G. Gremaud

Scanning local‐acceleration microscopy

By adapting a scanning force microscope to operate at frequencies above the highest tip–sample resonance, the sensitivity of the microscope to materials’ properties is greatly enhanced. The cantilever’s behavior in response to high‐frequency excitation from a transducer underneath the sample is fundamentally different than to its low‐frequency response. In this article, the motivations, instrumentation, theory, and first results for this technique are described.

Materials' properties measurements: Choosing the optimal scanning probe microscope configuration

Rheological models are used to represent different scanning probe microscope configurations. The solutions for their static and dynamic behavior are found and used to analyze which scanning probe microscope configuration is best for a given application. We find that modulating the sample at high frequencies results in the best microscope behavior for measuring the stiffness of rigid materials, and that by modulating the tip at low frequencies and detecting the motion of the tip itself (not its position relative to the tip holder) should be best for studying compliant materials in liquids.

The jamming route to the glass state in weakly perturbed granular media.

It has been suggested that a common conceptual framework known as ‘jamming’ (refs 1 and 2) may be used to classify a wide variety of physical systems; these include granular media, colloidal suspensions and glass-forming liquids, all of which display a critical slowdown in their dynamics before a sudden transition to an amorphous rigid state. Decreasing the relevant control parameter (such as temperature, drive or inverse density) may cause geometrical constraints to build up progressively and thus restrict the accessible part of the systems phase space. In glass-forming liquids (thermal molecular systems), jamming is provided by the classical vitrification process of supercooling, characterized by a rapidly increasing and apparently diverging viscosity at sufficiently low temperatures. In driven (athermal) macroscopic systems, a similar slowdown has been predicted to occur, notably in sheared foam or vibrated granular media. Here we report experimental evidence for dynamic behaviour, qualitatively analogous to supercooling, in a driven granular system of macroscopic millimetre-size particles. The granular medium is perturbed by isolated tapping or continuous vibration, with the perturbation intensity serving as a control parameter. We observe the random deflection of an immersed torsion oscillator that moves each time the grains rearrange, like a ‘thermometer’ sensing the granular noise. We caution that our granular analogy to supercooling is based on similarities in the dynamical behaviour, rather than quantitative theory.

High-frequency mechanical spectroscopy with an atomic force microscope

In this article we further develop local mechanical spectroscopy and extend the frequency range over which it can be used. Using a heterodyne method to measure the deflection of the cantilever enables one to measure the probe vibration at any frequency. Since the detection sensitivity of force gradients follows a f2 dependence, extending the frequency range from 1 to more than 5 MHz increases the sensitivity by over an order of magnitude. This setup is combined with a realistic model of the cantilever taking into account the geometry of the cantilever. The model is presented and discussed, and compared with experimental behavior measured on WC–Co and NiTi–epoxy samples. Experimental moduli of 730±50 and 260±40 GPa are obtained for WC and Co, respectively.

Dynamic mechanical analysis at the submicron scale

Dynamic mechanical analysis (DMA) is traditionally performed on bulk samples. However, studies of polymer blends would be enhanced if DMA could be applied on a local scale in order to enable a new form of microthermal analysis. Mounting a sample on a vibrating heating stage and observing the resulting amplitude and phase of the motion of an atomic force microscope cantilever allows the local elastic and visco-elastic properties to be studied. It is demonstrated in this article on samples of polyethersulfone/poly (acryonitrile-co-styrene) and polystyrene/poly(methyl methacrylate) (PS/PMMA) blends, and PMMA, PS and polytetrafluoroethylene homopolymers. Images at a specific temperature and spectroscopic data as a function of temperature of (nominally) a single point were collected. Primary and secondary relaxations were detected; the lateral resolution is better than 100 nm. We discuss the promising and limiting aspects of this new technique.

The coupling technique: a two-wave acoustic method for the study of dislocation dynamics

Progress in the study of dislocation dynamics has been achieved using a two‐wave acoustic method, which has been called the coupling technique. In this method, the attenuation α and the velocity v of ultrasonic waves are measured in a sample submitted simultaneously to a harmonic stress σ of low frequency. Closed curves Δα(σ) and Δv/v(σ) are drawn during each cycle of the applied stress. The shapes of these curves and their evolution are characteristic of each dislocation motion mechanism which is activated by the low‐frequency applied stress. For this reason, the closed curves Δα(σ) and Δv/v(σ) can be considered as signatures of the interaction mechanism which controls the low‐frequency dislocation motion. In this paper, the concept of signature is presented and explained with some experimental examples. It will also be shown that theoretical models can be developed which explain very well the experimental results.

Combination of scratch-test and acoustic microscopy imaging for the study of coating adhesion

The scratch-test is particularly well suited for the characterization of the adhesion of thin films and acoustic microscopy imaging for the in-depth observation of defects through opaque materials. This work proposes an original combination of these two different techniques for the study of adhesion of thick ductile coatings electrodeposited on a relatively rough substrate, as a function of surface preparation. On the tangential force curve, two distinct transitions have been recorded as a function of displacement along the scratch and correlated with substrate preparation. Interface acoustic images of scratches show pronounced acoustic contrast due to the propagation of delamination around the scratch. This is the only technique capable of imaging such interface defects which are due to the stress induced by the scratch experiment. The delaminated area was measured in these images and could be correlated with coating adhesion. The complementarity of the two methods used is emphasized. The combined approach described in this paper should be useful for the characterization of many other film-substrate systems.

Correlations between adhesion hysteresis and friction at molecular scales

Correlations between adhesion hysteresis and local friction are theoretically and experimentally investigated. The model is based on the classical theory of adhesional friction, contact mechanics, capillary hysteresis, and nanoscale roughness. Adhesion hysteresis was found to scale with friction through the scaling factor containing a varying ratio of adhesion energy over the reduced Youngs modulus. Capillary forces can offset the relationship between adhesion hysteresis and friction. Measurements on a wide range of engineering samples with varying adhesive and elastic properties confirm the model. Adhesion hysteresis is investigated under controlled, low humidity atmosphere via ultrasonic force microscopy. Friction is measured by the friction force microscopy.

Study of the dislocation mechanism responsible for the Bordoni relaxation in aluminum by the two‐wave acoustic coupling method

The most realistic model for the interpretation of the Bordoni relaxation observed by internal friction experiments is the mechanism of kink pair formation (KPF) on the dislocations. However, according to this model, high values of the critical resolved shear stress should also be measured at low temperature in face‐centered‐cubic (fcc) metals, but this has never been observed. Using the newly developed two‐wave acoustic coupling method, we have studied the reality of the KPF model as an explanation for the Bordoni relaxation in aluminum. The results are in very good agreement with the predictions of the KPF model and thus confirm this model. On the other hand, experimental evidence that the kink mobility is very high in aluminum have been found. Therefore, the diffusion time of the kinks is negligibly small for the KPF process in fcc metals. Values of the internal stress field in cold‐worked samples have also been obtained using the two‐wave acoustic coupling approach. A description of the experimental m...

Probing local water contents of in vitro protein films by ultrasonic force microscopy

By means of ultrasonic force microscopy and lateral force microscopy we measure adhesion hysteresis and friction on protein films of bovine serum albumin and concanavalin A at local scales. Our investigations at different relative humidities (less than 5% and at 50% relative humidity) correspond to dehydrated and hydrated states of proteins. We demonstrate that a substantial increase of adhesion hysteresis with relative humidity is sensitive measure of protein–water binding capacity at local scales.