Sliding friction and superlubricity of colloidal AFM probes coated by tribo-induced graphitic transfer layers

Colloidal probe Atomic Force Microscopy (AFM) allows to explore sliding friction phenomena in graphite contacts of nominal lateral size up to hundreds of nanometers. It is known that contact formation involves tribo-induced material transfer of graphite flakes from the graphitic substrate to the colloidal probe. In this context, sliding states with nearly-vanishing friction, i.e. superlubricity, may set in. A comprehensive investigation of the transfer layer properties is mandatory to ascertain the origin of superlubricity. Here we explore the friction response of micrometric beads, of different size and pristine surface roughness, sliding on graphite under ambient conditions. We show that such tribosystems undergo a robust transition towards a low-adhesion, low-friction state dominated by mechanical interactions at one dominant tribo-induced nanocontact. Friction force spectroscopy reveals that the nanocontact can be superlubric or dissipative, in fact undergoing a load-driven transition from dissipative stick-slip to continuous superlubric sliding. This behavior is excellently described by the thermally-activated, single-asperity Prandtl-Tomlinson model. Our results indicate that upon formation of the transfer layer, friction depends on the energy landscape experienced by the topographically-highest tribo-induced nanoasperity. Consistently we find larger dissipation when the tribo-induced nanoasperity is sled against surfaces with higher atomic corrugation than graphite, like MoS2 and WS2, in prototypical Van der Waals layered hetero-junctions.Comment: 35 pages, 6 figures, to be published in Langmui

Graphite superlubricity enabled by triboinduced nanocontacts

Colloidal probe Atomic Force Microscopy allows to explore sliding states of vanishing friction, i.e. superlubricity, in mesoscopic graphite contacts. Superlubricity is known to appear upon formation of a triboinduced transfer layer, originated by material transfer of graphene flakes from the graphitic substrate to the colloidal probe. Previous studies suggest that friction vanishes due to crystalline incommensurability at the newly formed interface. However this picture still lacks several details, such as the roles of the tribolayer roughness and of loading conditions. Hereafter we gain deeper insight into the tribological response of micrometric silica beads sliding on graphite under ambient conditions. We show that the tribotransferred flakes behave as lubricious nanoasperities with a twofold role. First, they decrease the silica-graphite true contact area, in fact causing a breakdown of adhesion and friction by one order of magnitude. Second, they govern mechanical dissipation through the specific energy landscape experienced by the topographically-highest triboinduced nanoasperity. Remarkably, such contact junctions can undergo a load-driven atomic-scale transition from continuous superlubric sliding to dissipative stick-slip, that agrees with the single-asperity Prandtl-Tomlinson model. Superlubricity in mesoscopic silica-graphite junctions may therefore arise from the load-controlled competition between interfacial crystalline incommensurability and contact pinning effects at one dominant nanoasperity.Comment: 55 pages, published in Carbon, this is the Accepted Manuscript version, it includes the revised Supplementary Data versio

Use of Untargeted Liquid Chromatography\u2013Mass Spectrometry Metabolome To Discriminate Italian Monovarietal Red Wines, Produced in Their Different Terroirs

The aim of this project was to register, in a liquid chromatography\u2013mass spectrometry-based untargeted single-batch analysis, the metabolome of 11 single-cultivar, single-vintage Italian red wines (Aglianico, Cannonau, Corvina, Montepulciano, Nebbiolo, Nerello, Primitivo, Raboso, Sagrantino, Sangiovese, and Teroldego) from 12 regions across Italy, each one produced in their terroirs under ad hoc legal frameworks to guarantee their quality and origin. The data provided indications regarding the similarity between the cultivars and highlighted a rich list of putative biomarkers of origin wines (pBOWs) characterizing each individual cultivar\u2013terroir combination, where Primitivo, Teroldego, and Nebbiolo had the maximum number of unique pBOWs. The pBOWs included anthocyanins (Teroldego), flavanols (Aglianico, Sangiovese, Nerello, and Nebbiolo), amino acids and N-containing metabolites (Primitivo), hydroxycinnamates (Cannonau), and flavonols (Sangiovese). The raw data generated in this study are publicly available and, therefore, accessible and reusable as a baseline data set for future investigations

Phenolic extraction and mechanical properties of skins and seeds during maceration of four main italian red wine grape varieties

Red grape varieties are characterized by different phenolic contents (prominently tannins and anthocyanins) found in skins and seeds. The extractability of these compounds varies during maceration, as well as the mechanical properties of skins and seeds. Four main Italian red winegrape varieties were tested to understand these differences during a simulated maceration process

The macromolecular diversity of Italian monovarietal red wines

13openInternationalItalian coauthor/editorWhile red wine phenolics have been extensively studied, polysaccharides and proteins have not received the same level of attention, especially when considering Italian wines. In this study, for the first time, quantitative and qualitative data on the macromolecular (proteins and polysaccharides) and tannin composition of 110 monovarietal red wines from 11 of the most important Italian grape varieties are reported. The winemaking did not include any filtration, oak contact, fining treatments, or ageing on yeast lees. Results highlighted a great inter- and intra- varietal diversity. The protein content ranged between 0 and 159 mg/L, polysaccharides between 211 and 1081 mg/L and total tannins between 171 and 3746 mg/L, with averages of 41 mg/L, 497 mg/L and 1687 mg/L, respectively. Six varieties with protein content representative of the variability observed were selected and submitted to electrophoresis. Within each variety, the SDS-PAGE mobility of protein-tannin complexes was similar but showed two distinct patterns for wines of different varieties (higher mobility for Corvina, Teroldego and Raboso Piave, lower mobility for Nebbiolo, Aglianico, Cannonau), suggesting that the Italian monovarietal wines can be diverse also in their colloidal-forming structures. This can be explained by looking at the different percentages of protein-reactive tannins (TBSA) on the total tannin content (TMCP), which is a varietal characteristicopenMarangon, Matteo; De Iseppi, Alberto; Gerbi, Vincenzo; Mattivi, Fulvio; Moio, Luigi; Piombino, Paola; Parpinello, Giuseppina Paola; Rolle, Luca; Slaghenaufi, Davide; Versari, Andrea; Vrhovsek, Urska; Ugliano, Maurizio; Curioni, AndreaMarangon, M.; De Iseppi, A.; Gerbi, V.; Mattivi, F.; Moio, L.; Piombino, P.; Parpinello, G.P.; Rolle, L.; Slaghenaufi, D.; Versari, A.; Vrhovsek, U.; Ugliano, M.; Curioni, A

The surface layer of Sr2_2RuO4_4: A two-dimensional model system for magnetic-field-tuned quantum criticality

Many of the exciting properties of strongly correlated materials are intricately linked to quantum critical points in their phase diagram. This includes phenomena such as high temperature superconductivity, unconventional superconductivity in heavy fermion materials, as well as exotic nematic states in Sr$_3$Ru$_2$O$_7$. One of the experimentally most successful pathways to reaching a quantum critical point is tuning by magnetic field allowing studies under well-controlled conditions on ultra-clean samples. Yet, spectroscopic evidence of how the electronic states change across a field-tuned quantum phase transition, and what the importance of quantum fluctuations is, is not available so far. Here we show that the surface layer of Sr$_2$RuO$_4$ is an ideal two-dimensional model system for a field-tuned quantum phase transition. We establish the existence of four van Hove singularities in close proximity to the Fermi energy, linked intricately to checkerboard charge order and nematicity of the electronic states. Through magnetic field, we can tune the energy of one of the van Hove singularities, with the Lifshitz transition extrapolated at ~32T. Our experiments open up the ability to directly study spectroscopically the role of quantum fluctuations at a field-tuned quantum phase transition in an effectively 2D strongly correlated electron material. Our results further have implications for what the leading instability in Sr$_2$RuO$_4$ is, and hence for understanding the enigmatic superconductivity in this material.Comment: 31 pages, 4 figure

Magnetic-field tunable intertwined checkerboard charge order and nematicity in the surface layer of Sr2RuO4

C.A.M. acknowledges funding from EPSRC through EP/L015110/1, LCR from the Royal Commission for the Exhibition of 1851, A.W.R. from EPSRC through EP/P024564/1, P.W. from EPSRC through EP/R031924/1, and C.M.Y. and P.W. through EP/S005005/1. V.G., R.F., R.B., A.G., A.V. and P.W. acknowledge support from the Bilateral Project "Atomic-scale imaging of the superconducting condensate in the putative triplet superconductor Sr2RuO4: a platform for topological quantum computations?" in a joint Royal Society of Edinburgh and CNR Bilateral Scheme CUP B56C18003920005.In strongly correlated electron materials, the electronic, spin, and charge degrees of freedom are closely intertwined. This often leads to the stabilization of emergent orders that are highly sensitive to external physical stimuli promising opportunities for technological applications. In perovskite ruthenates, this sensitivity manifests in dramatic changes of the physical properties with subtle structural details of the RuO6 octahedra, stabilizing enigmatic correlated ground states, from a hotly debated superconducting state via electronic nematicity and metamagnetic quantum criticality to ferromagnetism. Here, it is demonstrated that the rotation of the RuO6 octahedra in the surface layer of Sr2RuO4 generates new emergent orders not observed in the bulk material. Through atomic-scale spectroscopic characterization of the low-energy electronic states, four van Hove singularities are identified in the vicinity of the Fermi energy. The singularities can be directly linked to intertwined nematic and checkerboard charge order. Tuning of one of these van Hove singularities by magnetic field is demonstrated, suggesting that the surface layer undergoes a Lifshitz transition at a magnetic field of ≈32T. The results establish the surface layer of Sr2RuO4 as an exciting 2D correlated electron system and highlight the opportunities for engineering the low-energy electronic states in these systems.Publisher PDFPeer reviewe