Anomalous electrokinetics at hydrophobic surfaces: Effects of ion specificity and interfacial water structure

Modulation of Electrokinetic Potentials Using Graphene-Based Surfaces and Variable Substrate Charge Density

Enhanced electrokinetic phenomena, manifested through the observation of a large streaming potential (Vs), were obtained in microchannels with single-layer graphene (SLG)-coated and few-layer graphene (FLG)-coated surfaces. In comparison to silicon microchannels, the Vs obtained for a given pressure difference along the channel (ΔP) was higher by 75% for the graphene-based channels, with larger values in the SLG case. Computational modeling was used to correlate the surface charge density, tuned through plasma processing, and related zeta potential to measured Vs. The implications related to deploying lower dimensional material surfaces for modulating electrokinetic flows were investigated.

Confinement Dynamics of Nanodroplets between Two Surfaces: Effects of Wettability and Electric Field

The electrowetting effect and related applications of tiny droplets have aroused widespread research interest. In this work, we report molecular dynamics simulations of confinement dynamics of nanodroplets under different droplet-surface interactions and surface distances under an external electric field. So far, the effect of the surface-droplet interactions on electric field-induced dynamics behaviors of droplets in confined spaces has not been extensively studied. Our results show that in the absence of electric field there is a critical value of surface wettability for the shape transition of droplets. Above this value, the droplet is divided into small droplets adhered on the bottom and top surfaces; below this value, the droplets are detached from the surfaces. When an external electric field is applied parallel to the surfaces, the droplet spreads on the surface along the direction of the electric field. It was found that the surface separation significantly influences the transition of the droplet shape. The steady morphology of the droplets under the electric field depends on the surface-droplet interaction and surface separation. We explore the underlying mechanism causing the morphological transition through analyzing the molecular interactions, the number of interracial molecules and the interaction force between the droplets and surfaces. These results provide basic insights into the molecular interactions of nanodroplets under different confined environments, and clues for applications of confined nanodroplets under the control of electric field.

Hydration Structures on γ-Alumina Surfaces With and Without Electrolytes Probed by Atomistic Molecular Dynamics Simulations

A wide range of systems, both engineered and natural, feature aqueous electrolyte solutions at interfaces. In this study, the structure and dynamics of water at the two prevalent crystallographic terminations of gamma-alumina, [110] and [100], and the influence of salts─sodium chloride, ammonium acetate, barium acetate, and barium nitrate on such properties─were investigated using equilibrium molecular dynamics simulations. The resulting interfacial phenomena were quantified from simulation trajectories via atomic density profiles, angle probability distributions, residence times, 2-D density distributions within the hydration layers, and hydrogen bond density profiles. Analysis and interpretation of the results are supported by simulation snapshots. Taken together, our results show stronger interaction and closer association of water with the [110] surface, compared to [100], while ion-induced disruption of interfacial water structure was more prevalent at the [100] surface. For the latter, a stronger association of cations is observed, namely sodium and ammonium, and ion adsorption appears determined by their size. The differences in surface-water interactions between the two terminations are linked to their respective surface features and distributions of surface groups, with atomistic-scale roughness of the [110] surface promoting closer association of interfacial water. The results highlight the fundamental role of surface characteristics in determining surface-water interactions, and the resulting effects on ion-surface and ion-water interactions. Since the two terminations of gamma-alumina considered represent interfaces of significance to numerous industrial applications, the results provide insights relevant for catalyst preparation and adsorption-based water treatment, among other applications.

All-Weather Freshwater and Electricity Simultaneous Generation by Coupled Solar Energy and Convection

Integrating solar evaporation-driven desalination and electricity production has emerged as a promising approach to alleviate energy crisis and freshwater scarcity. However, there remain huge challenges to achieve high water productivity and steady power generation efficiency. Herein, a compact evaporation-induced water-electricity co-generation device was proposed using a bio-waste squid ink sphere-based cellulose fabric as an evaporator and a silicon nanowires array-based evaporation-driven moist-electric generator. The efficient localized solar thermal heating of the photothermal component leads to significant enhancement in freshwater yield, and the latent heat of vapor condensation is recycled to promote the electricity generation. More notably, the device is capable of harvesting wind energy toward all-weather water and power generation. The fabricated device demonstrated a high evaporation rate of 2.17 kg m^-2 h^-1 with a collection rate of 66.7% and a maximum output voltage of 1.48 V under one sun illumination with a wind speed of 4 m s^-1. The outdoor experiments display a maximum water evaporation rate of 1.84 kg m^-2 h^-1 with a maximum output voltage of 1.35 V even on cloudy days. Such superior performance of a comprehensive device has great potential for sustainable and practical application in freshwater and electricity generation.

Enhanced transport of ions by tuning surface properties of the nanochannel

Motivated by recent observations of anomalously large deviations of the conductivity currents in confined systems from the bulk behavior, we revisit the theory of ion transport in parallel-plate channels and also discuss how the wettability of a solid and the mobility of adsorbed surface charges impact the transport of ions. It is shown that depending on the ratio of the electrostatic disjoining pressure to the excess osmotic pressure at the walls two different regimes occur. In the thick channel regime this ratio is small and the channel effectively behaves as thick, even when the diffuse layers strongly overlap. The latter is possible for highly charged channels only. In the thin channel regime the disjoining pressure is comparable to the excess osmotic pressure at the wall, which implies relatively weakly charged walls. We derive simple expressions for the mean conductivity of the channel in these two regimes, highlighting the role of electrostatic and electrohydrodynamic boundary conditions. Our theory provides a simple explanation of the high conductivity observed experimentally in hydrophilic channels, and allows one to obtain rigorous bounds on its attainable value and scaling with salt concentration. Our results also show that further dramatic amplification of conductivity is possible if hydrophobic slip is involved, but only in the thick channel regime provided the walls are sufficiently highly charged and most of the adsorbed charges are immobile. However, for weakly charged surfaces the massive conductivity amplification due to hydrodynamic slip is impossible in both regimes. Interestingly, in this case the moderate slip-driven contribution to conductivity can monotonously decrease with the fraction of immobile adsorbed charges. These results provide a framework for tuning the conductivity of nanochannels by adjusting their surface properties and bulk electrolyte concentrations.

Electroosmotic Flow Grows with Electrostatic Coupling in Confining Charged Dielectric Surfaces

In this work, the effects of polarization of confining charged planar dielectric surfaces on induced electroosmotic flow are investigated. To this end, we use dissipative particle dynamics to model solvent and ionic particles together with a modified Ewald sum method to model electrostatic interactions and surfaces polarization. A relevant difference between counterions number density profiles, velocity profiles, and volumetric flow rates obtained with and without surface polarization for moderate and high electrostatic coupling parameters is observed. For low coupling parameters, the effect is negligible. An increase of almost 500% in volumetric flow rate for moderate/high electrostatic coupling and surface separation is found when polarizable surfaces are considered. The most important result is that the increase in electrostatic coupling substantially increases the electroosmotic flow in all studied range of separations when the dielectric constant of electrolytes is much higher than the dielectric constant of confining walls. For the higher separation simulated, an increase of around 340% in volumetric flow rate when the electrostatic coupling is increased by a factor of two orders of magnitude is obtained.

Interfacial Diffusion of Hydrated Ion on Graphene Surface: A Molecular Simulation Study

Hydration plays an important role in the diffusion and sieving of ions within nanochannels. However, it is hard to quantitatively analyze the contribution of hydration to the diffusion rates due to the complex hydrogen-bond and charge interactions between atoms. Here, we quantitatively investigated the interfacial diffusion rates of a single hydrated ion with different number of water molecules on graphene surface through molecular dynamics simulation. The simulation results show the ballistic diffusion mode by analyzing the mean-square displacement, and the diffusion rates change nonmonotonically with the hydration number. The potential energy profiles with the changing position of the hydrated ion on graphene surface were further analyzed, which shows the dominant factor for interfacial diffusion changing from ion-graphene interaction to water-graphene interaction as the number of water molecules increases. Besides, it was found that the surface hydrophilicity weakened the influence of hydration number on the diffusion rates of hydrated ion. Finally, the diffusion properties of different hydrated ions on graphene surface were investigated, and the hydrated Li⁺, Na⁺, and K⁺ containing three, four, and five water molecules, respectively, show the fastest diffusion rate. This work demonstrates the interfacial diffusion behavior and mechanism of hydrated ions at the molecular level, which can provide valuable guidance in nanosensors, seawater desalination, and other hydrated ion-related fields.

Correlation function approach for diffusion in confined geometries

This paper describes a formalism for extracting spatially varying transport coefficients from simulations of a molecular fluid in a nanochannel. This approach is applied to self-diffusion of a Lennard-Jones fluid confined between two parallel surfaces. A numerical grid is laid over the domain confining the fluid, and fluid properties are projected onto the grid cells. The time correlation functions between properties in different grid cells are calculated and can be used as the basis for a fitting procedure for extracting spatially varying diffusion coefficients from the simulation. Results for the Lennard-Jones system show that transport behavior varies sharply near the liquid-solid boundary and that the changes depend on the details of the liquid-solid interaction. A quantitative difference between the reduced and detailed models is discussed. It is found that the difference could be associated with assumptions about the form of the transport equations at molecular scales in lieu of problems with the method itself. The study suggests that this approach to fitting molecular simulations to continuum equations may guide the development of appropriate coarse-grained equations to model transport phenomena at nanometer scales.

Non-isothermal flow of an electrolyte in a charged nanochannel

Electrokinetic flows are generally analyzed, assuming isothermal conditions even though such situations are hard to be achieved in practice. In this paper, the flow of a symmetric electrolyte in a charged nanochannel subjected to an axial temperature gradient is investigated using molecular dynamics simulations. We analyze the relative contribution of the Soret effect, the thermoelectric effect, and the double layer potential in the electrical double layer for various surface charges and temperature gradients. We find the flow driven by thermal gradient is analogous to electroosmotic flow. The thermophoretic motion of the electrolyte is significant for negative surface charge than the positive surface charge. The vibrational spectrum of graphene is calculated to delineate the effect of the surface charge polarity on the observed thermophoretic motion of the electrolyte. A unique structure of interfacial water layer is observed for the positive and negative surface charges. We attribute the presence of these structures to the differences in water-carbon interactions existing for various surface charge polarity. For an applied thermal gradient in the range 2.6 K nm-1 to 8 K nm-1, we observe a continuous net flow with average velocities reaching up to 9.4 m s-1 inside the channel for a negative surface charge of -0.101 C m-2. The results indicate that in a charged graphene-based nanochannel, temperature gradients can be employed to induce streaming current, depending on the relative influence of the Soret effect and the double layer potential.

Electro-osmotic flow in hydrophobic nanochannels

An analytical theory of electroosmosis in hydrophobic nanochannels of large surface potential/charge density incorporates a mobility of adsorbed charges and hydrodynamic slip, and is valid both for thin and strongly overlapping diffuse layers.