Amplification of electro-osmotic flows by wall slippage: direct measurements on OTS-surfaces

Effects of salinity on the flow of dense colloidal suspensions

We experimentally study the effects of salt concentration on the flowing dynamics of dense suspensions of micrometer-sized silica particles in microfluidic drums. In pure water, the particles are fully sedimented under their own weight, but do not touch each other due to their negative surface charges, which results in a "frictionless" dense colloidal suspension. When the pile is inclined above a critical angle θc ∼ 5° a fast avalanche occurs, similar to what is expected for classical athermal granular media. When inclined below this angle, the pile slowly creeps until it reaches flatness. Adding ions in solution screens the repulsive forces between particles, and the flowing properties of the suspension are modified. We observe significant changes in the fast avalanche regime: a time delay appears before the onset of the avalanche and increases with the salt concentration, the whole dynamics becomes slower, and the critical angle θc increases from ∼5° to ∼20°. In contrast, the slow creep regime does not seem to be heavily modified. These behaviors can be explained by considering an increase in both the initial packing fraction of the suspension Φ0, and the effective friction between the particles μp. These observations are confirmed by confocal microscopy measurements to estimate the initial packing fraction of the suspensions, and AFM measurements to quantify the particles surface roughness and the repulsion forces, as a function of the ionic strength of the suspensions.

Flow effects on the surface properties of surfactant foam films

The surface electrostatic properties of liquid foam, involving the electrokinetic (EK) phenomena in the liquid-gas interface, have significant effects on the stability of the foam. Here, we established a theoretical model for ion transport in liquid films by combining the liquid flow and surface reaction. We found that the surface electrostatic properties of liquid foams were influenced unexpectedly by the pressure-induced flow. The liquid flow will induce the potential and concentration differences in the flow direction. When the pressure drop increases to a certain high value, the induced potential and salt concentration difference increases, leading to the change of the surface electrostatic properties such as zeta potential and the surface charge density. This change shows that the surface electrostatic properties of foam films depend on the coupling of various factors including ion distribution and pressure drop, which deepens our understanding of the electrostatic properties of the foam films.

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.

Nanoconfined Fluids: What Can We Expect from Them?

Nanoconfined fluids (NCFs), which are confined in nanospaces, exhibit distinctive nanoscale effects, including surface effects, small-size effects, quantum effects, and others. The continuous medium hypothesis in fluid mechanics is not valid in this context because of the comparable characteristic length of spaces and molecular mean free path, and accordingly, the classical continuum theories developed for the bulk fluids usually cannot describe the mass and energy transport of NCFs. In this Perspective, we summarize the nanoscale effects on the thermodynamics, mass transport, flow dynamics, heat transfer, phase change, and energy transport of NCFs and highlight the related representative works. The applications of NCFs in the fields of membrane separation, oil and gas production, energy harvesting and storage, and biological engineering are especially indicated. Currently, the theoretical description framework of NCFs is still missing, and it is expected that this framework can be established by adopting the classical continuum theories with the consideration of nanoscale effects.

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.

Large slippage and depletion layer at the polyelectrolyte/solid interface

The slippage of polymer solutions on solid surfaces is often attributed to a depletion layer whose origin, thickness, and interaction with the flow are poorly understood. Using a Dynamic Surface Force Apparatus we report a structural and nanorheological study of the interface between hydrolyzed poly-acrylamide solutions and platinum surfaces. Polyelectrolyte chains adsorb on the surfaces in a thin charged layer, acting as a nonattractive wall for the bulk solution. We investigate the flow of the visco-elastic solution on the adsorbed layer from the nanometer to 10 micrometers, bridging microscopic to macroscopic properties. At distances larger than 200 nanometers, the flow is well described by an apparent slip boundary condition. At smaller distance the apparent slip is found to decrease with the gap. In contrast to the apparent slip model, we show that a 2-fluids model taking into account the finite thickness of depletion layers at the non-attractive wall describes accurately the dynamic forces over 4 spatial decades of confinement. Depletion layers are found to be an equilibrium property of the interface, independent on the flow and on the confinement. Their thickness is phenomenologically described by ξ + 2l_D with ξ the correlation length of the semi-dilute solutions and l_D the Debye length. We interpret this result in terms of screened repulsion between the charged adsorbed layer and the bulk polyions.

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.

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.

What Controls Thermo-osmosis? Molecular Simulations Show the Critical Role of Interfacial Hydrodynamics

Thermo-osmotic and related thermophoretic phenomena can be found in many situations from biology to colloid science, but the underlying molecular mechanisms remain largely unexplored. Using molecular dynamics simulations, we measure the thermo-osmosis coefficient by both mechanocaloric and thermo-osmotic routes, for different solid-liquid interfacial energies. The simulations reveal, in particular, the crucial role of nanoscale interfacial hydrodynamics. For nonwetting surfaces, thermo-osmotic transport is largely amplified by hydrodynamic slip at the interface. For wetting surfaces, the position of the hydrodynamic shear plane plays a key role in determining the amplitude and sign of the thermo-osmosis coefficient. Finally, we measure a giant thermo-osmotic response of the water-graphene interface, which we relate to the very low interfacial friction displayed by this system. These results open new perspectives for the design of efficient functional interfaces for, e.g., waste-heat harvesting.

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.

Analysis of charging-induced structural damage in electrochemical systems

Electrochemical charging of an electrochemical system results in Maxwell stress and a variation in the surface energy of the electrode. The charging-induced variation in the surface energy of the electrode and Maxwell stress introduce mechanical deformation and create stresses in the electrode. To relax the strain energy stored in the electrode, surface cracking and buckling can occur. Under the condition that the charging-induced change in the surface energy is the dominant factor controlling the in-plane deformation of a planar electrode, i.e. the bonding between the adsorbate and the substrate is weak, the variation in the surface energy of the electrode can be described by the Lippmann equation. Using the Lippmann equation, both the surface charge density on the electrode and the surface energy of the electrode are formulated as a function of the electrode potential. Analytical relations between the critical electrode potential and average damage size have been obtained for the charging-induced cracking and buckling in a planar, thin-film electrode. The results show that surface cracking will prevail over local buckling in accordance with experimental observations. Both critical electrode potentials are inversely proportional to the square root of the magnitude of the differential capacity of the electrical double layer at the electrode potential of zero charge.

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.

Effects of Surfactants and Polyelectrolytes on the Interaction between a Negatively Charged Surface and a Hydrophobic Polymer Surface

We have measured and characterized how three classes of surface-active molecules self-assemble at, and modulate the interfacial forces between, a negatively charged mica surface and a hydrophobic end-grafted polydimethylsiloxane (PDMS) polymer surface in solution. We provide a broad overview of how chemical and structural properties of surfactant molecules result in different self-assembled structures at polymer and mineral surfaces, by studying three characteristic surfactants: (1) an anionic aliphatic surfactant, sodium dodecyl sulfate (SDS), (2) a cationic aliphatic surfactant, myristyltrimethylammonium bromide (MTAB), and (3) a silicone polyelectrolyte with a long-chain PDMS midblock and multiple cationic end groups. Through surface forces apparatus measurements, we show that the separate addition of three surfactants can result in interaction energies ranging from fully attractive to fully repulsive. Specifically, SDS adsorbs at the PDMS surface as a monolayer and modifies the monotonic electrostatic repulsion to a mica surface. MTAB adsorbs at both the PDMS (as a monolayer) and the mica surface (as a monolayer or bilayer), resulting in concentration-dependent interactions, including a long-range electrostatic repulsion, a short-range steric hydration repulsion, and a short-range hydrophobic attraction. The cationic polyelectrolyte adsorbs as a monolayer on the PDMS and causes a long-range electrostatic attraction to mica, which can be modulated to a monotonic repulsion upon further addition of SDS. Therefore, through judicious selection of surfactants, we show how to modify the magnitude and sign of the interaction energy at different separation distances between hydrophobic and hydrophilic surfaces, which govern the static and kinetic stability of colloidal dispersions. Additionally, we demonstrate how the charge density of silicone polyelectrolytes modifies both their self-assembly at polymer interfaces and the robust adhesion of thin PDMS films to target surfaces.

Anomalous ζ potential in foam films

Electrokinetic effects offer a method of choice to control flows in micro- and nanofluidic systems. While a rather clear picture of these phenomena exists now for the liquid-solid interfaces, the case of liquid-air interfaces remains largely unexplored. Here, we investigate at the molecular level electrokinetic transport in a liquid film covered with ionic surfactants. We find that the ζ potential, quantifying the amplitude of electrokinetic effects, depends on the surfactant coverage in an unexpected way. First, it increases upon lowering surfactant coverage from saturation. Second, it does not vanish in the limit of low coverage but instead approaches a finite value. This behavior is rationalized by taking into account the key role of interfacial hydrodynamics, together with an ion-binding mechanism. We point out implications of these results for the strongly debated measurements of the ζ potential at free interfaces and for electrokinetic transport in liquid foams.

The study of surface wetting, nanobubbles and boundary slip with an applied voltage: A review

The drag of fluid flow at the solid-liquid interface in the micro/nanoscale is an important issue in micro/nanofluidic systems. Drag depends on the surface wetting, nanobubbles, surface charge and boundary slip. Some researchers have focused on the relationship between these interface properties. In this review, the influence of an applied voltage on the surface wettability, nanobubbles, surface charge density and slip length are discussed. The contact angle (CA) and contact angle hysteresis (CAH) of a droplet of deionized (DI) water on a hydrophobic polystyrene (PS) surface were measured with applied direct current (DC) and alternating current (AC) voltages. The nanobubbles in DI water and three kinds of saline solution on a PS surface were imaged when a voltage was applied. The influence of the surface charge density on the nanobubbles was analyzed. Then the slip length and the electrostatic force on the probe were measured on an octadecyltrichlorosilane (OTS) surface with applied voltage. The influence of the surface charge on the boundary slip and drag of fluid flow has been discussed. Finally, the influence of the applied voltage on the surface wetting, nanobubbles, surface charge, boundary slip and the drag of liquid flow are summarized. With a smaller surface charge density which could be achieved by applying a voltage on the surface, larger and fewer nanobubbles, a larger slip length and a smaller drag of liquid flow could be found.

Influence of hydrophobic effects on streaming potential

We study the influence of hydrophobic effects on streaming potential mediated flow through a narrow confinement. In a clear departure from the approach used in prior works, we use a phase-field model to capture the hydrophobicity-induced depletion in the near wall region, and express the variation of viscosity and permittivity across the interfacial layer in terms of the phase-field variable. We then use these in the determination of the flow velocity, and highlight the sensitive interplay between the intrinsic length scale of the electrical double layer and that of the depletion in terms of the variations of an effective normalized viscosity that captures the electroviscous effect. We expect that this work will be an important step forward in the realistic continuum modeling of interfacial physics in the particular context of streaming potential mediated flows.

Capacitive detection of buried interfaces by a dynamic surface force apparatus

We present here a new type of distance sensor mounted on a Surface Force Apparatus (SFA), able to detect the position of a buried interface and therefore the thickness of a thin solid or soft matter film coating the SFA surface(s). This sensor relies on the capacitance created by the two metallized surfaces of the SFA. An harmonic oscillation of these polarized surfaces creates a pico- to femto-amps current indicating their relative position. One of the specificities of this sensor is the relatively weak polarization used for the measurements, minimizing the electrical forces and their impact on other interactions, hydrodynamical and mechanical forces measured by the SFA. This original and simple design is of high interest for studying the viscoelastic properties of thin films, to detect adsorbed liquid layers or slippage at liquid-solid interfaces, or even to study complex fluids such as ionic liquids under polarization. We demonstrate the use of this sensor to study the flow boundary condition of silicon oil on a metal surface, and the elastic modulus of a thin elastomer layer.

Soft nanofluidic transport in a soap film

We investigate experimentally the electrokinetic properties of soft nanofluidic channels that consist in soap films with nanometric thickness, covered with charged surfactants. Both the electric and fluidic responses of the system are measured under an applied voltage drop along the film. The electric field is shown to induce an electro-osmotic hydrodynamic flow in the film. However, in contrast to systems confined between solid surfaces, the soft nature of the nanochannel results furthermore in a thickening of the film. This effect accordingly increases the total electro-osmotic flow rate, which behaves nonlinearly with the applied electric field. This behavior is rationalized in terms of an analogy with a Landau-Levich film withdrawn from a reservoir, with the driving velocity identified here with the electro-osmotic one.

Consistent description of electrohydrodynamics in narrow fluidic confinements in the presence of hydrophobic interactions

Electrohydrodynamics in the presence of hydrophobic interactions in narrow confinements is traditionally represented from a continuum viewpoint by a Navier slip-based conceptual paradigm, in which the slip length carries the sole burden of incorporating the effects of substrate wettability on interfacial electromechanics, precluding any explicit dependence of the interfacial potential distribution on the substrate wettability. Here we show that this traditional way of treating electrokinetics-wettability coupling may lead to serious discrepancies in predicting the resultant transport characteristics as manifested through an effective zeta potential. We suggest that an alternative consistent description of the underlying physics through a free-energy-based formalism, in conjunction with considerations of hydrodynamic and electrical property variations consistent with the pertinent phase-field description, may represent the underlying consequences in a more rational manner, as compared to the traditional slip-based model coupled with a two-layer description. Our studies further reveal that the above discrepancies may not occur solely due to the slip-based route of representing the interfacial wettability, but may be additionally attributed to the act of "discretizing" the interfacial phase fraction distribution through an artificial two-layer route.

A flux monitoring method for easy and accurate flow rate measurement in pressure-driven flows

We propose a low-cost and versatile method to measure flow rate in microfluidic channels under pressure-driven flows, thereby providing a simple characterization of the hydrodynamic permeability of the system. The technique is inspired by the current monitoring method usually employed to characterize electro-osmotic flows, and makes use of the measurement of the time-dependent electric resistance inside the channel associated with a moving salt front. We have successfully tested the method in a micrometer-size channel, as well as in a complex microfluidic channel with a varying cross-section, demonstrating its ability in detecting internal shape variations.

A deliberation on nanobubbles at surfaces and in bulk

Surface and bulk nanobubbles are two types of nanoscopic gaseous domain that have recently been discovered in interfacial physics. Both are expected to be unstable to dissolution because of the high internal pressure driving diffusion and the surface tension which squeezes the gas out, but there is a rapidly growing body of experimental evidence that demonstrates both bubble types to be stable. However, the two types of bubbles also differ in many respects: surface nanobubble stability is most probably assisted by the nearby wall, which can repel the water (in the case of hydrophobicity), accept physisorbed gas molecules, and reduce the surface area through which outfluxing can occur; bulk nanobubbles, on the other hand, must stabilise themselves. This is perhaps through ionic shielding, perhaps through diffusive shielding, or perhaps through both. Herein, the features of both bubble types are described individually, their common and disparate features are discussed, and emerging applications are examined.