Using the atomic force microscope (AFM), the pull-off forces between flat glass or silicon surfaces and silicon AFM tips or glass microspheres of different sizes have been extensively studied as a function of relative humidity (RH) in the range 5-90%, as model systems for the behavior of cohesive powders. The glass and silicon substrates were treated to render them either hydrophobic or hydrophilic. All the hydrophilic surfaces gave simple force curves and pull-off forces increasing uniformly with RH. Small contacts (R ∼ 20 nm) gave pull-off forces close to values predicted by simple Laplace-Kelvin theory (∼20 nN), but the values with microspheres (R ∼ 20 µm) fell well below predictions for sphere-flat or sphere-sphere geometry, due to roughness and asperity contacts. The hydrophobic silicon surfaces also exhibited simple behavior, with no significant RH dependence. The pull-off force again fell well below predicted values (Johnson-Kendall-Roberts contact mechanics theory) for the larger contacts. Hydrophobic glass gave similar adhesion to silicon over most of the RH range, but against both silicon tips and glass microspheres, there was an anomalously large adhesion in the RH range 20-40%, accompanied by a long-range noncontact force. The adhesion on fully hydrophilic surfaces and its RH dependence can be mostly explained by current theories of capillary bridges, but the interpretation is complicated by the sensitivity of theoretical predictions to contact geometry (and hence to roughness effects) and by uncertainties in the thickness of adsorbed water layers. The anomalous behavior on hydrophobic glass surfaces at intermediate values of RH is not fully understood, but possible causes are (1) dipole layers in the partially formed water film, giving rise to patch charges and long-range forces, or (2) fixed charges at a reactive glass surface, involving specific bonding reactions. The results may be useful in explaining the behavior of cohesive powders with different coatings or those which show a large humidity dependence (e.g., zeolites) or show electrostatic charging effects (e.g., silica aerogels).

There is a demand for good theoretical understanding of the response of an atomic force microscope cantilever to the extremely nonlinear impacts received while tapping a sample. A model and numerical simulations are presented in this paper which provide a very pleasing comparison with experimental results. The dependence of the cantilever amplitude and phase upon the sample stiffness, adhesion and damping are investigated using these simulations, and it is found that 'topographic' tapping images are not independent of sample properties, nor will it be trivial to measure materials' properties from the tapping data. The simulation can be applied to other probe microscope configurations as well.

Since the introduction of force microscopy in 1986 as a tool for imaging insulators, it has increasingly been acclaimed as a quantitative probe of surface forces such as van der Waals, capillary, electrostatic, capacitive, double-layer, magnetic or adhesive forces. A plot of the force interaction between two surfaces-typically a tip mounted on a cantilever beam and a flat surfac&as a function of relative tipsample separation constitutes a force curve, and such measurements have been termed 'force spectroscopy' (FS). We descfibe how to interpret force curves so as to gain information about (i) the instant of tipsample contact, (ii) the magnitude and functional dependence of adhesive and long-range attractive forces for different tip/sample combinations, (iii) possible mechanisms of long-range force interaction (surface layers, fNed dipoles, patch charges), (i v) tipsample contact area, which relates to the imaging mechanism in contact-mode force microscopy, (v) the elastic modulus and plasticity of thin and thick films, and (vi) how the pull-off force varies as a function of the maximum load.

The terms micro-thermal analysis and micro-spectroscopic analysis are used to include any form of localized characterization or analysis combined with microscopy that uses a near-field thermal probe to exploit the benefits of using thermal excitation. Individual regions of a solid sample are selected by means of surface or sub-surface imaging (atomic force microscopy and/or scanning thermal microscopy), so as to add spatial discrimination to four well-established methods of chemical fingerprinting, namely thermomechanometry, calorimetry, spectroscopy and analytical pyrolysis. We begin by describing the state of the art of scanning microscopy that uses resistive thermal probes, followed by an account of the various techniques of micro-thermal analysis. Modern materials technology is increasingly concerned with the control of materials at the mesoscale. The ability to add an extra dimension of, say, chemical composition information to high-resolution microscopy, or microscopic information to spectroscopy, plays an increasingly useful part in applied research. Micro-thermal analysis is now being used commercially to visualize the spatial distribution of phases, components and contaminants in polymers, pharmaceuticals, foods, biological materials and electronic materials. This review outlines various applications that have been described in the literature to date, the topics ranging from multi-layer packaging materials and interphase regions in composites, to the use of the technique as a means of surface treatment.