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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.

International audience; 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.

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., Comment: 6 pages, 4 figures

International audience; When a liquid drop is gently deposited on a wetting solid surface, it spreads due to capillary forces until it reaches a thermodynamical equilibrium set by the relative surface energies of the system. We investigate here experimentally the spreading ability of drops made of yield stress fluids, which flow only if the applied stress is above a finite value. We observe that in this case, after a spreading phase, the motion stops and a well-defined contact angle can be measured. This contact angle depends on the rheological properties of the fluid and in particular on its yield stress, on the drop radius and on the hydrodynamic boundary condition at the surface. These results are quantitatively compared to an analysis showing that, due to the yield stress of the fluid, a mechanical equilibrium is indeed reached which does not correspond to the thermodynamical equilibrium.

International audience; 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.

International audience; Liquid drop impact dynamics depends on the liquid–substrate interaction. In particular, when liquid–solid friction is decreased, the spreading of the impacting drop lasts longer. We characterise this effect by using two types of superhydrophobic surfaces, with similar wetting properties but different friction coefficients. It is found that, for large enough impact velocities, a reduced friction delays the buildup of a viscous boundary layer, and leads to an increase of the time required to reach the maximal radius of the impacting drop. An asymptotic analysis is carried out to quantify this effect, and agrees well with the experimental findings. Interestingly, this novel description complements the general picture of drop impact on solid surfaces, and more generally addresses the issue of drag reduction in the presence of slippage for non-stationary flows.

International audience; 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.

International audience; A liquid foam, a dense assembly of gas bubbles in a surfactant solution, is a deformable porous material. As classically observed in divided systems, electrokinetic transport can be induced, for example, when an electric field is applied from either side of the foam sample. We determine here the liquid fraction profile obtained when a foam is submitted to an electro-osmotic flow, when the surfactant induces so-called rigid or mobile interfaces. We show that the main governing equation for the liquid fraction repartition in space and time is diffusivelike and similar to the one describing pressure-induced drainage only. The electric field however significantly affects the general profile by modifying the boundary conditions. A capillary number that compares electro-osmotic stress and capillary pressure in the foam geometry is introduced and characterizes the magnitude of electrically induced flow in a macroscopic foam. In particular, the ability of a foam to capture liquid from a reservoir is quantitatively estimated

Surface charge controls many static and dynamic properties of soft matter and micro/nanofluidic systems, but its unambiguous measurement forms a challenge. Standard characterization methods typically probe an effective surface charge, which provides limited insight into the distribution and dynamics of charge across the interface, and which cannot predict consistently all surface-charge-governed properties. New experimental approaches provide local information on both structure and transport, but models are typically required to interpret raw data. Conversely, molecular dynamics simulations have helped showing the limits of standard models and developing more accurate ones, but their reliability is limited by the empirical interaction potentials they are usually based on. This review highlights recent developments and limitations in both experimental and computational research focusing on the liquid-solid interface. Based on recent studies, we make the case that coupling of experiments and simulations is pivotal tomitigate methodological shortcomings and address open problems pertaining to charged interfaces., Current Opinion in Colloid & Interface Science (2018)

International audience; 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.

A liquid foam is a dispersion of gas bubbles in a liquid matrix containing surface-active agents. Its flow involves the relative motion of bubbles, which switch neighbours during a so-called topological rearrangement of type 1 (T1). The dynamics of T1 events, as well as foam rheology, have been extensively studied, and experimental results point to the key role played by surfactants in these processes. However, the complex and multiscale nature of the system has so far impeded a complete understanding of the mechanisms involved. In this work, we investigate numerically the effect of surfactants on the rheological response of a 2D sheared bubble cluster. To do so, a level-set method previously employed for simulation of two-phase flow has been extended to include the effects of surfactants. The dynamical processes of the surfactants – diffusion in the liquid and along the interface, adsorption/desorption at the interface – and their coupling with the flow – surfactant advection and Laplace and Marangoni stresses at the interface – are all taken into account explicitly. Through a systematic study of the Biot, capillary and Péclet numbers that characterise the surfactant properties in the simulation, we find that the presence of surfactants can affect the liquid/gas hydrodynamic boundary condition (from a rigid-like situation to a mobile one), which modifies the nature of the flow in the volume from a purely extensional situation to a shear. Furthermore, the work done by surface tension (the 2D analogue of the work by pressure forces), resulting from surfactant and interface dynamics, can be interpreted as an effective dissipation, which reaches a maximum for a Péclet number of order unity. Our results, obtained at high liquid fraction, should provide a reference point, with which experiments and models of T1 dynamics and foam rheology can be compared.

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.

In this article, the fabrication of a single stable cement bubble is investigated. To achieve this goal, the stability of model particle covered bubbles is firstly experimentally studied, by bubbling in a pool filled with micrometric silica particles. Bubble stability is shown to be governed mainly by particle covering rate, which is maximized when particle wetting angle prior to liquid approaches π/2. This angle can be adjusted in situ by electrostatic adsorption of cationic surfactant on silica if proper amount of surfactant is added in the silica suspension. The covering rate is also shown to be governed by the time spent by the bubble in the pool, allowing us to define a timescale for particle adsorption at the liquid/gas interface. In the end, this method is shown to be successful with other types of foamed granular materials such as cement, and the fabrication of a stable and fully covered solid cement bubble is for the first time demonstrated.

International audience; 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 de-formable 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.

International audience; New models of fluid transport are expected to emerge from the confinement of liquids at the nanoscale1, 2, with potential applications in ultrafiltration, desalination and energy conversion3. 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.

International audience; 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.

Decisive advances in the field of nanosciences and nanotechnologies are intimately related to the development of new instruments and of related writing schemes and methodologies. Therefore we have recently proposed the exploitation of the nano-structuring potential of a highly focused ion beam (FIB) as a tool, to overcome intrinsic limitations of current nano-fabrication techniques and to allow innovative patterning schemes that are urgently needed in many nanoscience challenges. In this work, we will first detail a very high-resolution FIB instrument we have developed specifically to meet these nano-fabrication requirements. Then we will introduce and illustrate an advanced FIB processing scheme that is the fabrication of artificial nanopores.

Nanoengraving of membranes as a template for nanopores fabrication is an application field of growing interest. Similarly to the formation of ion tracks in membranes created when high-energetic ions pass trough thin foils, it is possible with a FIB system to fabricate, design and organise nanodevices within thin membranes. In this work, we detail the advanced methodology we have carefully optimised for such deep sub-10nm nanodevices fabrication using our high-performance FIB instrument.

The classical problem of foam film rupture dynamics has been investigated when the film interfaces exhibit very high rigidity due to the presence of specific surfactants. Two new features are reported. First, a strong deviation from the well-known Taylor–Culick law is observed. Second, crack-like patterns can be visualized in the film; these patterns are shown to appear at a well-defined film shrinkage. The key role of surface-active material on these features is quantitatively investigated, pointing to the importance of surface elasticity to describe these fast dynamical processes and thus providing an alternative tool to characterize surface elasticity in conditions extremely far from equilibrium. The origin of the cracks and their consequences on film rupturing dynamics are also discussed.

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.

T1 topological rearrangement, i.e. switching of neighbouring bubbles in a liquid foam, is the elementary process of foam dynamics, and it involves film disappearance and generation. It has been studied extensively as it is crucial in foam rheology or foam collapse. T1 dynamics depends mainly on the surfactants used to generate the foam, and several models taking into account surface viscosity and/or elasticity have been proposed. By performing experiments in a cubic assembly of films, we go a step forward in this global analysis and investigate experimentally the mechanism of formation of the new film. In particular, the flow velocity field is probed by particle tracking and the film thickness is measured by light absorption and interferometric measurements. Two limit behaviours for the film are reported: it may (i) undergo an homogeneous extension, or (ii) resist elongation and remain at rest, new film being created from liquid exchange with connecting meniscus. Both T1 dynamics and film thickness are shown to depend on the competition between these two behaviours. Interestingly, their balance is set by the surfactant solution used, but it is also shown to vary during a single T1 relaxation process.

© The Royal Society of Chemistry 2015. 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.

High resolution FIB offers nowadays the capability to sculpt matter at the nanometer scale. Using FIB to engrave membranes opens new emerging routes for nanoscience tooling. We propose here a way to fabricate tensile SiC membranes (with thicknesses adjustable between 10 and 100nm), in order to drill them with high resolution focused ion beam, to obtain holes below 15nm. Moreover, we show how we can, in some specific conditions, take advantage of the behavior of the irradiated matter (sputtering/redeposition processes) to reduce the size of the drilled hole below 10nm. Eventually, we present two applications of these objects in biophysics and nanoscience tooling.

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.

International audience; The propagation and distribution of oil inside the aqueous network of a foam is investigated in the casewhere oil can invade the foam without breaking it. The oil is injected into an elementary foamarchitecture of nine foam films and four vertices obtained by plunging a cubic frame in a foamingsolution. 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 andwater, different behaviours are observed. For small amounts of oil, a globule is trapped in one singlenode whereas for large oil volumes, it invades the four nodes of the foam film assembly. In both thesecases, a T1 process does not change the oil distribution. However, for intermediate volumes, oil initiallytrapped in one node is able to propagate to the neighbouring nodes after the T1 process. This importantobservation shows that topological rearrangements, which naturally occur in foams when they evolvewith time or when they flow, do affect the distribution of the third phase that they carry. These differentregimes are captured by simple modeling based on the capillary pressure balance inside the foamnetwork. Moreover, in the large-oil-volume limit, a transient situation is evidenced where an oil film istrapped within the freshly formed water film. This oil film modifies the dynamics of the T1 process andcan be stable for up to a few minutes. We expect this mechanism to have consequences on therheological properties of oil-laden foams. Film rupture dynamics is also experimentally captured.

International audience; Osmosis across membranes is intrinsically associated with the concept of semipermeability. Here, however, we demonstrate that osmotic flow can be generated by solute gradients across nonselective, fully permeable nanochannels. Using a fluorescence imaging technique, we are able to measure the water flow rate inside single nanochannels to an unprecedented sensitivity of femtoliters per minute flow rates. Our results indicate the onset of a convective liquid motion under salinity gradients, from the higher to lower electrolyte concentration, which is attributed to diffusio-osmotic transport. To our knowledge, this is the first experimental evidence and quantitative investigation of this subtle interfacially driven transport, which need to be accounted for in nanoscale dynamics. Finally, diffusio-osmotic transport under a neutral polymer gradient is also demonstrated. The experiments highlight the entropic depletion of polymers that occurs at the nanochannel surface, resulting in convective flow in the opposite direction to that seen for electrolytes.

The shape of impacting drops onto a solid surface is investigated by probing the local flow velocity and the local thickness profile of the spreading lamella during the drop impact. First, as a model situation of no viscous coupling between the liquid and the substrate, the impact of a drop onto hot plates, above the Leidenfrost temperature, is considered. In this case, we demonstrate that the velocity and thickness profiles are in good agreement with inviscid convective flow theory. This local description allows us to revisit the modelling of well-studied global behaviour such as drop spreading. Building from this idealized situation, viscous boundary-layer effects emerging from frictional coupling on a cold surface are then captured.

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.

The integration of the nanopore drilled in a nanometric thin and therefore fragile membrane in a macroscopic set-up necessitates a multiscale approach. An integration technique to select and support such a nanopore is described, using a Pyrex device with a micropore. Therefore, we developed a method to fabricate micrometer sized apertures in Pyrex wafers relying on a specific control of HF etching. The integration of FIB nanodrilled membrane in this Pyrex set-up is here discussed.

In this chapter we briefly review the techniques available to researchers in the nanofluidic domain to fabricate nanopores and nanochannels. In this context the focused ion beam (FIB) technique will be introduced as a useful and versatile tool for nanofluidics. We illustrate it with two specific examples involving nanopores as building blocks for nanofluidics.

International audience; 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.

International audience; Armored interfaces refer to fluid interfaces on which a compact monolayer of particles is adsorbed. In this paper, we probe their robustness under impact. For such an investigation, the impact of a drop (covered or not by particles) on a flat armored interface is considered. Two regimes are observed: small drops impacting at low velocities do not coalesce, while bigger drops falling at higher velocities lead to coalescence. The coalescence which occurs when the impacting drop has just reached its maximum extension directly results from the formation of bare regions within the armor. We therefore propose a geometric criterion to describe this transition. This simple modeling is able to capture the dependence of the measured velocity threshold with particle size and drop diameter. The additional robustness experienced by double armors (both drop and puddle covered) results in an increase of the measured velocity threshold, which is quantitatively predicted. © 2013 AIP Publishing LLC.

International audience; 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.

International audience

International audience; We study how shearing clusters of two or four bubbles induces bubble separation or topological rearrangement. The critical deformation at which this yielding occurs is measured as a function of shear rate, liquid composition, and liquid content in the cluster. We establish a geometrical yield criterion in the quasistatic case on the basis of these experimental data as well as simulations. In the dynamic regime, the deformation where the cluster yields increases with the strain rate, and we derive a scaling law describing this phenomenon based on the dynamical inertial rupture of the liquid meniscus linking the two bubbles. Our experiments show that the same scaling law applies to two- and four-bubble clusters.

We investigate the entrance of single poly(ethylene glycol) chains into an $\ensuremath{\alpha}$-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.

International audience; The mechanical properties of particle laden interfaces are investigated by studying capillary wave propagation along the interface. Interfaces are coated with monodisperse silica hydrophobic particles with diameters ranging from 35 mm to 159 mm. The surfaces are prepared with a particle density just above that required for random close packing, so that no macroscopic wrinkles can be observed. Measurements of wave celerity versus wavelength and a comparison of the results with theory allow us to define the stretching modulus and the bending stiffness of such an interface. These quantities are for the first time measured independently and under a large range of frequencies (100 Hz-900 Hz). Particle diameters and contact angles have been varied systematically. We observe that the stretching modulus does not depend on the particle size, while the bending stiffness exhibits variations with the square of particle diameter. These results are confronted with previous results on quasistatic assays and with theoretical predictions on the capillary origin of bending stiffness, showing a good agreement. Similarities and differences are discussed.

International audience; Liquid marbles are liquid droplets covered with hydrophobic particles. This particular layer physically isolates them from the substrate on which they are deposited. In this study, we investigate the properties of such liquid marbles when impacting onto a solid substrate. The different behaviors during impact (non-bouncing, bouncing and rupture) are experimentally characterized and scenarios for understanding the transitions between the three regimes are proposed. Eventually, we highlight the importance of particle surface coverage by comparing the impact of a liquid marble on a smooth surface with the impact of a bare water drop on a rough superhydrophobic microtextured surface. Copyright (C) EPLA, 2012

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.

International audience; The stability of foam is investigated experimentally through coalescence events. Instability (coalescence) occurs when the system is submitted to external perturbations (T1) and when the liquid amount in the film network is below a critical value. Microscopically, transient thick films are observed during film rearrangements. Film rupture, with coalescence and eventual collapse of the foam, occurs when the available local liquid amount is too small for transient films to be formed. Similar experiments and results are shown in the two-bubble case.

International audience; Drop impacts on a smooth plate heated above the Leidenfrost temperature are investigated in the range of large Weber number. Liquid fragmentation due to the rupture of the expanding lamella during the impact—by hole nucleation and subsequent growth—is studied. Control of this rupturing process is achieved experimentally through the use of single model-defects attached to the substrate which act as an initiating spots for the hole formation, whereas the liquid does not contact the substrate. Overall, the lamella rupture is shown to take place above a critical impact velocity, the value of which decreases with increasing defect size. Comparing this rupture mechanism to classical splash, it is shown to be the relevant fragmentation phenomenon below a critical ratio between drop R0 and defect d sizes of R0 / d < 40.

International audience; We investigate the flow of a foam injected through a cylindrical inlet into a quiescent liquid which is miscible with the foaming solution. Depending on a Reynolds number, combining inlet diameter, liquid viscosity, and flow rate, the jet disperses into a conical plume, takes a stable cylindrical straight shape whose radius swells with flow rate or disintegrates into blobs. We compare this behavior to that reported for other complex fluid jets and present a simple physical model for the straight jet regime.

Symposium OO – Materials and Strategies for Lab-on-a-Chip–Biological Analysis, Cell-Material Interfaces and Fluidic Assembly of Nanostructures); International audience; Here we propose to detail an innovative FIB instrumental approach and processing methodologies we have developed for sub-10 nm nanopore fabrication. The main advantage of our method is first to allow direct fabrication of nanopores in relatively large quantities with an excellent reproducibility. Second our approach offers the possibility to further process or functionalize the vicinity of each pore on the same scale keeping the required deep sub-10 nm scale positioning and patterning accuracy. We will summarise the optimisation efforts we have conducted aiming at fabricating thin (10-100 nm thick) and high quality dielectric films to be used as a template for the nanopore fabrication, and at performing efficient and controlled FIB nanoengraving of such a delicate media. Finally, we will describe the method we have developed for integrating these “single nanopore devices” in electrophoresis experiments and our preliminary measurements.

In recent years, nanopores nave become an important tool to study the properties of single biomolecules in ionic solutions. The technique is based on the translocation of biomolecules through pores of comparable size, which are recorded as changes in the pore conductance. Among the various topics that can be studied with this technique are DNA unzipping and protein denaturation Various types of nanopores are used for these experiments, including biological, chemical, and solid‐state pores. Here we present a way to fabricate sub‐5‐nm pores in 20–100 nm thick SiC membranes by direct drilling with a focused ion beam. In order to obtain a user‐friendly setup allowing low‐noise measurements, these membranes have to be carefully integrated into an electrolytic cell. For this purpose, the membranes are fixed on a piece of Pyrex containing a micrometer‐sized aperture, which is used to select the desired pore and at the same time reduces the area of the membrane that is exposed to the liquid, thus reducing the cap...

International audience; We study experimentally the dynamics of strain-induced T1 neighbour switching in clusters of 3D bubbles. To determine the physico-chemical processes that set the time scale of such rearrangements, foaming solutions of a wide range of well characterized bulk and interfacial rheological properties are used. At low strain rates, the time scale is set by a balance between surface tension and surface viscous forces, and we present a simple physical model that explains these findings, on the basis of previous experimental and theoretical work with 2D foams.1 At higher strain rates, rearrangement dynamics are driven by the applied strain. In a wider context, our study of T1s in bubble clusters provides insight into the structural relaxations that accompany the flow of 3D foams.

Controlled and reproducible fabrication of nano-structured materials is one of the main scientific and industrial challenges for the next few years. We have recently proposed exploitation of the nano-structuring potential of a high resolution focused ion beam tool to overcome basic limitations of current nano-fabrication techniques. The aim of this article is to present our research efforts in combining ion source and ion optics specifically for high-resolution applications. First, we detail the very high resolution FIB instrument we have developed specifically to meet nano-fabrication requirements. Finally, we present the instrument now capable of fabricating directly nano-pores with diameters below 5 nm.

A specific experimental set-up has been installed in a large centrifuge facility in order to study different aspects of Leidenfrost drops under high-gravity conditions (5, 10, 15 and 20 times the Earth gravity). In particular, the drop lifetime and more precisely the variations of drop diameter vs. time have shown to be in good agreement with previous experiments and scaling analysis (Biance A.-L. et al., Phys. Fluids, 15 (2003) 1632). Moreover, so-called chimneys are expectedly observed in the large puddles, the distance between two chimneys depending linearly on the capillary length. Finally, the Leidenfrost point, i.e. the temperature above which the Leidenfrost effect takes place, was unexpectedly found to increase slightly with gravity. A qualitative explanation based on a refined model (Sobac B. et al., Phys. Rev. E, 90 (2014) 053011) recognizing the non-trivial shape of the vapor film under the drop is proposed to explain this observation.

We describe the first steps of spreading of a liquid droplet brought in contact with a solid that it wets completely. Usually, it is assumed that the dynamics of the droplet results from a balance between the spreading forces and viscosity. But before this classical stage, inertia resists to the motion, which leads to a very different dynamic law. We study experimentally the nature of this law, compare our results with recent theoretical predictions, and determine the duration of this inertial regime.

New paradigms for fluid transport are expected to emerge from the confinement of liquids at the nanoscales [1- 3], with potential breakthroughs in ultra-filtration, desalination, and energy conversion [4]. Nevertheless, advancing the fundamental understanding of fluid transport at the smallest scales requires mass and ion dynamics to be ultimately characterized across an individual channel to avoid averaging over many pores. To this end, a major challenge for nanofluidics consists in building distinctive and well-controlled nano-channels, amenable for systematic exploration of their properties. In this work [5] we describe the elaboration and exploitation of a new hierarchical nanofluidic device, made of a unique boron-nitride (BN) nanotube that transpierces an ultrathin membrane and connects two fluid reservoirs. Such a transmembrane geometry allows the versatile exploration of fluidic transport through a single nanotube under diverse forcing, including electric fields, pressure drops, and chemical gradients. Using this device, we discover huge osmotically-induced electric currents generated by salinity gradients, exceeding by two orders of magnitude their pressure-driven counterpart. We show that this result originates from the anomalously high surface charge carried by the BN internal surface in water at large pH, which was independently quantified from conductance measurements. The nano-assembling route using nanostructures as building blocks opens new perspectives to explore fluid, ionic and molecule transport at nanoscales, paving the way towards biomimetic functionalities. Our results furthermore suggests that boron-nitride nanotubes could be advantageously exploited as membranes for osmotic power harvesting under salinity gradients, making it a prime renewable source of energy. [1] W. Sparreboom, et al., Nature Nano, 4, 713-720 (2009). [2] L. Bocquet and E. Charlaix, Chem. Soc. Rev. 39, 1073-1095 (2010). [3] J. C. Rasaih, et al., Annu. Rev. Phys. Chem. 59 713-740 (2008). [4] B. E. Logan and M. Elimelech, Nature, 488 313-319 (2012). [5] A. Siria, et al. Nature, 494, 455-458, (2013).