Olivier Noel

Force Curve Measurements with the AFM: Application to the In Situ Determination of Grafted Silicon-Wafer Surface Energies

The atomic force microscope (AFM) can be used to perform surface force measurements in the quasi-static mode (cantilever is not oscillating) to investigate nanoscale surface properties. Nevertheless, there is still a lack of literature proposing a complete systematic and rigorous experimental procedure that enables one to obtain reproducible and significant quantitative data. This article focuses on the fundamental experimental difficulties arising when making force curve measurements with the AFM in air. On the basis of this AFM calibration procedure, quantitative assessment values were used to determine, in situ, SAM (or Self Assembled Monolayer)-tip thermodynamic work of adhesion at a local scale, which have been found to be in good agreement with quoted values. Finally, determination of surface energies of functionalised silicon wafers (as received, CH3, OH functionalised silicon wafers) with the AFM (at a local scale) is also proposed and compared with the values obtained by wettability (at a macroscopic scale). In particular, the effect of the capillary forces is discussed.

Circular mode: A new scanning probe microscopy method for investigating surface properties at constant and continuous scanning velocities

In this paper, we introduce a novel scanning probe microscopy mode, called the circular mode, which offers expanded capabilities for surface investigations especially for measuring physical properties that require high scanning velocities and/or continuous displacement with no rest periods. To achieve these specific conditions, we have implemented a circular horizontal displacement of the probe relative to the sample plane. Thus the relative probe displacement follows a circular path rather than the conventional back and forth linear one. The circular mode offers advantages such as high and constant scanning velocities, the possibility to be combined with other classical operating modes, and a simpler calibration method of the actuators generating the relative displacement. As application examples of this mode, we report its ability to (1) investigate the influence of scanning velocity on adhesion forces, (2) measure easily and instantly the friction coefficient, and (3) generate wear tracks very rapidly for tribological investigations.

Quantitative Spreading Kinetics of a Three Molecular Layer Liquid Patch

The late stage kinetics of the spreading of a smectic nanodrop on a solid surface was investigated by direct and real time imaging of a three molecular layer patch using the SEEC microscopy. Experimental data do not conform to the only available theory, which covers only weakly stratified liquids. A new model is proposed, in remarkable agreement with experiments, in which the spreading mechanism appears to be a quasi-static process ruled by solid/liquid interactions, 2D Laplace pressure, and separate edge and surface permeation coefficients.

Contact Atomic Force Microscopy: A Powerful Tool in Adhesion Science

Adhesion between two objects appears confusing or ambiguous, because the term is employed generally for two things: first, the formation of the interface between a pair of materials, i.e. the establishment of interfacial bonds through forces at the interface which cause materials to attract one another and second, the breaking stress or energy required to break the formed assembly. One can easily see that both interfacial forces and mechanical properties of adherents in the vicinity of the interface and in the bulk contribute to the global mechanical response of the assembly. Such a fundamental issue reflects a paradox that has stimulated intensive research for decades: what is the interplay between surface forces, surface rheology, and adhesive strength? In recent years, Atomic Force Microscopy (AFM) has become a powerful tool, sensitive enough, to detect small surface forces and to study adhesion at the nanoscale. Precise analysis of adhesion forces and surface mechanical properties of model polymer surfaces can be achieved with such a nanometer probe. The purpose and scope of this chapter is to highlight the experimental methods that enable one to dissociate the different contributions (chemical and mechanical) included in an AFM force-distance curve in order to establish quantitative relationships between interfacial tip–polymer interactions and surface viscoelastic properties of a polymer surface. New relationships are proposed that provide a complete understanding of how the adhesion separation energy depends on both surface chemistry and rheological behavior of the surface and thus at a local scale.

From Permeation to Pore Nucleation in Smectic Stacks

The last stage of the spreading of a stratified droplet in the odd wetting case is the evolution from a trilayer to a monolayer, that is, vanishing of the last bilayer in the stack. We studied it in the case of 8CB smectic liquid crystal on a hydrophilic surface. Receding of the last bilayer is accompanied by formation of pores in it, which appear in the outer part of it. From analysis of real-time experimental observations of this phenomenon, we demonstrate that the dislocation loops which border these pores are not located at the same height in the trilayer stack as the dislocation lines that border the bilayer. Also, careful analysis of our results using a recently developed theoretical approach of smectic liquid nanodrop spreading strongly suggests that pore nucleation is triggered by differences in chemical potential between adjacent layers, which contrasts with the classical scheme where it is attributed to lateral tension along the layers.

Surface Mechanical Property Determination of Soft Materials Through an AFM Nanoindentation Experiment

In this paper, we have studied the capability of the AFM to perform nanoindentation experiments. We mainly focused on the importance of a rigorous experimental procedure to get quantitative and reproducible data with the Atomic Force Microscopy (AFM). Systematic calibration procedure of AFM measurements, as well as a complete description of the mechanical behavior of the soft material is necessary before producing reliable quantitative data. In particular, the influences of the creep and of the strain rate have been studied. Then, this technique was used to probe model cross-linked polydimethylsiloxane (PDMS) and to extract their surface mechanical properties at the nanoscale. Young modulus of each sample was calculated by comparing different contact mechanics theories (Hertz and JKR theories and a power law expression). In conclusion, the contact mechanics relationships have to be redefined and adapted to soft materials. In particular, it appears necessary to consider the specific mechanical contribution of the polymers.

Exploring wear at the nanoscale with circular mode atomic force microscopy

The development of atomic force microscopy (AFM) has allowed wear mechanisms to be investigated at the nanometer scale by means of a single asperity contact generated by an AFM tip and an interacting surface. However, the low wear rate at the nanoscale and the thermal drift require fastidious quantitative measurements of the wear volume for determining wear laws. In this paper, we describe a new, effective, experimental methodology based on circular mode AFM, which generates high frequency, circular displacements of the contact. Under such conditions, the wear rate is significant and the drift of the piezoelectric actuator is limited. As a result, well-defined wear tracks are generated and an accurate computation of the wear volume is possible. Finally, we describe the advantages of this method and we report a relevant application example addressing a Cu/Al2O3 nanocomposite material used in industrial applications.

Decoupling of the Chemical and Mechanical Surface Contributions in a Force Curve Measurement with AFM

Atomic Force Microscope (AFM) was used to perform surface force measurements in contact mode to investigate surface properties of model systems at the nanoscale. Model systems were considered and compared. The first one was related to systems of controlled chemical surface properties with identical mechanical properties (chemically modified silicon substrates with hydroxyl, amine, methyl and ester functional groups). The second one deals with model polymer networks (Cross-linked polydimethylsiloxane or PDMS) of controlled mechanical properties and identical surface chemistry. The third system consists in a model polymer network, whose surface is chemically controlled with the same groups as before with silicon substrates. The results show that the viscoelastic contribution is dominating in the adhesion force measurement. Finally, we propose a relationship (derived from the Gent and Schultzs one), which expresses the AFM adhesion force as a function of mechanical energy dissipated in the contact and the surface properties of the material.