Anomalous ionic transport in tunable angstrom-size water films on silica.

Liquid and ionic transport through nanometric structures is central to many phenomena, ranging from cellular exchanges to water resource management or green energy conversion. While pushing down toward molecular scales progressively unveils novel transport behaviors, reaching ultimate confinement in controlled systems remains challenging and has often involved 2D Van der Waals materials. Here, we propose an alternative route which circumvents demanding nanofabrication steps, partially releases material constraints, and offers continuously tunable molecular confinement. This soft-matter-inspired approach is based on the spontaneous formation of a molecularly thin liquid film onto fully wettable substrates in contact with the vapor phase of the liquid. Using silicon dioxide substrates, water films ranging from angstrom to nanometric thicknesses are formed in this manner, and ionic transport within the film can then be measured. Performing conductance measurements as a function of confinement in these ultimate regimes reveals a one-molecule thick layer of fully hindered transport nearby the silica, above which continuum, bulk-like approaches account for experimental results. Overall, this work paves the way for future investigations of molecular scale nanofluidics and provides insights into ionic transport nearby high surface energy materials such as natural rocks and clays, building concretes, or nanoscale silica membranes used for separation and filtering.

Microscopic Origins of the Viscosity of a Lennard-Jones Liquid.

Unlike crystalline solids or ideal gases, transport properties remain difficult to describe from a microscopic point of view in liquids, whose dynamics result from complex energetic and entropic contributions at the atomic scale. Two scenarios are generally proposed: one represents the dynamics in a fluid as a series of energy-barrier crossings, leading to Arrhenius-like laws, while the other assumes that atoms rearrange themselves by collisions, as exemplified by the free volume model. To assess the validity of these two views, we computed, using molecular dynamics simulations, the transport properties of the Lennard-Jones fluid and tested to what extent the Arrhenius equation and the free volume model describe the temperature dependence of the viscosity and of the diffusion coefficient at fixed pressure. Although both models reproduce the simulation results over a wide range of pressure and temperature covering the liquid and supercritical states of the Lennard-Jones fluid, we found that the parameters of the free volume model can be estimated directly from local structural parameters, also obtained in the simulations. This consistency of the results gives more credibility to the free volume description of transport properties in liquids.

Interfacial Characterization of Ruthenium-Based Amphiphilic Photosensitizers.

Nonreactive surfactant molecules have long been used and characterized for a wide range of applications in industries, life science, and everyday life. Recently, new types of functional amphiphilic molecules have emerged that bear another function, for example, a light-absorbing action, or catalytic properties. However, the surfactant properties of these molecules remain to date essentially unknown. In this context, we investigated here the interfacial activity of photocatalytic surfactants based on a ruthenium(II) tris-bipyridine core, functionalized with two alkyl tails. We realized a systematic characterization of the surfactant properties of these molecules at a water-air interface and studied the effect of the alkyl chain length and of the counterions (hexafluorophosphate or chloride) on these properties. Our data demonstrate that ruthenium surfactants with chloride counteranions form a denser layer at the interface, but their surfactant properties can dramatically deteriorate when the chain length of the alkyl tail increases, leading to simple hydrophobic molecules with poor surfactant properties for the longest chains (C17). These findings pave the way for a better use and understanding of photocatalytic soft interfaces.

Electrokinetic sweeping of colloids at a reactive magnesium oxide interface.

Investigating the electrokinetic (EK) response in the vicinity of interfaces has regained interest due to the development of new membrane based processes for energy harvesting or soil depollution. However, the case of reactive interfaces, ubiquitous in these processes, remains scarcely explored. Here we experimentally investigate the EK response of a model interface between an aqueous electrolyte and a bulk MgO crystal surface (100), for different pH. For that purpose, we use a lab-scale non invasive method to monitor the zeta potential of the interface versus time, by confocal fluorescent particle tracking. An unexpected motion of the particles, repelled and then attracted again by the interface is observed. We attributed this motion to the surface reactivity, inducing ion concentration gradients perpendicular to the interface and subsequent diffusiophoresis of the charged particle. Accordingly, we could describe at a semi-quantitative level the particle dynamics by solving numerically the Poisson-Nernst-Planck equations to establish concentration profile in the system and subsequent diffusiophoretic motion. These experiments open the way to the characterization of both the EK response and the reaction rate in the vicinity of reactive interfaces.

Generation and stability of cement soap films.

Foaming a cementitious suspension is a complex process that involves many multiscale chemical, physical and dynamical mechanisms. As a first step, we investigate here experimentally the possibility of withdrawing a single liquid soap film from a suspension of cement. We then determine the film lifetime and if particles are entrained or not. We vary the cement concentration, grain size, rheological properties and withdrawing velocity. We observed that the rheology of the cement paste, characterized through its yield stress, plays a key role in the film formation. We show that an optimum exists, as a low yield stress promotes film creation but is detrimental to the film stability. Another key result is that the rheology alone is not enough to describe film formation: the particle size in the suspension is also crucial, with large particles promoting film creation. Finally, we found that the withdrawing velocity also affects the ability to create films and the possibility to drag particles in them. Experiments performed with a silica suspension for comparison confirm these findings.

Rupture of granular rafts: effects of particle mobility and polydispersity.

Reversible encapsulation of liquid materials is a technical challenge in many applications such as for the transport and controlled delivery of active ingredients. In contrast to most state-of-the-art processes, capillary adsorbed solid particles can achieve chemical-reaction-free encapsulation by forming dense rafts which isolate the liquid from its surroundings. While the production conditions of such capsules have been characterized, the control of the armor robustness remains poorly described and understood. In this paper, we probe the armor robustness via impacts of droplets on encapsulated materials. Thereby, we establish the mechanisms and conditions of armor rupture and derive models that predict the rupturing thresholds or probabilities. Using monodisperse sized particles and gradually increasing the impacting drop velocity, a sharp transition from sustained to coalescing drops is observed. On mobile rafts made of particles at the water/air interface, the velocity threshold increases with increasing particle diameter while an opposite trend is observed on immobile rafts made of particles trapped at a gelified interface. Two models based on particle pair and triplet interactions, respectively, quantitatively match the experiments. Assembling rafts with particles of two different sizes significantly smoothens the coalescence transition, regardless of particle mobility. Beyond apparent similarities, rationalizing the rupturing probability of mobile and immobile armor evidences very different sensitivity to heterogeneities. On immobile armor, drop coalescence remains random and thus well described by the statistical particle distribution while on mobile armor the ruptures are preferably localized at the non-percolated parts of the granular network.

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.

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.

Bending modulus of bidisperse particle rafts: Local and collective contributions.

The bending modulus of air-water interfaces covered by a monolayer of bidisperse particles is probed experimentally under quasistatic conditions via the compression of the monolayer, and under dynamical conditions studying capillary-wave propagation. Simple averaging of the modulus obtained solely with small or large particles fails to describe our data. Indeed, as observed in other configurations for monodisperse systems, bidisperse rafts have both a granular and an elastic character: chain forces and collective effects must be taken into account to fully understand our results.

Two types of Cassie-to-Wenzel wetting transitions on superhydrophobic surfaces during drop impact.

Despite the fact that superhydrophobic surfaces possess useful and unique properties, their practical application has remained limited by durability issues. Among those, the wetting transition, whereby a surface gets impregnated by the liquid and permanently loses its superhydrophobicity, certainly constitutes the most limiting aspect under many realistic conditions. In this study, we revisit this so-called Cassie-to-Wenzel transition (CWT) under the broadly encountered situation of liquid drop impact. Using model hydrophobic micropillar surfaces of various geometrical characteristics and high speed imaging, we identify that CWT can occur through different mechanisms, and at different impact stages. At early impact stages, right after contact, CWT occurs through the well established dynamic pressure scenario of which we provide here a fully quantitative description. Comparing the critical wetting pressure of surfaces and the theoretical pressure distribution inside the liquid drop, we provide not only the CWT threshold but also the hardly reported wetted area which directly affects the surface spoiling. At a later stage, we report for the first time to our knowledge, a new CWT which occurs during the drop recoil toward bouncing. With the help of numerical simulations, we discuss the mechanism underlying this new transition and provide a simple model based on impulse conservation which successfully captures the transition threshold. By shedding light on the complex interaction between impacting water drops and surface structures, the present study will facilitate designing superhydrophobic surfaces with a desirable wetting state during drop impact.

Ultra-sensitive flow measurement in individual nanopores through pressure--driven particle translocation.

A challenge for the development of nanofluidics is to develop new instrumentation tools, able to probe the extremely small mass transport across individual nanochannels. Such tools are a prerequisite for the fundamental exploration of the breakdown of continuum transport in nanometric confinement. In this letter, we propose a novel method for the measurement of the hydrodynamic permeability of nanometric pores, by diverting the classical technique of Coulter counting to characterize a pressure-driven flow across an individual nanopore. Both the analysis of the translocation rate, as well as the detailed statistics of the dwell time of nanoparticles flowing across a single nanopore, allow us to evaluate the permeability of the system. We reach a sensitivity for the water flow down to a few femtoliters per second, which is more than two orders of magnitude better than state-of-the-art alternative methods.

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.

Thermodynamic and mechanical timescales involved in foam film rupture and liquid foam coalescence.

Recent advances in the coalescence in liquid foams are reviewed, with a special focus on the multiscale structure of foams. Studies concerning the stability of isolated foam films, on the one hand, and the coalescence process in macroscopic foams, on the other hand, are not always in good agreement. This discrepancy reveals that two routes can induce coalescence in a foam. The first route is thermodynamic and shows that coalescence is governed by a stochastic rupture of foam films. The second route relies on a mechanically induced rupture of the films, due to the spontaneous evolution of foams. From a literature review, the evaluation of the different timescales involved in these mechanisms allows defining the limiting parameters of foam coalescence.

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.

Oil repartition in a foam film architecture.

The propagation and distribution of oil inside the aqueous network of a foam is investigated in the case where oil can invade the foam without breaking it. The oil is injected into an elementary foam architecture of nine foam films and four vertices obtained by plunging a cubic frame in a foaming solution. The frame is then deformed to trigger a film switching (topological rearrangement named T1) and oil redistribution through this process is reported. Depending on the relative ratio of injected oil and water, different behaviours are observed. For small amounts of oil, a globule is trapped in one single node whereas for large oil volumes, it invades the four nodes of the foam film assembly. In both these cases, a T1 process does not change the oil distribution. However, for intermediate volumes, oil initially trapped in one node is able to propagate to the neighbouring nodes after the T1 process. This important observation shows that topological rearrangements, which naturally occur in foams when they evolve with time or when they flow, do affect the distribution of the third phase that they carry. These different regimes are captured by simple modeling based on the capillary pressure balance inside the foam network. Moreover, in the large-oil-volume limit, a transient situation is evidenced where an oil film is trapped within the freshly formed water film. This oil film modifies the dynamics of the T1 process and can be stable for up to a few minutes. We expect this mechanism to have consequences on the rheological properties of oil-laden foams. Film rupture dynamics is also experimentally captured.

Giant osmotic energy conversion measured in a single transmembrane boron nitride nanotube.

New models of fluid transport are expected to emerge from the confinement of liquids at the nanoscale, with potential applications in ultrafiltration, desalination and energy conversion. Nevertheless, advancing our fundamental understanding of fluid transport on the smallest scales requires mass and ion dynamics to be ultimately characterized across an individual channel to avoid averaging over many pores. A major challenge for nanofluidics thus lies in building distinct and well-controlled nanochannels, amenable to the systematic exploration of their properties. Here we describe the fabrication and use of a hierarchical nanofluidic device made of a boron nitride nanotube that pierces an ultrathin membrane and connects two fluid reservoirs. Such a transmembrane geometry allows the detailed study of fluidic transport through a single nanotube under diverse forces, including electric fields, pressure drops and chemical gradients. Using this device, we discover very large, osmotically induced electric currents generated by salinity gradients, exceeding by two orders of magnitude their pressure-driven counterpart. We show that this result originates in the anomalously high surface charge carried by the nanotube's internal surface in water at large pH, which we independently quantify in conductance measurements. The nano-assembly route using nanostructures as building blocks opens the way to studying fluid, ionic and molecule transport on the nanoscale, and may lead to biomimetic functionalities. Our results furthermore suggest that boron nitride nanotubes could be used as membranes for osmotic power harvesting under salinity gradients.

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.

Large apparent electric size of solid-state nanopores due to spatially extended surface conduction.

Ion transport through nanopores drilled in thin membranes is central to numerous applications, including biosensing and ion selective membranes. This paper reports experiments, numerical calculations, and theoretical predictions demonstrating an unexpectedly large ionic conduction in solid-state nanopores, taking its origin in anomalous entrance effects. In contrast to naive expectations based on analogies with electric circuits, the surface conductance inside the nanopore is shown to perturb the three-dimensional electric current streamlines far outside the nanopore in order to meet charge conservation at the pore entrance. This unexpected contribution to the ionic conductance can be interpreted in terms of an apparent electric size of the solid-state nanopore, which is much larger than its geometric counterpart whenever the number of charges carried by the nanopore surface exceeds its bulk counterpart. This apparent electric size, which can reach hundreds of nanometers, can have a major impact on the electrical detection of translocation events through nanopores, as well as for ionic transport in biological nanopores.

Transport of long neutral polymers in the semidilute regime through a protein nanopore.

We investigate the entrance of single poly(ethylene glycol) chains into an α-hemolysin channel. We detect the frequency and duration of the current blockades induced by large neutral polymers, where chain radius is larger than pore diameter. In the semidilute regime, these chains pass only if the monomer concentration is larger than a well-defined threshold. Experiments are performed in a very large domain of concentration and molecular mass, up to 35% and 200 kDa, respectively, which was previously unexplored. The variation of the dwell time as a function of molecular mass shows that the chains are extracted from the semidilute solution in contact with the pore by a reptation mechanism.

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.