Anomalous ζ potential in foam films

Impart of Heterogeneous Charge Polarization and Distribution on Friction at Water-Graphene Interfaces: a Density-Functional-Theory based Machine Learning Study

Accurately characterizing friction behaviors at water-solid interfaces remains a challenge because of the dynamic nature of water molecules and temporal variations in solid surface charges. By using a density-functional-theory (DFT) based machine learning (ML) technique and long-time ML-parametrized molecular dynamics simulations, we have systematically investigated water-induced charge polarization and redistribution on graphene, as well as its impact on friction at water-graphene interfaces. Heterogeneous charge polarization and distribution are observed for water-covered graphene accompanied by the formation of electric double layers (EDLs). The introduction of defects into graphene significantly enhances the heterogeneity in charge polarization and distribution. Compared to pristine graphene, defected graphene exhibits reduced friction at water-graphene interfaces due to stronger charge heterogeneity, resulting in lower surface charge density and the inverse relationship between slip length and surface charge density for EDLs. Our results highlight the pivotal roles of defects and charge heterogeneity in reducing friction at water-graphene interfaces.

Energy harvesting from water streaming at charged surface

Water flowing at a charged surface may produce electricity, known as streaming current/potentials, which may be traced back to the 19th century. However, due to the low gained power and efficiencies, the energy conversion from streaming current was far from usable. The emergence of micro/nanofluidic technology and nanomaterials significantly increases the power (density) and energy conversion efficiency. In this review, we conclude the fundamentals and recent progress in electrical double layers at the charged surface. We estimate the generated power by hydrodynamic energy dissipation in multi-scaling flows considering the viscous systems with slipping boundary and inertia systems. Then, we review the coupling of volume flow and current flow by the Onsager relation, as well as the figure of merits and efficiency. We summarize the state-of-the-art of electrokinetic energy conversions, including critical performance metrics such as efficiencies, power densities, and generated voltages in various systems. We discuss the advantages and possible constraints by the figure of merits, including single-phase flow and flying droplets.

Electrokinetic, electrochemical, and electrostatic surface potentials of the pristine water liquid-vapor interface

Although conceptually simple, the air-water interface displays rich behavior and is subject to intense experimental and theoretical investigations. Different definitions of the electrostatic surface potential as well as different calculation methods, each relevant for distinct experimental scenarios, lead to widely varying potential magnitudes and sometimes even different signs. Based on quantum-chemical density-functional-theory molecular dynamics (DFT-MD) simulations, different surface potentials are evaluated and compared to force-field (FF) MD simulations. As well explained in the literature, the laterally averaged electrostatic surface potential, accessible to electron holography, is dominated by the trace of the water molecular quadrupole moment, and using DFT-MD amounts to +4.35 V inside the water phase, very different from results obtained with FF water models which yield negative values of the order of -0.4 to -0.6 V. Thus, when predicting potentials within water molecules, as relevant for photoelectron spectroscopy and non-linear interface-specific spectroscopy, DFT simulations should be used. The electrochemical surface potential, relevant for ion transfer reactions and ion surface adsorption, is much smaller, less than 200 mV in magnitude, and depends specifically on the ion radius. Charge transfer between interfacial water molecules leads to a sizable surface potential as well. However, when probing electrokinetics by explicitly applying a lateral electric field in DFT-MD simulations, the electrokinetic ζ-potential turns out to be negligible, in agreement with predictions using continuous hydrodynamic models. Thus, interfacial polarization charges from intermolecular charge transfer do not lead to significant electrokinetic mobility at the pristine vapor-liquid water interface, even assuming these transfer charges are mobile in an external electric field.

Harnessing bubble behaviors for developing new analytical strategies

Gas bubbles are easily accessible and offer many unique characteristic properties of a gas/liquid two-phase system for developing new analytical methods. In this minireview, we discuss the newly developed analytical strategies that harness the behaviors of bubbles. Recent advancements include the utilization of the gas/liquid interfacial activity of bubbles for detection and preconcentration of surface-active compounds; the employment of the gas phase properties of bubbles for acoustic imaging and detection, microfluidic analysis, electrochemical sensing, and emission spectroscopy; and the application of the mass transport behaviors at the gas/liquid interface in gas sensing, biosensing, and nanofluidics. These studies have demonstrated the versatility of gas bubbles as a platform for developing new analytical strategies.

Liquid-Solid Slip on Charged Walls: The Dramatic Impact of Charge Distribution

Nanofluidic systems show great promise for applications in energy conversion, where their performance can be enhanced by nanoscale liquid-solid slip. However, efficiency is also controlled by surface charge, which is known to reduce slip. Combining molecular dynamics simulations and analytical developments, we show the dramatic impact of surface charge distribution on the slip-charge coupling. Homogeneously charged graphene exhibits a very favorable slip-charge relation (rationalized with a new theoretical model correcting some weaknesses of the existing ones), leading to giant electrokinetic energy conversion. In contrast, slip is strongly affected on heterogeneously charged surfaces, due to the viscous drag induced by counterions trapped on the surface. In that case slip should depend on the detailed physical chemistry of the interface controlling the fraction of bound ions. Our numerical results and theoretical models provide new fundamental insight into the molecular mechanisms of liquid-solid slip, and practical guidelines for searching new functional interfaces with optimal energy conversion properties, e.g., for blue energy or waste heat harvesting.

Nanomolar Surface-Active Charged Impurities Account for the Zeta Potential of Hydrophobic Surfaces

The electrification of hydrophobic surfaces is an intensely debated subject in physical chemistry. We theoretically study the ζ potential of hydrophobic surfaces for varying pH and salt concentration by solving the Poisson-Boltzmann and Stokes equations with individual ionic adsorption affinities. Using the ionic surface affinities extracted from the experimentally measured surface tension of the air-electrolyte interface, we first show that the interfacial adsorption and repulsion of small inorganic ions such as H₃O⁺, OH^-, HCO₃^-, and CO₃^2- cannot account for the ζ potential observed in experiments because the surface affinities of these ions are too small. Even if we take hydrodynamic slip into account, the characteristic dependence of the ζ potential on pH and salt concentration cannot be reproduced. Instead, to explain the sizable experimentally measured ζ potential of hydrophobic surfaces, we assume minute amounts of impurities in the water and include the impurities' acidic and basic reactions with water. We find good agreement between our predictions and the reported experimental ζ potential data of various hydrophobic surfaces if we account for impurities that consist of a mixture of weak acids (pK_a = 5-7) and weak bases (pK_b = 12) at a concentration of the order of 10^-7 M.

Electrokinetic Transport of a Charged Dye in a Freely Suspended Liquid Film: Experiments and Numerical Simulations

Electrokinetic transport of a charged dye within a free liquid film stabilized by a cationic surfactant, trimethyl(tetradecyl)ammonium bromide, subjected to an external electric field was investigated. Confocal laser scanning microscopy was used to visualize fluorescein isothiocyanate (FITC) separation within the stabilized liquid film. Numerical simulations were performed using the finite element method to model the dynamics of charged dye separation fronts observed in the experiments. Because of the electrochemical reactions at the electrodes, significant spatial and temporal pH changes were observed within the liquid film. These local pH changes could affect the local zeta potential at the gas-liquid and solid-liquid film boundaries; hence, the flow field was found to be highly dynamic and complex. The charged dye (FITC) used in the experiments is pH-sensitive, and therefore, electrophoresis of the dye also depended on the local pH. The pH and the electroosmotic flow field predicted from the numerical simulations were useful for understanding charged dye separation near both the anode and the cathode.

Dimension-reconfigurable bubble film nanochannel for wetting based sensing

Dimensions and surface properties are the predominant factors for the applications of nanofluidic devices. Here we use a thin liquid film as a nanochannel by inserting a gas bubble in a glass capillary, a technique we name bubble-based film nanofluidics. The height of the film nanochannel can be regulated by the Debye length and wettability, while the length independently changed by applied pressure. The film nanochannel behaves functionally identically to classical solid state nanochannels, as ion concentration polarizations. Furthermore, the film nanochannels can be used for label-free immunosensing, by principle of wettability change at the solid interface. The optimal sensitivity for the biotin-streptavidin reaction is two orders of magnitude higher than for the solid state nanochannel, suitable for a full range of electrolyte concentrations. We believe that the film nanochannel represents a class of nanofluidic devices that is of interest for fundamental studies and also can be widely applied, due to its reconfigurable dimensions, low cost, ease of fabrication and multiphase interfaces.

Adsorption of ionic surfactants at the air-water interface: The gap between theory and experiment

We review the experimental and theoretical results for the adsorption and structure of ionic surfactants at the air-liquid interface. The results show that ionic surfactants form thick adsorption layers at the interfacial region. We also review several adsorption models for ionic surfactants, which become increasingly complex as they capture the many features of adsorption layers. However, the adsorption layer structures determined by experiments and the structures predicted by models do not match because most models assume very thin adsorption layers. We show the discrepancies between measured and predicted surface properties and provide several explanations. We conclude that the mismatch in the adsorption layer structure provided by experiments and the structure provided by adsorption models is the main reason for the discrepancies in the surface excess and the surface potential.

Giant Thermoelectric Response of Nanofluidic Systems Driven by Water Excess Enthalpy

Nanofluidic systems could in principle be used to produce electricity from waste heat, but current theoretical descriptions predict a rather poor performance as compared to thermoelectric solid materials. Here we investigate the thermoelectric response of NaCl and NaI solutions confined between charged walls, using molecular dynamics simulations. We compute a giant thermoelectric response, 2 orders of magnitude larger than the predictions of standard models. We show that water excess enthalpy-neglected in the standard picture-plays a dominant role in combination with the electro-osmotic mobility of the liquid-solid interface. Accordingly, the thermoelectric response can be boosted using surfaces with large hydrodynamic slip. Overall, the heat harvesting performance of the model systems considered here is comparable to that of the best thermoelectric materials, and the fundamental insight provided by molecular dynamics suggests guidelines to further optimize the performance, opening the way to recycle waste heat using nanofluidic devices.

Flow effects on silicate dissolution and ion transport at an aqueous interface

Flow effects on chemical reactions at a solid-liquid interface are fundamental to diverse technological applications but remain poorly understood from a molecular perspective. In this work, we demonstrate that the coupling between laminar flow and surface chemistry can be adequately described using classical density functional theory for ion distributions near the surface in conjunction with kinetics modeling and the Navier-Stokes equation. In good agreement with recent experiments, we find that flowing of fresh water over a silica surface may result in drastic changes in the rate of silica dissolution and, consequently, the surface charge density and the interfacial structure. A nonlinear streaming current is predicted when the surface reactions are disturbed by a laminar flow.

Interfacial transport with mobile surface charges and consequences for ionic transport in carbon nanotubes

In this paper, we explore the effect of a finite surface charge mobility on the interfacial transport: conductance, streaming currents, electro- and diffusio-osmotic flows. We first show that the surface charge mobility modifies the hydrodynamic boundary condition for the fluid, which introduces a supplementary term depending on the applied electric field. In particular, the resulting slip length is found to decrease inversely with the surface charge. We then derive expressions for the various transport mobilities, highlighting that the surface charge mobility merely moderates the amplification effect of interfacial slippage, to the noticeable exception of diffusio-osmosis and surface conductance. Our calculations, obtained within Poisson-Boltzmann framework, highlight the importance of non-linear electrostatic contributions to predict the small concentration/large charge limiting regimes for the transport mobilities. We discuss these predictions in the context of recent electrokinetic experiments with carbon nanotubes.

Electroosmosis near surfactant laden liquid-air interfaces

Generation of an electroosmostic (EO) flow near a liquid-gas interface covered with ionic surfactants is experimentally investigated. A combination of microscopic flow measurements with a molecular characterization of the interface by second harmonic generation (SHG) shows that under an electrical forcing, although a liquid flow is generated below the free surface, the surfactants remain immobile. The zeta potential was then determined and compared to the surfactant surface coverage. This combination of experimental techniques opens the route to simultaneously probe the liquid flow near a soapy interface and the corresponding surfactant repartition affecting the hydrodynamic boundary condition.

Viscous interfacial layer formation causes electroosmotic mobility reversal in monovalent electrolytes

Using molecular dynamics simulations, the ion density, shear viscosity and electroosmotic mobility of an aqueous monovalent electrolyte at a charged solid surface are studied as a function of the surface charge density.

Electrokinetic transport in liquid foams

Investigating electrokinetic transport in a liquid foam is at the confluence of two well developed research areas. On one hand, the study of electrokinetic flows (i.e. surface-driven flows generated close to a charged interface) is fairly well understood in regards the solid/liquid interface. On the other hand, the flow of liquid in a 3D deformable network, i.e a foam, under a volume force such as gravity has been thoroughly studied over the past decade. The overlapping zone of these two frameworks is of great interest for both communities as it gives rise to challenging new questions such as: what is the importance of the nature of the charged interface, created by mobile and soluble surfactants in the case of foam, on electrokinetic transport? How does a foam behave when submitted to a surface-driven flow? Can we compensate a volume-driven flow, i.e. gravity, by a surface-driven flow, i.e. electroosmosis? In this review, we will explore these questions on three different scales: a surfactant laden interface, a foam film and a macroscopic foam.

Electro-osmosis at surfactant-laden liquid-gas interfaces: beyond standard models

Electro-osmosis (EO) is a powerful tool to manipulate liquids in micro and nanofluidic systems. While EO has been studied extensively at liquid-solid interfaces, the case of liquid-vapor interfaces, found e.g. in foam films and bubbles, remains to be explored. Here we perform molecular dynamics (MD) simulations of EO in a film of aqueous electrolyte covered with fluid layers of ionic surfactants and surrounded by gas. Following the experimental procedure, we compute the zeta potential from the EO velocity, defined as the velocity difference between the middle of the liquid film and the surrounding gas. We show that the zeta potential can be smaller or larger than existing predictions depending on the surfactant coverage. We explain the failure of previous descriptions by the fact that surfactants and bound ions move as rigid bodies and do not transmit the electric driving force to the liquid locally. Considering the reciprocal streaming current effect, we then develop an extended model, which can be used to predict the experimental zeta potential of surfactant-laden liquid-gas interfaces.

Strong Coupling between Nanofluidic Transport and Interfacial Chemistry: How Defect Reactivity Controls Liquid-Solid Friction through Hydrogen Bonding

Defects are inevitably present in nanofluidic systems, yet the role they play in nanofluidic transport remains poorly understood. Here, we report ab initio molecular dynamics (AIMD) simulations of the friction of liquid water on defective graphene and boron nitride sheets. We show that water dissociates at certain defects and that these "reactive" defects lead to much larger friction than the "nonreactive" defects at which water molecules remain intact. Furthermore, we find that friction is extremely sensitive to the chemical structure of reactive defects and to the number of hydrogen bonds they can partake in with the liquid. Finally, we discuss how the insight obtained from AIMD can be used to quantify the influence of defects on friction in nanofluidic devices for water treatment and sustainable energy harvesting. Overall, we provide new insight into the role of interfacial chemistry on nanofluidic transport in real, defective systems.

Rapid capillary filling via ion-water interactions over the nanoscale

Giant frictional resistances are grand challenges against the rapid filling of nanoscale capillaries, as encountered in a wide variety of applications ranging from nature to energy. It is commonly believed that partially wettable charged nanocapillaries fill up considerably slower, compared to completely wettable ones, under the influence of a complex interplay between interfacial tension and electrical interactions. In sharp contrast to this common belief, here we discover a new non-intuitive regime of rapid filling of charged capillaries over the nanometer scale, by virtue of which a partially wettable capillary may fill up comparatively faster than a completely wettable one. We attribute the fundamental origin of this remarkable behavior to ion-water interactions over interfacial scales. The underlying novel electro-hydrodynamic mechanism, as unveiled here, may provide deeper insights into the physico-chemical interactions leading to augmentations in the rates of nanocapillary filling over hydrophobic regimes, bearing far-reaching implications in the transport of biological fluids, enhanced oil recovery, and miniaturized energy harvesting applications.

Surface conductivity measurements in nanometric to micrometric foam films

Foam films (a liquid lamella in air covered by surfactants) are tools of choice for nanofluidic characterization as they are intrinsically nanometric. Their size is indeed fixed by a balance between external pressure and particular molecular interactions in the vicinity of interfaces. To probe the exact nature of these interfaces, different characterizations can be performed. Among them, conductivity in confined systems is a direct probe of the electrostatic environment in the vicinity of the surface. Therefore, we designed a dedicated experiment to measure this conductivity in a cylindrical bubble coupled to interferometry for film thickness characterization. We then show that this conductivity depends on the surfactant nature. These conductivity measurements have been performed in an extremely confined system, the so called Newton black foam films. Unexpectedly in this case, a conductivity close to surface conductivity is recovered.

Electrohydrodynamics near hydrophobic surfaces

We show that an electro-osmotic flow near the slippery hydrophobic surface depends strongly on the mobility of surface charges, which are balanced by counterions of the electrostatic diffuse layer. For a hydrophobic surface with immobile charges, the fluid transport is considerably amplified by the existence of a hydrodynamic slippage. In contrast, near the hydrophobic surface with mobile adsorbed charges, it is also controlled by an additional electric force, which increases the shear stress at the slipping interface. To account for this, we formulate electrohydrodynamic boundary conditions at the slipping interface, which should be applied to quantify electro-osmotic flows instead of hydrodynamic boundary conditions. Our theoretical predictions are fully supported by dissipative particle dynamics simulations with explicit charges. These results lead to a new interpretation of zeta potential of hydrophobic surfaces.