Atomic force microscopy studies of nanotribology and nanomechanics

This thesis comes within the scope of tribology studies at the nanometer scale. The experimental techniques used in this work are essentially related to atomic force microscopy, which gives a direct access to the topography and forces of the studied systems. Two principal aspects of the nanotribology have kept our attention. They are the nanofriction and the nanomechanics. These two domains divide the thesis in two main parts that remain however intimately bound. The first experimental part of the thesis describes the friction of a nanoscopic tip sliding on hydrophilic surfaces. The dependence of the friction force on the sliding velocity is studied for various applied normal loads and surrounding relative humidity levels. We found a logarithmic dependence of the friction force on both the scanning velocity and the relative humidity. For the first time, a transition from a positive to a negative slope of the friction force versus the logarithm of the sliding velocity has been observed for the very same tip-surface contact, by varying the relative humidity. The role of the relative humidity on the friction force has then been studied more deeply, leading to a two-thirds power law dependence of the capillary force on the normal load. Finally, analytical expressions for the friction phenomenon and the capillary condensation between the asperities of the tip and the surface have been developed to explain the phenomenological behaviors. The second experimental part describes themechanics of carbon nanotubes and tobacco mosaic viruses. In order to stay in the linear elasticity regime, small indentation amplitudes have been applied in the radial direction of multiwalled carbon nanotubes adsorbed on a silicon oxide surface. Using a theory based on the Hertz model, the radial stiffness has been evaluated and compared to molecular dynamics simulations. We found a radial Young modulus strongly decreasing with increasing radius and reaching an asymptotic value of 30 ± 10 GPa. The tobacco mosaic viruses have been adsorbed on a polyimide porous membrane. Evidence for the softness of the viruses has been obtained by imaging the tubes with the atomic force microscope in non-contact mode. Viruses hanging like ropes over the pores of the surface served as basis to measure their bending Young modulus. Using a model of a clamped beam loaded by a discrete gradient of van der Waals forces, we found a bending Young modulus of 3.1 ± 0.1 MPa.

Radial elasticity of multiwalled carbon nanotubes

Harald Brune, Stephan Fedrigo, Ismaël Palaci

We report an experimental and a theoretical study of the radial elasticity of multiwalled carbon nanotubes as a function of external radius. We use atomic force microscopy and apply small indentation amplitudes in order to stay in the linear elasticity regime. The number of layers for a given tube radius is inferred from transmission electron microscopy, revealing constant ratios of external to internal radii. This enables a comparison with molecular dynamics results, which also shed some light onto the applicability of Hertz theory in this context. Using this theory, we find a radial Young modulus strongly decreasing with increasing radius and reaching an asymptotic value of 30&PLUSMN; 10 GPa.

2005

Characterization of Solid-Liquid Interfaces with High-Resolution Atomic Force Microscop

Maria Ricci

Solid-liquid interfaces are ubiquitous. Despite the unquestioned relevance, many scientific research fields rely on simplified or macroscopic descriptions, discarding the molecular nature of the constituents. However, it is the specific molecular interactions and the complex chemistry in the interfacial region that are fundamental in phenomena such as heterogeneous catalysis, electrochemistry, crystal growth, membrane transport and body-antibody recognition. In particular, the models used to describe the so-called electrical double layer of ionic distributions at charged surfaces in liquid, are based on continuum assumptions. For example, the Gouy-Chapman-Stern model assumes that a dense, homogenously distributed layer with a thickness of a hydrated ion (referred to as the "Stern layer"), is adsorbed onto the charged surface, proceeded by an exponentially decaying diffuse ionic cloud, in which ions are point charges in a continuous dielectric media. In this thesis, I will first illustrate how the assumption of a homogeneous ion lateral density in the Stern Layer fails to describe the actual solid-water interface of several relevant surfaces. Then I will show how the lateral organization of the ions indeed plays a role as important as the vertical modulation of the charge distribution. This is because the molecular nature of the ions, the surface, and the water molecules themselves each contribute to the precise lateral organization of the constituents. As result, the ions can reside on discrete surface sites, defined both laterally and vertically in space. Moreover, the water-mediated interaction can impart a correlation in the reciprocal lateral distribution of the ions. To achieve this level of description, high-resolution atomic force microscopy (AFM) is used. Unlike most of other experimental techniques, AFM is able to characterize interfaces locally at the atomic/molecular level. In fact, in the AFM small-amplitude regime, detection of even single ions adsorbed at the surface of the solid is possible by measuring the perturbation they induce on the local solvation environment. This capability represents an invaluable tool for the exploration of phenomena occurring at the Stern layer of solid-liquid interfaces. Following, I will investigate the properties of several solid-aqueous interfaces in the presence of different ionic concentrations and species, ranging from hard (mica) to soft systems such as organic self-assembled monolayers, lipid bilayers, and the dynamic surface restructuring of calcite when covered with fatty acid molecules. The experimental study, combined with molecular dynamic simulations, reveals a water-induced ion-ion attractive interaction for some ionic species. These results show that water alone is the driving force to induce order within the Stern layer, creating hydration-correlation effects on mica, self-assembled monolayers and possibly lipid bilayers. The local modulation of the hydration properties of the surface is fundamental to highly dynamic systems such as calcite, where steps act as nucleating points for the processes of dissolution and growth. The specific interaction of foreign ions and fatty acids modify these processes as well as the equilibrium between the crystal and the solution. This thesis provides clear experimental evidence of new mechanisms developing at solid-liquid interfaces paving the way for a deeper and more conscious understanding of the colloidal properties of materials.

EPFL2015

Physical properties and interactions of biomolecules

Mélanie Favre

The aim of this thesis was to explore the statistical physics and the mechanical properties of individual biomolecules, to study their interactions, and to develop new techniques based on atomic force microscopy (AFM). First, we exploit the high resolution imaging of AFM over long DNA molecules adsorbed on a surface to measure the average end-to-end distance as a function of the DNA length, along with its full distribution function. The latter is almost never determined experimentally, and is not available with ensemble techniques, such as light scattering. We found that the scaling exponents are closer to the values predicted by the polymer theory for a 3D self-avoiding random walk (SAW) than for a 2D SAW. These results suggest that the adsorption process of the DNA molecules is akin to a geometric projection from 3D to 2D, known to preserve the scaling properties of fractal objects of dimension df < 2. Second, taking advantage of the possibility of AFM to also manipulate biomolecules, we investigate the specific interactions between the glycoprotein avidin and the vitamin biotin, the ribonuclease barnase and its inhibitor barstar, and the West Nile Virus surface glycoprotein E and the laminin binding protein. The dependence of the most probable unbinding force on the force loading rate has revealed details of the energy landscapes of the unbinding process of these highly efficient and specific interactions. Beside this standard method, we have measured the lifetime of the avidin-biotin bond, kept stretched at a constant force, as a function of this force, and combined this approach with a small dithering of the AFM tip in order to further explore the avidin-biotin energy landscape. Finally, the viscoelastic properties of biomolecules, such as dextran and titin, have been studied using standard force spectroscopy combined with a small oscillation of the AFM tip. A theoretical description for excitation frequencies near the first and the second resonance of the cantilever has been developed to be able to extract, from the amplitude and phase measurement of the oscillations, the stiffness and friction coefficients of the biomolecule being stretched. We conclude that an excitation frequency of 3 kHz gives reliable results for the stiffness measurement whereas frequencies of 15-25 kHz are more appropriate for the dissipation measurements.