed from both the electronic and ionic fluids. For example, it is found that there exists an excitation threshold value on angular wavenumber, $K \approx3$ in the $K$ -space on the Debye scale, beyond which only dispersive characteristic features prevail. Further, it is demonstrated that the viscoelastic relaxation time plays a stabilizing influential role on the wave dynamics. In contrast, it is just opposite for the effective viscoelastic relaxation effect. Consistency with the usual viscoelasticity-free situations, with and without plasma approximation taken into account, is also established and explained. It is identified and conjectured that the plasma fluid viscoelasticity acts as unavoidable dispersive agency in attributing several new characteristics to acoustic wave excitation and propagation. The analysis is also exploited to derive a quantitative glimpse on the various basic properties and dimensionless numbers of the viscoelastic plasma. Finally, extended implications of our results tentative to different cosmic, space and astrophysical situations, amid the entailed facts and faults, are highlighted together with indicated future directions.

The line tension of two-dimensional ionic fluids.

Pressure tensor components are very useful in the calculation of the tension associated with a liquid-vapor interface. In this work, we present expressions for the pressure tensor components of two-dimensional ionic fluids, modeled at the level of the primitive model. As an application, we carried out molecular dynamics simulations of liquid-vapor interfaces to calculate the line tension of the 1:1 two-dimensional ionic fluid, whose liquid-vapor coexistence curve had already been obtained in a previous work. The pressure tensor components were validated by simulating states of one phase and reproducing the scalar pressure, previously obtained from bulk simulations and reported in the literature. The effects on the line tension and the coexisting densities, originated by the choice of the Ewald parameters, the cutoff radius, and the interfacial length were also evaluated.

Fourth Moment Sum Rule for the Charge Correlations of a Two-Component Classical Plasma

We consider an ionic fluid made with two species of mobile particles carrying either a positive or a negative charge. We derive a sum rule for the fourth moment of equilibrium charge correlations. Our method relies on the study of the system response to the potential created by a weak external charge distribution with slow spatial variations. The induced particle densities, and the resulting induced charge density, are then computed within density functional theory, where the free energy is expanded in powers of the density gradients. The comparison with the predictions of linear response theory provides a thermodynamical expression for the fourth moment of charge correlations, which involves the isothermal compressibility as well as suitably defined partial compressibilities. The familiar Stillinger-Lovett condition is also recovered as a by-product of our method, suggesting that the fourth moment sum rule should hold in any conducting phase. This is explicitly checked in the low density regime, within the Abe-Meeron diagrammatical expansions. Beyond its own interest, the fourth-moment sum rule should be useful for both analyzing and understanding recently observed behaviours near the ionic critical point.

Surface deformation and shear flow in ligand mediated cell adhesion.

We present a unified, multiscale model to study the attachment/detachment dynamics of two deforming, charged, near spherical cells, coated with binding ligands and subject to a slow, homogeneous shear flow in a viscous, ionic fluid medium. The binding ligands on the surface of the cells experience both attractive and repulsive forces in an ionic medium and exhibit finite resistance to rotation via bond tilting. The microscale drag forces and couples describing the fluid flow inside the small separation gap between the cells, are calculated using a combination of methods in lubrication theory and previously published numerical results. For a selected range of material and fluid parameters, a hysteretic transition of the sticking probability curves (i.e., the function [Formula: see text]) between the adhesion phase (when [Formula: see text]) and the fragmentation phase (when [Formula: see text]) is attributed to a nonlinear relation between the total nanoscale binding forces and the separation gap between the cells. We show that adhesion is favoured in highly ionic fluids, increased deformability of the cells, elastic binders and a higher fluid shear rate (until a critical threshold value of shear rate is reached). Within a selected range of critical shear rates, the continuation of the limit points (i.e., the turning points where the slope of[Formula: see text] changes sign) predict a bistable region, indicating an abrupt switching between the adhesion and the fragmentation regimes. Although, bistability in the adhesion-fragmentation phase diagram of two deformable, charged cells immersed in an ionic aqueous environment has been identified by some in vitro experiments, but until now, has not been quantified theoretically.

Bi-directional ACET micropump for on-chip biological applications.

The ability to control and pump high ionic strength fluids inside microchannels forms a major advantage for clinical diagnostics and drug screening processes, where high conductive biological and physiological buffers are used. Despite the known potential of AC electro-thermal (ACET) effect in different biomedical applications, comparatively little is known about controlling the velocity and direction of fluid inside the chip. Here, we proposed to discretize the conventional electrodes to form various asymmetric electrode structures in order to control the fluid direction by simple switching the appropriate electric potential applied to the discretized electrodes. The ACET pumping effect was numerically studied by solving electrical, thermal and hydrodynamic multi-physic coupled equations to optimize the geometrical dimensions of the discretized system. PBS solutions with different ionic strength were seeded with 1 μm sized fluorescent particles and electrothermally driven fluid motion was observed inside the channel for different electrode structures. Experimental analyses confirm that the proposed micropump is efficient for a conductivity range between 0.1 and 1 S/m and the efficiency improves by increasing the voltage amplitude. Behavior of the proposed electrode-electrolyte system is discussed by lumped circuit model. Frequency response of system illustrated that the optimal frequency range increases by increasing the conductivity of medium. For 0.18 S/m PBS solution, the constant pumping effect was observed at frequency range between 100 kHz and 1 MHz, while frequency range of 100 kHz to 5 MHZ was observed for 0.42 S/m. The characteristics of experimental results were in good agreement with the theoretical model.

Electrophoretic mobility of charged porous shells or microcapsules and electric conductivity of their dilute suspensions

Abstract An analysis is presented for the electrophoresis and electric conduction in a dilute suspension of charged spherical porous shells or permeable microcapsules with electric double layers of arbitrary thickness in an electrolyte solution. With the assumption that the system is slightly perturbed from equilibrium, the linearized electrokinetic equations governing the ionic electrochemical potential energy and fluid velocity distributions are solved as series expansions in the small fixed charge density of the porous shells. Explicit formulas for the electrophoretic mobility of the porous shells and effective electric conductivity of the suspension are derived. Both the electrophoretic mobility and the effective conductivity decrease monotonically with a decrease in the relative thickness of the porous shells, but these decreases are not conspicuous until the porous shells are quite thin. When the fluid permeability of the porous shells is smaller or the electric double layers are thicker, the effect of this relative thickness on the electrophoretic mobility and effective conductivity becomes more significant. In the limiting case of zero inner radius of the porous shells, our formulas reduce to the corresponding results obtained for the electrophoresis and electric conduction in a suspension of charged porous spheres.

Configurable ACET micro-manipulator for high conductive mediums by using a novel electrode engineering

The ability to control and manipulate the high ionic strength fluids is major advantage for fluidic on-chip biological analysis. In this study we proposed an AC electrothermally-driven configurable flow for forward/reverse pumping and also clockwise/anti-clockwise rotational effects. Electrode discretization allow us to generate such a configurable AC electrothermal effects by simple switching the electric potential over the various asymmetric electrodes. Taguchi optimization method have been applied for discretized electrode dimensions and pumping/rotational effects was numerically studied by solving the coupled multi-physic fluidic, convection---diffusion, thermal and electric field equations. High efficient mixing process (above 90 %) achieved by exciting the electrodes in time-varying clockwise and anti-clockwise rotational mode. Thereafter, three particles released inside the channel and the particles were stretched, folded and finally separated by exciting the electrodes in rotational mode (a proof of chaotic regime). We investigate the proposed ACET characteristic, the results confirmed that the proposed system is efficient for high conductive mediums and the efficiency increase by increasing the non-uniform electric field intensity. Based on the frequency analysis from 100 to 500 MHz, the working frequency range of the configurable system is increases by increasing the electric conductivity of solution.

Polarization induced dynamic photoluminescence in carbon quantum dot-based ionic fluid

We have recently reported the synthesis of water-dispersible, polymer-passivated and redox-active carbon quantum dots (CQDs).

Exploiting hydrophobic borohydride-rich ionic liquids as faster-igniting rocket fuels.

A family of hydrophobic borohydride-rich ionic liquids was developed, which exhibited the shortest ignition delay times of 1.7 milliseconds and the lowest viscosity (10 mPa s) of hypergolic ionic fluids, demonstrating their great potential as faster-igniting rocket fuels to replace toxic hydrazine derivatives in liquid bipropellant formulations.

Predicting critical temperatures of ionic and non-ionic fluids from thermophysical data obtained near the melting point.

In the correlation and prediction of thermophysical data of fluids based on a corresponding-states approach, the critical temperature Tc plays a central role. For some fluids, in particular ionic ones, however, the critical region is difficult or even impossible to access experimentally. For molten salts, Tc is on the order of 3000 K, which makes accurate measurements a challenging task. Room temperature ionic liquids (RTILs) decompose thermally between 400 K and 600 K due to their organic constituents; this range of temperatures is hundreds of degrees below recent estimates of their Tc. In both cases, reliable methods to deduce Tc based on extrapolations of experimental data recorded at much lower temperatures near the triple or melting points are needed and useful because the critical point influences the fluid's behavior in the entire liquid region. Here, we propose to employ the scaling approach leading to universal fluid behavior [Román et al., J. Chem. Phys. 123, 124512 (2005)] to derive a very simple expression that allows one to estimate Tc from the density of the liquid, the surface tension, or the enthalpy of vaporization measured in a very narrow range of low temperatures. We demonstrate the validity of the approach for simple and polar neutral fluids, for which Tc is known, and then use the methodology to obtain estimates of Tc for ionic fluids. When comparing these estimates to those reported in the literature, good agreement is found for RTILs, whereas the ones for the molten salts NaCl and KCl are lower than previous estimates by 10%. The coexistence curve for ionic fluids is found to be more adequately described by an effective exponent of βeff = 0.5 than by βeff = 0.33.

Colloid Deposit Morphology and Clogging in Porous Media: Fundamental Insights Through Investigation of Deposit Fractal Dimension

Experiments reveal a wide discrepancy between the permeability of porous media containing colloid deposits and the available predictive equations. Evidence suggests that this discrepancy results, in part, from the predictive equations failing to account for colloid deposit morphology. This article reports a series of experiments using static light scattering (SLS) to characterize colloid deposit morphology within refractive index matched (RIM) porous media during flow through a column. Real time measurements of permeability, specific deposit, deposit fractal dimension, and deposit radius of gyration, at different vertical positions, were conducted with initially clean porous media at various ionic strengths and fluid velocities. Decreased permeability (i.e., increased clogging) corresponded with higher specific deposit, lower fractal dimension, and smaller radius of gyration. During deposition, fractal dimension, radius of gyration, and permeability decreased with increasing specific deposit. During flushing with colloid-free fluid, these trends reversed, with increased fractal dimension, radius of gyration, and permeability. These observations suggest a deposition scenario in which large and uniform aggregates become deposits, which reduce porosity, lead to higher fluid shear forces, which then decompose the deposits, filling the pore space with small and dendritic fragments of aggregate.

A novel micro/nano fabrication process based on the combined use of dielectrophoresis, electroosmotic flow, and electrodeposition for surface patterning

In this work, a novel application of electrokinetic forces is presented. Employing a gold interdigitated electrode array (IDEA), dielectrophoresis (a force acting on polarizable material) and electroosmosis (a force acting on ionic fluids) were utilized as microfabrication tools. Through electroosmotic flow, particles were dragged toward dielectrophoretic trapping zones, where they were held. Then polypyrrole, an electroconductive material with good mechanical and electrical properties, wide electrochemical window, and ease of fabrication, was electrodeposited onto the gold IDEA surface, permanently entrapping the microparticles within the structure. The process was tested employing organic (polystyrene), inorganic (silicon), and biologic (yeast cells) microparticles, all of which were successfully trapped. Computational models were developed to predict the electrokinetic response of the microparticles and compared to experimental observations. It was demonstrated that this process can be used to produce hierarchical structures through sequential repetition of entrapment cycles. Additionally, it is compatible with the Carbon-MEMS process. The fabrication process presented in this paper opens a wide range of applications that include: energy storage devices, scaffolds for cell culture, biomedical devices, electrochemical sensors, electrokinetic devices, among many others.

CFTR, bicarbonate, and the pathophysiology of cystic fibrosis.

The gene that encodes for the cystic fibrosis transmembrane regulator protein (CFTR) was identified in 1989, yet major pathophysiologic questions remain unanswered. There is emerging evidence that CFTR is a bicarbonate channel, a driver of chloride-bicarbonate exchange and through its action on local pH, a regulator of other ion channels and of proteins that function optimally in a neutral environment. In both the respiratory and gastrointestinal (GI) tracts, bicarbonate drives ionic content and fluid on epithelial surfaces, allows mucins to unfold and become slippery, and contributes to innate immunity. In the GI tract bicarbonate neutralizes gastric acid to support digestion and absorption. When CFTR is dysfunctional, lack of bicarbonate secretion disrupts these normal processes and thus leads directly to the clinical symptoms and signs of CF. This article synthesizes evidence from cell, animal, and human investigations that support these concepts. Bicarbonate secretion does not seem to be the same in all tissues and varies with physiologic demand. Thus, tissue type and whether conditions are baseline or stimulated needs to be taken into account when evaluating the evidence concerning the role of bicarbonate in the pathophysiology of CF as a regulator of local pH. Basic and applied research that focuses on the role of CFTR-mediated bicarbonate secretion helps explain many of the diverse clinical manifestations that are CF.

P10. Magnetic vestibular stimulation (MVS) influences fMRI resting-state fluctuations. The modulation of the default-mode network as an exemplary case

Introduction Dizziness in the presence of strong magnetic fields has been noticed ever since the first magnetic resonance experiments at high field strengths (>1 Tesla) have been conducted. It was suggested that this was due to the changing magnetic flux in the inner ear induced by the gross subject-motion within the magnetic field and that the motionless presence in the static magnetic field could not affect subjects significantly ( Schenck, 1992 ). Recently however, Roberts et al. (2011) showed that healthy subjects at rest who are kept in total darkness and exposed to the static magnetic field of an MR tomograph developed a persistent nystagmus, while patients with bilateral peripheral vestibular failure did not. Furthermore, the nystagmus’ slow phase velocity was modulated systematically depending on the subject’s head orientation relative to the field. The authors argued that the ionic fluids in the inner ear, which are constantly flowing as cells maintain resting activity, will be diverted by a (magnetic) Lorentz-force and this creates a pressure onto the cupula “the rotatory motion sensor” in the inner ear, thus leading to a nystagmus akin to a constant (accelerating) rotatory stimulation. This model was further supported by a simulation study ( Antunes et al., 2012 ) and a recent study of patients with unilateral labyrinthine disorders ( Ward et al., 2014 ). It was speculated that this magnetic vestibular stimulation (MVS) effect might influence fMRI results, because a nystagmus is indicative for a vestibular imbalance. Methods The aim of the current study was to investigate if this MVS effect does indeed modulate the fluctuations of the BOLD signal in areas related to the vestibular system. Here we treat the case of MVS influencing resting-state networks, presenting results from resting-state experiments conducted at two different field strengths (1.5 Tesla & 3 Tesla) and focus in particular on the influence of MVS on the default mode network. Results Those subparts of the default mode network that were significantly modulated between field strengths were also commonly associated with vestibular functions. Furthermore, the signal-scaling behavior of these subparts was different from the other parts of the default mode network that were not significantly modulated. In fact, the signal-scaling behavior was in agreement with a modulation due to MVS as expected from a Lorentz-model perspective (i.e. linear increase) while the other parts that were not significantly modulated by the MVS effect showed a scaling behavior that was expected simply from considering fMRI signal-scaling behavior due to increased field strength (i.e. approximately square root increase, e.g. see ( Duyn, 2012 )). Conclusion The MVS effect significantly affected BOLD signal modulation. Those parts which were more closely connected to the vestibular peripheral sense-organs did modulate in accordance with the Lorentz-model prediction and those subparts which modulated less, but still modulated significantly above the expected increase due to field-strength related signal-increase, were further removed in terms of connectivity from the vestibular peripheral sense-organs and therefore were modulated less strongly by MVS.

Transient electroosmosis in the transverse direction of a fibrous porous medium

Abstract The transient electroosmotic response in the porous medium constituted by a homogeneous array of parallel, charged, circular cylindrical fibers filled with an electrolyte solution to the step application of an electric field in the transverse direction is analytically studied. The thickness of the electric double layers surrounding the dielectric cylinders is assumed to be small relative to the radius of the cylinders and the gap width between two adjacent cylinders, but the effect of time-evolving electroosmosis within the thin but finite double layers is incorporated. Through the use of a unit cell model, the transient equation of conservation of the fluid momentum outside the double layer is solved for each cell, in which a single cylinder is enveloped by a coaxial shell of the ionic fluid. Explicit expressions for the transient electroosmotic velocity of the bulk fluid in the Laplace transform as functions of the porosity of the fibrous medium and the electrokinetic radius of the constitutive cylinders are obtained for two different conditions at the virtual boundary of the cell. Our results indicate that the time scale for the growth of electroosmosis is significantly smaller for a fiber matrix with a lower porosity and the electroosmotic velocity increases with a decrease in the double-layer thickness relative to the cylinder radius.