Competition between ion-ion electrostatic correlations and hydrodynamic slip radically changes diffusioosmosis.

Analyses of microchannel and microgap cooling show that galinstan, a recently developed nontoxic liquid metal that melts at −19 °C, may be more effective than water for direct liquid cooling of electronics. The thermal conductivity of galinstan is nearly 28 times that of water. However, since the volumetric specific heat of galinstan is about half that of water and its viscosity is 2.5 times that of water, caloric, rather than convective, resistance is dominant. We analytically investigate the effect of using structured surfaces (SSs) to reduce the overall thermal resistance of galinstan-based microgap cooling in the laminar flow regime. Significantly, the high surface tension of galinstan, i.e., 7 times that of water, implies that it can be stable in the nonwetting Cassie state at the requisite pressure differences for driving flow through microgaps. The flow over the SS encounters a limited liquid–solid contact area and a low viscosity gas layer interposed between the channel walls and galinstan. Consequent reductions in friction factor result in decreased caloric resistance, but accompanying reductions in Nusselt number increase convective resistance. These are accounted for by expressions in the literature for apparent hydrodynamic and thermal slip. We develop a dimensionless expression to evaluate the tradeoff between the pressure stability of the liquid–solid–gas system and hydrodynamic slip. We also consider secondary effects including entrance effects and temperature dependence of thermophysical properties. Results show that the addition of SSs enhances heat transfer.

At concentrated electrolytes, the ion-ion electrostatic correlation effect is considered an important factor in electrokinetics. In this paper, we compute, in theory and simulation, the dipole moment for a spherical particle (charged, dielectric) under the action of an alternating electric field using the modified continuum Poisson-Nernst-Planck (PNP) model by Bazant et al. [ Double Layer in Ionic Liquids: Overscreening Versus Crowding . Phys. Rev. Lett. 2011 , 106 , 046102 ] We investigate the dependency of the dipole moment in terms of frequency and its variation with such quantities like ζ-potential, electrostatic correlation length, and double-layer thickness. With thin electric double layers, we develop simple models through performing an asymptotic analysis of the modified PNP model. We also present numerical results for an arbitrary Debye screening length and electrostatic correlation length. From the results, we find a complicated impact of electrostatic correlations on the dipole moment. For instance, with increasing the electrostatic correlation length, the dipole moment decreases and reaches a minimum and then it goes up. This is because of initially decreasing of surface conduction and finally increasing due to the impact of ion-ion electrostatic correlations on ion's convection and migration. Also, we show that in contrast to the standard PNP model, the modified PNP model can qualitatively explain the data from the experimental results in multivalent electrolytes.

Influence of extracellular polymeric substances on the aggregation kinetics of TiO2 nanoparticles

Aggregation kinetics of UV irradiated nanoplastics in aquatic environments

The stability of functionalized nanoparticles generally results from both steric and electrostatic interactions. Viruses like bacteriophage MS2 have adopted similar strategies for stability against aggregation, including a net negative charge under natural water conditions and using polypeptides that form loops extending from the surface of the protein capsid for stabilization. In natural systems, dissolved organic matter can adsorb to and effectively functionalize nanoparticle surfaces, affecting the fate and transport of these nanoparticles. We used time-resolved dynamic light scattering to measure the aggregation kinetics of a model virus, bacteriophage MS2, across a range of solution chemistries to determine what factors might destabilize viruses in aquatic systems. In monovalent electrolytes (LiCl, NaCl, and KCl), aggregation of MS2 could not be induced within a reasonable kinetic time frame, and MS2 was stable even at salt concentrations greater than 1.0 M. Aggregation of MS2 could be induced in divalent electrolytes when we employed Ca(2+). This trend was also observed in solutions containing 10 mg/L Suwannee River organic matter (SROM) reference material. Even at Ca(2+) concentrations as high 200 mM, diffusion-controlled aggregation was never achieved, demonstrating an additional barrier to aggregation. These results were confirmed by small-angle X-ray scattering experiments, which indicate a transition from repulsive to attractive interactions between MS2 virus particles as monovalent salts are replaced by divalent salts.

Aggregation behavior of graphitic C3N4 nanosheets in aqueous environment: Kinetics and mechanisms

Unsteady MHD Falkner-Skan flow of Casson nanofluid with generative/destructive chemical reaction

The accurate characterization of the electrical double layer properties of nanoparticles is of fundamental importance for optimizing their physicochemical properties for specific biotechnological and biomedical applications. In this article, we use classical solvation density functional theory and a surface complexation model to investigate the effects of the pH and the nanoparticle size on the structural and electrostatic properties of an electrolyte solution surrounding a spherical silica oxide nanoparticle. The formulation has been particularly useful for identifying dominant interactions governing the ionic driving force at a variety of pH levels and nanoparticle sizes. As a result of the energetic interplay displayed between electrostatic potential, ion-ion correlation and particle crowding effects on the nanoparticle surface titration, rich, non-trivial ion density profiles and mean electrostatic potential behavior have been found.

The aggregation and deposition kinetics of two multiwalled carbon nanotubes (MWNTs) with different degrees of surface oxidation are investigated using time-resolved dynamic light scattering (DLS) and quartz crystal microbalance with dissipation monitoring (QCM-D), respectively. Carboxyl groups are determined to be the predominant oxygen-containing surface functional groups for both MWNTs through X-ray photoelectron spectroscopy (XPS). The aggregation and deposition behavior of both MWNTs is in qualitative agreement with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The critical coagulation concentration (CCC) of the highly oxidized MWNTs (HO-MWNTs) is significantly higher than the lowly oxidized MWNTs (LO-MWNTs) in the presence of NaCl (210 and 53 mM, respectively) since HO-MWNTs have a higher surface charge density. In contrast, the aggregation inverse stability profiles of HO-MWNTs and LO-MWNTs are identical and yield comparable CCCs (0.9 and 1.0 mM, respectively) in the presence of CaCl(2). Similar to the results obtained from the aggregation study, HO-MWNTs are considerably more stable to deposition on silica surfaces compared to LO-MWNTs in the presence of NaCl. However, both MWNTs have the same propensity to undergo deposition in the presence of CaCl(2). The remarkable similarity in the aggregation and deposition kinetics of HO-MWNTs and LO-MWNTs in CaCl(2) may be due to Ca(2+) cations having a higher affinity to form complexes with adjacent carboxyl groups on HO-MWNTs than with isolated carboxyl groups on LO-MWNTs.

Aggregation of engineered nanoparticles (NPs) plays a crucial role in their environmental transport, fate,bioavailability and biological effects. This study investigated the temperature effect on the aggregation kinetics of CeO2 NPs in KCl and CaCl2 solutions using time-resolved dynamic light scattering. The results show that in KCl and CaCl2, the aggregation rate became faster as the temperature increased. The critical coagulation concentration (CCC) of CeO2 NPs went down from approximately 100 to 10 mM in KCl and from approximately 10 to 2 mM in CaCl2 solutions when the temperature increased from 4 to 37°C. The observations were analyzed in the framework of extended Derjaguin-Landau-Verwey-Overbeek (EDLVO) theory in order to find out the mechanisms underlying the temperature effect. Moreover, a theoretical model developed on the basis of EDLVO theory and von Smoluchowski’s population balance equation was used to predict the aggregation kinetics of CeO2 NPs under different temperature. The model predictions agreed well with experimental data, suggesting that the model could be employed to predict the size change of NPs in solution. Overall, this work provides insights into NP aggregation using experimental and modeling approaches, and allows people to better understand and theoretically predict the environmental behavior and risk of NPs.

The surface charge of calcite in aqueous environments is essential to many industrial and environmental applications. Electrokinetic measurements are usually used to assess the calcite charging behavior and characterize its electrical double layer (EDL). Numerous surface complexation models (SCMs) have been proposed to interpret the effect of different surface interactions on the zeta potential. Because of their versatility, SCMs have also become important tools in reactive transport modeling. The research on enhanced oil recovery within the last decade has led to an increased number of publications reporting both zeta potential measurements and SCMs for calcite. Nonetheless, the measurements are often inconsistent and the reasons for choosing one model over another are unclear. In this work, we review the models proposed for calcite and address their main differences. We first collect a large number of published zeta potential measurements and then we fit a Diffuse Layer, Basic Stern, and Charge-Distribution Multi-Site Complexation models to a selected reliable dataset. For each model, we maintain a similar number of adjustable parameters. After optimizing the parameters of the models, we systematically compare their prediction capabilities against data obtained in monovalent and divalent electrolyte systems containing calcium, magnesium, sulfate, or carbonate. We show that, often, the discrepancies between the models and the experimental data can be explained by different levels of disequilibrium. Nonetheless, assumptions used in the development of the models may significantly reduce their extrapolability to variable chemical conditions. The poor agreement between the models tuned to electrokinetic data with surface charge measurements and dynamic retention from single-phase flowthrough tests show that zeta potential may not be the best type of data to characterize ion binding at the calcite surface. Including the effect of mineral impurities and temperature on the calcite surface speciation and electrokinetic behavior prevail as main challenges for reactive transport modeling.

Separation of electrically neutral, mutually noninteracting passive solutes via AC electroosmotic slit channel flows is investigated for general asymmetric wall surface zeta potentials and apparent hydrodynamic slip lengths. We consider the nontrivial coupling between the surface potentials (or charge densities) and the apparent slip lengths, and focus our attention on the occurrence of a so called "crossover phenomenon" for separating out the slow diffusers when both slow and fast diffusers are present. Results show that regardless of the potential-slip coupling, wider bandwidths become available for crossover phenomenon to occur when the electroosmotic velocity gradient (magnitude) is greater. Contrarily, plug-like velocity profiles inhibit crossover phenomenon, and the potential-slip parametric combinations leading to such profiles can be easily identified by the conditions for minimal transport enhancement reported in recent literature. When separating out the slow diffuser or crossover phenomenon is desired, we recommend incorporating significant asymmetry in the surface potential and apparent slip boundary conditions such that the operating frequency and flow oscillation amplitude may be lowered to more practical values. Our results also agree with and strengthen the physical picture for explaining crossover phenomenon in macroscopic pressure-driven oscillatory flows.

Theoretical calculation on the membrane potential of charged porous membranes in 1-1, 1-2, 2-1 and 2-2 electrolyte solutions

We report the comparative aggregation behavior of three emerging inorganic 2D nanomaterials (NMs): MoS2, WS2, and h-BN in aquatic media. Their aqueous dispersions were subjected to aggregation under varying concentrations of monovalent (NaCl) and divalent (CaCl2) electrolytes. Moreover, Suwanee River Natural Organic Matter (SRNOM) has been used to analyze the effect of natural macromolecules on 2D NM aggregation. An increase in electrolyte concentration resulted in electrical double-layer compression of the negatively charged 2D NMs, thus displaying classical Derjaguin-Landau-Verwey-Overbeek (DLVO)-type interaction. The critical coagulation concentrations (CCC) have been estimated as 37, 60, and 19 mM NaCl and 3, 7.2, and 1.3 mM CaCl2 for MoS2, WS2, and h-BN, respectively. Theoretical predictions of CCC by modified DLVO theory have been found comparable to the experimental values when dimensionality of the materials is taken into account and a molecular modeling approach was used for calculating molecular level interaction energies between individual 2D NM nanosheets. Electrostatic repulsion has been found to govern colloidal stability of MoS2 and WS2 while the van der Waals attraction has been found to govern that of h-BN. SRNOM stabilizes the 2D NMs significantly possibly by electrosteric repulsion. The presence of SRNOM completely stabilized MoS2 and WS2 at both low and high ionic strengths. While h-BN still showed appreciable aggregation in the presence of SRNOM, the aggregation rates were decreased by 2.6- and 3.7-fold at low and high ionic strengths, respectively. Overall, h-BN nanosheets will have higher aggregation potential and thus limited mobility in the natural aquatic environment when compared to MoS2 and WS2. These results can also be used to mechanistically explain fate, transport, transformation, organismal uptake, and toxicity of inorganic 2D NMs in the natural ecosystems.

Electrokinetic properties of biodegradable conducting polyaniline-graft-chitosan copolymer in aqueous and non-aqueous media