Romain Lhermerout

Durant cette thèse, nous avons d'abord développé un dispositif expérimental permettant de mesurer la dynamique de l'angle de contact avec une précision record de 0,01° sur 7 décades de vitesses de la ligne triple, gamme jamais atteinte auparavant. Pour la première fois, la résolution numérique des équations de lubrification a permis de déduire l'angle de contact à l'échelle microscopique de ces mesures macroscopiques, découplant donc le problème hydrodynamique multi-échelles de la physique de la ligne de contact à petite échelle. Avec ces outils, nous avons montré qu'une pseudo-brosse - une couche nanométrique de polymères - peut complètement piloter la dynamique, en produisant des hystérésis les plus faibles jamais mesurées (<0,07° !) et des surdissipations massives provenant de la nature visco-élastique de la couche. Cette étude ouvre la voie à la nano-rhéologie, permettant de sonder la dynamique extrêmement rapide (~100 ns) de polymères confinés à l'échelle nanométrique. Grâce à un travail collaboratif fructueux, nous avons ensuite développé un modèle permettant de décrire quantitativement et de façon unifiée la dissipation hydrodynamique, l'hystérésis et l'activation thermique. Enfin, beaucoup d'efforts ont été fournis pour la fabrication de surfaces aux défauts nanométriques contrôlés en taille, forme et concentration. La dynamique s'est révélée insensible à cette échelle de désordre, la présence des défauts n'affectant que l'hystérésis. Ces résultats ont été interprétés semi-quantitativement avec des lois d'échelle, et la caractérisation complète des défauts devrait permettre à terme de développer des modèles plus quantitatifs.

This article shows the differentiation between water adsorbed outside of a single-walled carbon nanotube (CNT) from that confined inside. This distinction is made possible by tracking the electronic transport of a CNT-based field effect transistor constructed with an individual nanotube and exposed to controlled environments. Presence of water shifts the electrical neutrality point, indicating charge transfer between the nanotube and its environment. We identify three types of water molecules: (i) chemically adsorbed on the SiO2 surface, forming silanol groups, (ii) physically adsorbed outside the nanotube, and (iii) confined inside. The first type is eliminated only by high-temperature treatment under vacuum, while the latter two desorb at room temperature under moderate or high vacuum, i.e. 10^-3 mbar. We observe that water confinement inside the nanotube is fast and thermodynamically favorable, with no qualitative influence from the metallicity of the nanotube.

Quadrature phase differential interferometry is a technique of choice for measuring arbitrary large displacements with record resolution. The so-called Heydemann corrections are generally employed to take account of the unavoidable imperfections due to the optics or the electronics, and a linearisation of the signals is usually performed when the displacement to be determined is the superposition of a quasi-static component and a small harmonic component. Here we show that this standard analysis can lead to non-linearities of two types. The first type of non-linearities is due to optical and electronic imperfections that were not reported in the past and not included in the Heydemann corrections. These imperfections appear as amplitude difference and phase shift between each pair of intensity signals, and produce periodic non-linearities of nanometer amplitude when the intensity signals are combined to obtain contrats, i.e. quantities that are insensitive to intensity fluctuations of the light source. The second type of non-linearities is due to large amplitudes of harmonic oscillation, which become significant when the oscillations are typically larger than 20 nm for a red-emitting laser source, and can lead to an underestimation of the measured amplitude of ∼ 20 % . In order to correct these non-linearities, new procedures of calibration and analysis are presented, implemented and successfully tested over a wide range of quasi-static velocities, frequencies and amplitudes of harmonic oscillation. The proposed corrections reduce non-linearities below the noise level, making them smaller than 100 pm and 10 pm for the quasi-static and harmonic displacement, respectively.

Water-in-salt electrolytes are a fascinating new class of highly concentrated aqueous solutions with wide electrochemical stability windows that make them viable as aqueous battery electrolytes. However, the high ion concentration of water-in-salt electrolytes means that these systems are poorly understood when compared to more dilute electrolyte solutions. Here, we present direct surface force measurements across thin films of a water-in-salt electrolyte at several concentrations. We find that the electrolyte adopts a layered structure at charged interfaces composed of a nanostructure of a hydrated cation and nonaqueous anion-rich domains. These observations will aid in the interpretation of capacitance and double-layer behavior of water-in-salt electrolytes with consequences for their use in energy storage devices.

Experimental investigations of surface forces generally involve two solid bodies of simple and well-defined geometry interacting across a medium. Direct measurement of their surface interaction can be interpreted to reveal fundamental physics in confinement, i.e. independent of the particular geometry. However real solids are deformable - they can change shape due to their mutual interaction - and this can influence force measurements. These aspects are frequently not considered, and remain poorly understood. We have performed experiments in a dry atmosphere and across an ionic liquid with a Surface Force Balance (SFB), combining measurement of the surface interactions and simultaneous in-situ characterization of the geometry. First we find that, whilst the variation of the contact radius with the force across dry nitrogen can be interpreted by the Johnson-Kendall-Roberts (JKR) model, for the (ionic) liquid it is well described only by the Derjaguin-Muller-Toporov (DMT) model. Secondly, we find that mica does not only bend but also experiences a compression. By performing experiments with substantially thicker mica than usual we were able to investigate this with high precision, and find compression of order 1 nm with 7 um mica. These findings imply that, in some cases structural forces measured across nanoconfined liquids must be interpreted as a convolution of the surface forces across the liquid and the mechanical response of the confining solids. We discuss the influence of mica thickness, and propose a scaling criterion to distinguish situations where the solid deformation is negligible and when it is dominant.

When confined at the nanoscale between smooth surfaces, an ionic liquid forms a structured film responding to shear in a quantized way, i.e., with a friction coefficient indexed by the number of layers in the gap. So far, only a few experiments have been performed to study this phenomenon, because of the delicate nature of the measurements. We propose a new methodology to measure friction with a surface force balance, based on the simultaneous application of normal and lateral motions to the surfaces, allowing for a more precise, comprehensive and rapid determination of the friction response. We report on proof-of-concept experiments with an ionic liquid confined between mica surfaces in dry or wet conditions, showing the phenomenon of quantized friction with an unprecedented resolution. First, we show that the variation of the kinetic friction force with the applied load for a given layer is not linear, but can be quantitatively described by two additive contributions that are respectively proportional to the load and to the contact area. Then, we find that humidity improves the resistance of the layers to be squeezed-out and extends the range of loads in which the liquid behaves as a superlubricant, interpreted by an enhanced dissolution of the potassium ions on the mica leading to a larger surface charge. There, we note a liquid-like friction behavior, and observe in certain conditions a clear variation of the kinetic friction force over two decades of shearing velocities, that does not obey a simple Arrhenius dynamics.

Buckminsterfullerenes (C60) are near-spherical molecules which freely rotate at room temperature in the solid state and when dissolved in solution. An intriguing question arises as to whether C60 molecules can act as 'molecular ball bearings', i.e. preventing direct contact between two solid surfaces whilst simultaneously dissipating shear stress through fast rotation. To explore this, we performed measurements of friction across a solution of C60 in the boundary lubrication regime. High resolution shear and normal force measurements between mica sheets separated by the C60 solution were made using a Surface Force Balance to provide a single-asperity contact and sub-nanometer resolution in film thickness. We find that, even at small volume fraction, C60 forms a solid-like amorphous boundary film sustaining high normal load, suggesting that this system undergoes a glass transition in confinement. The C60 film gives rise to a low friction coefficient up to moderate applied loads, and we discuss the possible relevance of the ball bearing effect at the molecular scale.