Non-Newtonian Dynamics in Water-in-Salt Electrolytes.

Water-in-salt electrolytes have attracted considerable interest in the past decade for advanced lithium-ion batteries, possessing important advantages over the non-aqueous electrolytes currently in use. A battery with a LiTFSI-water electrolyte was demonstrated in which an operating window of 3 V is made possible by a solid-electrolyte interface. Viscosity is an important property for such electrolytes, because high viscosity is normally associated with low ionic conductivity. Here, we investigate shear and longitudinal viscosities using shear stress and compressional longitudinal stress measurements as functions of frequency and concentration. We find that both viscosities are frequency-dependent and exhibit almost identical frequency and concentration dependences in the high-concentration region. A comparison to quasielastic neutron scattering experiments suggests that both are governed by structural relaxation of the TFSI- network. Thus, LiFTSI-water electrolytes appear to be an unusual case of a non-Newtonian fluid, where shear and longitudinal viscosities are determined by the same relaxation mechanism.

Overlapping double layers in electrokinetics of concentrated dispersions.

The traditional model of the Isolated Double Layer (DL) becomes inadequate in concentrated dispersions when the distance between particles becomes comparable to the Debye length and DLs overlap. The notion of "overlapping DLs" was introduced to Electrophoretic theory 60 years ago by Prof. T. Overbeek's group. The theory posed in that paper and a subsequent publication by Long and Ross predicts a decline of electrophoretic mobility due to DLs overlap. There is also more recent theory that describes electrokinetics at the extreme case of complete DLs overlap. We call it "quasi-homogeneous DL theory". It predicts an increase of electrophoretic mobility. We conducted an experimental study to verify which theoretical prediction is correct. We used an equilibrium dilution protocol, which allowed us to maintain the same interface-bulk electrochemical equilibrium at all volume fractions, ranging from 1 %vl up to 36 %vl (56 %wt). Electroacoustics were applied for measuring electrophoretic mobility and zeta potential. They remain constant up to 15% vl. At the higher volume fraction, where DLs become overlapping, they increase. This confirms prediction of "quasi-homogeneous DL theory" at least on qualitative level.

Rheology of non-Newtonian liquid mixtures and the role of molecular chain length.

We have examined mixtures of toluene, a Newtonian liquid, with several non-Newtonian liquids (i.e. surfactants) across the full composition range, from pure toluene to pure surfactant, for the purpose of exploring their rheological properties. Surfactants of similar viscosities but significantly different molecular weights were chosen, in order to verify the role of the molecular chain length. We discovered that the classical mixing rule, based on excess activation energy, succeeds for mixtures with shorter molecular chain, but fails for mixtures with longer ones. We suggest here a hypothesis which explains this difference as the result of expanding-collapsing of flexible long-chain molecules in shear flow. In order to support this hypothesis, we applied longitudinal rheology measurements which reproduce such oscillatory effects under the controlled conditions of an ultrasound longitudinal wave. This method confirms that there are qualitative differences in the rheology of these liquids mixtures with short and long molecular chains.

Adsorption of surfactant from non-polar liquid into porous material characterized using conductivity measurement.

HYPOTHESIS: Surfactants cause ionization in non-polar liquids, enabling such liquids to become electrically conductive with that conductivity being a linear function of the surfactant concentration. Consequently, measurement of the conductivity can be used as a tool for monitoring surfactant concentration. EXPERIMENTS: We describe here a simple method for studying surfactant adsorption from oil into a porous material. The conductivity of solutions containing toluene and porous particles was measured as a function of time after the addition of surfactant, at various concentrations. We applied this method for characterizing surfactant (SPAN 20) adsorption by porous particles (silica gel Davisil) suspended in the non-polar liquid (toluene). We the suggested a simple theoretical model for the initial stage of this adsorption process and tested its prediction experimentally. FINDINGS: The experimental data confirms all predicted theoretical trends both qualitatively and quantitatively. This method can be used for understanding surfactant behavior in rock formations during oil recovery, optimizing surfactant concentration, and analyzing chemical composition.

Optimal MEMS device for mobility and zeta potential measurements using DC electrophoresis.

We have developed a novel microchannel geometry that allows us to perform simple DC electrophoresis to measure the electrophoretic mobility and zeta potential of analytes and particles. In standard capillary geometries, mobility measurements using DC fields are difficult to perform. Specifically, measurements in open capillaries require knowledge of the hard to measure and often dynamic wall surface potential. Although measurements in closed capillaries eliminate this requirement, the measurements must be performed at infinitesimally small regions of zero flow where the pressure driven-flow completely cancels the electroosmotic flow (Komagata Planes). Furthermore, applied DC fields lead to electrode polarization, further questioning the reliability and accuracy of the measurement. In contrast, our geometry expands and moves the Komagata planes to where velocity gradients are at a minimum, and thus knowledge of the precise location of a Komagata plane is not necessary. Additionally, our microfluidic device prevents electrode polarization because of fluid recirculation around the electrodes. We fabricated our device using standard MEMS fabrication techniques and performed electrophoretic mobility measurements on 500 nm fluorescently tagged polystyrene particles at various buffer concentrations. Results are comparable to two different commercial dynamic light scattering based particle sizing instruments. We conclude with guidelines to further develop this robust electrophoretic tool that allows for facile and efficient particle characterization.

Ionization of a nonpolar liquid with an alcohol.

Nonpolar liquids whose dielectric permittivities are close to 2 have very low conductivities, usually below 10 × 10(-10) S/m. Their ionization is suppressed by the lack of solvation resulting from the negligible dipole moment of such liquids' molecules. Ionization could be enhanced by the addition of other substances that could serve as solvating agents, creating inverse micelles around ions and preventing them from reassociating into ion pairs and neutral molecules. Surfactants are normally used for this purpose, but we show here that alcohols could perform a similar function. However, the mechanism of ionization by alcohols turns out to be quite different compared to the mechanism of ionization by surfactant. For instance, the conductivity of poly-α-olefin oil (PAO) depends on the concentration of added octanol (alcohol) as an exponential function above 10% of the octanol content. At concentrations below approximately 10%, octanol does not affect the conductivity at all. This phenomenon has never been observed for surfactant solutions. Apparently, octanol is completely dissolved at concentrations below 10% and forms micelles only above this concentration, which is the cmc for octanol-PAO mixtures. Below the cmc, octanol molecules do not dissociate, despite being able to dissociate in pure octanol, which has a conductivity of about 10 × 10(-7) S/m. This again stresses the importance of the solvating factor in the ionization of liquids. Above 10% concentration, octanol molecules form micelles, which become charged by the disproportionation mechanism when they collide. To explain the exponential dependence of conductivity on octanol content, we assume that charged micelles grow in volume with increasing octanol content faster than neutral ones. Ion-dipole interactions are responsible for the preferential adsorption of octanol molecules onto charged micelles. Additional ionization occurs in such larger micelles, which then break down into smaller ones carrying individual electric charges.

Critical concentration of ion-pairs formation in nonpolar media.

It is known that nonpolar liquids can be ionized by adding surfactants, either ionic or nonionic. Surfactant molecules serve as solvating agents, building inverse micelles around ions, and preventing their association back into neutral molecules. According to the Bjerrum-Onsager-Fuoss theory, these inverse micelle ions should form "ion pairs." This, in turn, leads to nonlinear dependence of the conductivity on the concentration. Surprisingly, ionic surfactants exhibit linear conductivity dependence, which implies that these inverse micelle ions do not form ion pairs. Theory predicts the existence of two ionic strength ranges, which are separated by a certain critical ion concentration. Ionic strength above the critical one is proportional to the square root of the ion concentration, whereas it becomes linear below the critical concentration. Critical ion concentration lies within the range of 10(-11) -10(-7) mol/L when ion size ranges from 1 to 3 nm. Critical ion concentration is related, but not equal, to a certain surfactant concentration (critical concentration of ion-pairs formation (CIPC)) because only a fraction of the surfactant molecules is incorporated into the micelles ions. The linear conductivity dependence for ionic surfactants indicates that the corresponding CIPC is above the range of studied concentrations, perhaps, due to rather large ion size. The same linearity is a sign that charged inverse micelles structure and fraction are concentration independent due to strong charge-dipole interaction in the charge micelle core. This also proves that CIPC is independent of critical concentration of micelle formation. Nonionic surfactants, on the other hand, exhibit nonlinear conductivity dependence apparently due to smaller ion sizes.

Seismoelectric effect: a non-isochoric streaming current. 1. Experiment.

Propagation of ultrasound through a porous body saturated with liquid generates an electric response. This electroacoustic effect is called the "seismoelectric current"; the reverse process, when an electric field is the driving force, is called the "electroseismic current." Seismoelectric currents can be measured with electroacoustic devices originally designed for characterizing liquid dispersions. Such electroacoustic devices must first be calibrated with a liquid dispersion and then used to characterize a porous body. We demonstrated such measurements of the seismoelectric current with electroacoustic devices in three different types of porous bodies. The first porous body was a deposit of solid submicrometer particles. We monitored the kinetics of the deposit formation on the surface of the electroacoustic probe. It allowed us to unambiguously confirm that the measured signal was generated by the deposit. We were also able to extract information about the porosity of the forming deposits. The second type of porous body was again a deposit, but instead of solid submicrometer particles, we used very large, porous glass spheres. According to classical theory, these glass particles are not supposed to generate any electroacoustic signal because colloid vibration current decays with increasing particle size due to the particles inertia. Nevertheless, we measured a strong signal, which was apparently associated with the pores of the particles. We were able to derive some conclusions about the dependence of the seismoelectric current on the pore size. The last tests were performed with cylindrical sandstone cores. These porous bodies have a very high hydrodynamic resistance that prevents measurement of the classical streaming current. We are able to measure a strong seismoelectric current that correlates with porosity of the cores.

Bulk viscosity and compressibility measurement using acoustic spectroscopy.

Bulk viscosity is a somewhat obscure parameter that appears in the hydrodynamic equations for Newtonian liquids when compressibility is important and, together with the dynamic viscosity, controls sound attenuation. Whereas dynamic viscosity reflects only "translational" molecular motion, in contrast the bulk viscosity reflects the relaxation of both "rotational" and "vibrational" degrees of molecular freedom. Several molecular theories yield predictive expressions for both bulk and dynamic viscosities, but experimentally the situation is quite out of balance, in that there is extensive data for the dynamic viscosity of all sorts of liquids, but a paucity of data for bulk viscosity, just a few values for water and a handful of exotic liquids. We compare three possible experimental techniques for measuring bulk viscosity, namely, Brillouin spectroscopy, Laser transient grating spectroscopy, and acoustic spectroscopy. We then formulate some arguments suggesting that acoustic spectroscopy is not only the most suitable for measuring bulk viscosity, but that it also offers a verification procedure that can confirm that the measured parameter agrees with theoretical definition of bulk viscosity for a Newtonian liquid. In addition, acoustic spectroscopy provides a measurement of sound speed, which cannot only improve the attenuation measurement but as a side benefit can also be used to calculate liquid compressibility. We apply this technique for measuring the bulk viscosity and compressibility of twelve commonly assumed Newtonian liquids, two of which surprisingly fails to pass a verification test described here to test the Newtonian hypothesis. Then, we test correlation between measured bulk viscosity and several other intensive properties of these liquids, such as density, dynamic viscosity, dielectric permittivity, and compressibility. We have not discovered any meaningful correlation. This suggests that bulk viscosity is an independent parameter that reflects peculiar properties of liquids and can be used in the set of independent equations describing molecular interaction in liquids.

Observation of sol-gel transition for carbon nanotubes using electroacoustics: colloid vibration current versus streaming vibration current.

Propagation of ultrasound through a porous body generates an electric signal, similarly to the well-known electroacoustic effect in dispersions of mobile particles. This obscure version of electroacoustics has been known since 1948, when M. Williams published his paper on electrokinetic transducers [M. Williams, Rev. Sci. Instrum. 19 (10) (1948) 640-646]. We observe this effect in a 1 wt% aqueous dispersion of carbon nanotubes. Magnitude and phase of the electroacoustic signal, as well as conductivity, are sensitive to sonication and mixing. Sonication with no mixing leads to phase rotation by up to 180 degrees comparing to the traditional colloid vibration current (CVI) in sols. This is explained by the fact that sonication terminates motion of the carbon nanotubes by building up a continuous network gel. Propagation of ultrasound through the immobile carbon nanotube network generates a streaming vibration current (SVI), but not a CVI, which requires free motion of the particles relative to the liquid. Theoretical analysis indicates that the SVI has 180 degrees difference in phase from the CVI. The magnitude of the SVI after sonication with no mixing depends on the shifts of the measuring probe position. Apparently this occurs due to inhomogeneity of the carbon nanotube gel, which might have clusters with higher density and gaps with no solids at all. This effect can be used for testing homogeneity of the carbon nanotube gel. Sonication with continuous mixing also affects the electroacoustic signal and conductivity. However, the electroacoustic phase does not reach 360 degrees , which corresponds to the SVI in gel. The measured signal is the vector sum of the CVI and SVI under these conditions. It is possible to use data on the electroacoustic phase to monitoring the number of carbon nanotube segments that retain independent motion.

Ultrasonic characterization of proteins and blood cells.

Ultrasound changes its intensity and speed when propagating through a liquid or a suspension containing particles. In addition it generates a weak electric signal by altering the motion of ions and charged particles. Hence acoustic and electroacoustic measurements provide information about the properties of suspended particles and molecules. Here we present both acoustic and electroacoustic results on blood suspensions and protein solutions, relevant to life sciences. For blood cells a strong increase in acoustic attenuation with volume fraction is found, from which the speed of sound in an erythrocyte is found to be about 1900 m/s, assuming the attenuation is due to scattering only. A similar value of 1700 m/s is found from the increase in sound speed of the dispersion with concentration. Electroacoustic measurements on bovine serum albumin (BSA) yield a charge of about seven elementary charges per BSA molecule. These results show the power and usefulness of acoustic and electroacoustic measurement techniques for biological systems.

Aperiodic capillary electrophoresis method using an alternating current electric field for separation of macromolecules.

Switching from direct current (DC) to alternating current (AC) electric fields has provided substantial improvements in various instrument techniques that use electric fields for manipulating with various liquid-based systems. For example, AC fields are now used in both light scattering and electroacoustic instruments for measuring xi-potential, largely replacing more traditional microelectrophoresis techniques that use DC fields. In this paper, we suggest a novel way to make a similar transition in the area of separation techniques, capillary electrophoresis (CE) in particular. Dielectrophoresis is one well-known separation effect in which a drifting motion of particles is produced in a "spatially nonhomogeneous" AC electric field. However, there is another field effect that also causes a similar drift of particles. Instead of a "spatially nonhomogeneous" field, this method relies on a "temporally nonhomogeneous" field, normally referred to as "aperiodic electrophoresis". Despite a number of recently published experimental and theoretical papers describing this effect, it is less well-known than dielectrophoresis. We present a short overview of some of the relevant papers. We point out for the first time the idea that "aperiodic electrophoresis" might be useful for separation of macromolecules. We suggest several new mechanisms that could induce this effect in a sufficiently strong AC electric field. This effect can be used as a basis for a new separation method having several important advantages over traditional CE. We present a simple scheme as an example illustrating this new method.

Multilaboratory study of the shifts in the IEP of anatase at high ionic strengths.

The zeta-potentials of anatase at pH 2-11 in 0.1, 0.3, 0.5, and 1 moldm(-3) NaI were studied using the DT 1200 in three laboratories. At [NaI]=1 moldm(-3) the zeta-potentials were positive over the entire pH range. The previously observed tendency of the isoelectric point of anatase to shift to high pH at high ionic strength (M. Kosmulski, J.B. Rosenholm, J. Phys. Chem. 100 (1996) 11681) and the salt specificity of this effect were confirmed. The zeta-potentials obtained in different laboratories using DT 1200 are consistent within 3 mV.