Organic crystal is described that can be bent plastically and twisted elastically, and can self-heal to 67%, an efficiency that is an order-of-magnitude higher compared to the only previously reported example.

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SummaryCdS quantum dots were grown on mesoporous TiO2 films by successive ionic layer adsorption and reaction processes in order to obtain CdS particles of various sizes. AFM analysis shows that the growth of the CdS particles is a two-step process. The first step is the formation of new crystallites at each deposition cycle. In the next step the pre-deposited crystallites grow to form larger aggregates. Special attention is paid to the estimation of the CdS particle size by X-ray photoelectron spectroscopy (XPS). Among the classical methods of characterization the XPS model is described in detail. In order to make an attempt to validate the XPS model, the results are compared to those obtained from AFM analysis and to the evolution of the band gap energy of the CdS nanoparticles as obtained by UV–vis spectroscopy. The results showed that XPS technique is a powerful tool in the estimation of the CdS particle size. In conjunction with these results, a very good correlation has been found between the number of deposition cycles and the particle size.

The propensity for adherence to solid surfaces of asphaltenes, a complex solubility class of heteropolycyclic aromatic compounds from the heavy fraction of crude oil, has long been the root cause of scale deposition and remains an intractable problem in the petroleum industry. Although the adhesion is essential to understanding the process of asphaltene deposition, the relationship between the conformation of asphaltene molecules on mineral substrates and its impact on adhesion and mechanical properties of the deposits is not completely understood. To rationalize the primary processes in the process of organic scale deposition, here we use atomic force microscopy (AFM) to visualize the morphology of petroleum asphaltenes deposited on model mineral substrates. High imaging contrast was achieved by the differential adhesion of the tip between asphaltenes and the mineral substrate. While asphaltenes form smooth continuous films on all substrates at higher concentrations, they deposit as individual nanoparticles at lower concentrations. The size, shape, and spatial distribution of the nanoaggregates are strongly affected by the nature of the substrate; while uniformly distributed spherical particles are formed on highly polar and hydrophilic substrates (mica), irregular islands and thicker patches are observed with substrates of lower polarity (silica and calcite). Asphaltene nanoparticles flatten when adsorbed on highly oriented pyrolytic graphite due to π-π interactions with the polycyclic core. Force-distance profiles provide direct evidence of the conformational changes of asphaltene molecules on hydrophilic/hydrophobic substrates that result in dramatic changes in adhesion and mechanical properties of asphaltene deposits. Such an understanding of the nature of adhesion and mechanical properties tuned by surface properties, on the level of asphaltene nanoaggregates, would contribute to the design of efficient asphaltene inhibitors for preventing asphaltene fouling on targeted surfaces. Unlike flat surfaces, the AFM phase contrast images of defected calcite surfaces show that asphaltenes form continuous deposits to fill the recesses, and this process could trigger the onset for asphaltene deposition.

International audienceNatural fiber-reinforced polymers or biocomposites are becoming increasingly popular as an environment friendly alternative to traditional glass fiber-reinforced thermoplastics. The mechanical properties of reinforced biocomposites, such as flax/polylactic acid (PLA), are largely governed by the level of interfacial interactions between the two constituents apart from their intrinsic properties. The hierarchical organization of various polysaccharides present in natural fibers results in complex mechanisms at the interface which are still poorly understood and difficult to analyze through a traditional approach that rely on indirect assessments. The possibility of measuring direct adhesion force between individual particles using the colloidal force microscopy has been exploited here by developing an experimental set-up in which a micrometer colloidal PLA bead is brought into close contact with molecularly smooth polysaccharide surfaces that mimic the main constituents of flax fibers, cellulose, hemicellulose, and pectins. Adhesion force measurements performed under ambient and low relative humidity conditions indicate that cellulose/PLA is the weakest interface in the biocomposite. Moreover, the results emphasize the important role of water molecules for the more hydrophilic polymers in flax fibers that takes place in the fundamental forces involved in the adhesion phenomena at the biocomposite interface

The effect of alkali and enzymatic treatments on flax fibre morphology, mechanical, and adhesion properties was investigated. The multilength scale analysis allows for the correlation of the fibre's morphological changes induced by the treatments with mechanical properties to better explain the adherence properties between flax and PLA. The atomic force microscopy (AFM) images revealed the removal of primary layers, upon treatments, down to cellulose microfibrils present in the secondary layers. The variation in mechanical properties was found to be dependent, apart from the crystalline content, on interaction between cellulose microfibrils and encrusting polysaccharides, pectins and hemicelluloses, in the secondary layers. Finally, microbond tests between the modified fibres and PLA emphasize the important role of the outer fibre's surface on the overall composite properties. It was observed here that gentle treatments of the fibres, down to the oriented microfibrils, are favourable to a better adherence with a PLA drop. This paper highlights the important role of amorphous polymers, hemicellulose and pectin, in the optimisation of the adhesion and mechanical properties of flax fibres in the biocomposite.

Photoexcitation can lead to either homogeneous or heterogeneous transformations of a reactive surface. Homogeneous transformations result in a statistical mixture of reactants and products, whereas the outcome of heterogeneous transformations is a coexistence of macroscopic reactant and product domains, separated by a phase boundary. Heterogeneous photoinduced changes are also typically restricted to the surface, have individual phase structures that are inaccessible with classical diffraction methods, and possess surface properties that cannot readily be measured by the traditional wetting (water contact angle) technique. In this study, we demonstrate application of Atomic Force Microscopy (AFM) to obtain high spatial resolution surface energy distribution in the trans and cis domains on the surface of azobenzene single crystal. UV excitation of single crystals of 3′,4′-dimethyl-4-(dimethylamino)azobenzene results in domino-like trans-to-cis isomerization on their surface. In the AFM phase channel, this affords contrasting domains with different physicochemical properties. Small amplitude small set point (SASS) method and bimodal AFM operated in the attractive regime provide maps of the tip−sample adhesion force and the Hamaker constant, respectively. The results show that the Hamaker constant of the cis domains (∼1 × 10 −19 J) is higher than that of the trans domains (∼7 × 10 −20 J). After UV irradiation, the calculated surface energies of the domains were ∼40% higher based on the Hamaker constant. Within a broader context, the results presented here demonstrate the potency of AFM-based surface-sensitive techniques for probing of the dynamic changes in surface properties upon photoinduced isomerization of molecular switches.

Plastic bending of organic crystals is a well-known, yet mechanistically poorly understood phenomenon. On three structurally related epimers, derivatives of galactose, glucose, and mannose, it is demonstrated here that small changes in the molecular structure can have a profound effect on the mechanical properties. While the galactose derivative affords crystals which can be easily bent, the crystals of the derivatives of glucose and mannose are brittle and do not bend. Structural, microscopic, and mechanical evidence is provided showing that hydrogen bonding of water molecules is the key element for sliding over the slip planes in the crystal and accounts for the plastic bending.