Frictional Duality Observed during Nanoparticle Sliding

One of the most fundamental questions in tribology concerns the area dependence of friction at the nanoscale. Here, experiments are presented where the frictional resistance of nanoparticles is measured by pushing them with the tip of an atomic force microscope. We find two coexisting frictional states: While some particles show finite friction increasing linearly with the interface areas of up to 310,000nm^2, other particles assume a state of frictionless sliding. The results further suggest a link between the degree of surface contamination and the occurrence of this duality.Comment: revised versio

Expansion rates of bubble clusters in superheated liquids

[EN] The present work analyses the growth of multiple bubbles in superheated liquid jets by means of direct numerical simulations (DNS). A discontinuous Galerkin approach is used to solve the Euler equations and an adequate interface resolution is ensured by applying finite-volume sub-cells in cells with interfaces. An approximate Riemann solver has been adapted to account for evaporation and provides consistency of all conserved quantities across the interface. The setup mimics conditions typical for orbital manoeuvring systems when liquid oxygen (LOX) is injected into the combustion chamber prior to ignition. The liquid oxygen will then be in a superheated state, bubble nucleation will occur and the growth of the bubbles will determine the break-up of the liquid jet. The expansion rates of bubble groups under such conditions are not known and standard models rely on single bubble assumptions. This is a first DNS study on bubble-bubble interactions in flash boiling sprays and on the effects of these interactions on the growth rates of the individual bubbles. The present simulations resolve a small section of the jet close to the nozzle exit and the growth of bubble groups inside of the jet is analysed. The results suggest that an individual bubble within the group grows more slowly than conventional models for single bubble growth would predict. The reduction in bubble growth can amount to up to 30% and depends on the distances between the bubbles and the number of bubbles within the bubble group. In the present case, the volume expansion of the superheated liquid decreases by approximately 50% if the distance between the bubbles is doubled.The authors acknowledge the financial support by the Deutsche Forschungsgemeinschaft (DFG) as part of the Collaborative Research Center SFB TRR 75 “Droplet dynamics under extreme ambient conditions” held by University of Stuttgart and University of Technology Darmstadt.Dietzel, D.; Hitz, T.; Munz, C.; Kronenburg, A. (2017). Expansion rates of bubble clusters in superheated liquids. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 1068-1075. https://doi.org/10.4995/ILASS2017.2017.4714OCS1068107

Friction anomalies at first-order transition spinodals: 1T-TaS2

Revealing phase transitions of solids through mechanical anomalies in the friction of nanotips sliding on their surfaces, a successful approach for continuous transitions, is still an unexplored tool for first-order ones. Owing to slow nucleation, first-order structural transformations occur with hysteresis, comprised between two spinodal temperatures where, on both sides of the thermodynamic transition, one or the other metastable free energy branches terminates. The spinodal transformation, a collective one-shot event without heat capacity anomaly, is easy to trigger by a weak external perturbation. Here we show that even the gossamer mechanical action of an AFM-tip can locally act as a trigger, narrowly preempting the spontaneous spinodal transformation, and making it observable as a nanofrictional anomaly. Confirming this expectation, the CCDW-NCCDW first-order transition of the important layer compound 1T-TaS2 is shown to provide a demonstration of this effect

DNS of Multiple Bubble Growth and Droplet Formation in Superheated Liquids

Flash boiling can occur in rocket thrusters used for orbital manoeuvring of spacecraft as the cryogenic propellants are injected into the vacuum of space. For reliable ignition, a precise control of the atomization process is required as atomization and mixing of fuel and oxidizer are crucial for the subsequent combustion process. This work focuses on the microscopic process leading to the primary break-up of a liquid oxygen jet, caused by homogeneous nucleation and growth of vapour bubbles in superheated liquid. Although large levels of superheat can be achieved, sub-critical injection conditions ensure distinct gas and liquid phases with a large density ratio. Direct numerical simulations (DNS) are performed using the multiphase solver FS3D. The code solves the incompressible Navier-Stokes equations using the Volume of Fluid (VOF) method and PLIC reconstruction for the phase interface treatment. The interfaces are tracked as multiple bubbles grow, deform and coalesce, leading to the formation of a spray. The evaporation rate at the interface and approximate vapour properties are based on pre-computed solutions resolving the thermal boundary layer surrounding isolated bubbles, while liquid inertia and surface tension effects are expected to play a major role in the final spray characteristics which can only be captured by DNS. Simulations with regular arrays of bubbles demonstrate how the initial bubble spacing and thermodynamic conditions lead to distinct spray characteristics and droplet size distributions

DNS of multiple bubble growth and droplet formation in superheated liquids

Flash boiling can occur in rocket thrusters used for orbital manoeuvring of spacecraft as the cryogenic propellants are injected into the vacuum of space. For reliable ignition, a precise control of the atomization process is required as atomization and mixing of fuel and oxidizer are crucial for the subsequent combustion process. This work focuses on the microscopic process leading to the primary break-up of a liquid oxygen jet, caused by homogeneous nucleation and growth of vapour bubbles in superheated liquid. Although large levels of superheat can be achieved, sub-critical injection conditions ensure distinct gas and liquid phases with a large density ratio. Direct numerical simulations (DNS) are performed using the multiphase solver FS3D. The code solves the incompressible Navier-Stokes equations using the Volume of Fluid (VOF) method and PLIC reconstruction for the phase interface treatment. The interfaces are tracked as multiple bubbles grow, deform and coalesce, leading to the formation of a spray. The evaporation rate at the interface and approximate vapour properties are based on pre-computed solutions resolving the thermal boundary layer surrounding isolated bubbles, while liquid inertia and surface tension effects are expected to play a major role in the final spray characteristics which can only be captured by DNS. Simulations with regular arrays of bubbles demonstrate how the initial bubble spacing and thermodynamic conditions lead to distinct spray characteristics and droplet size distributions

Fuel purification, Lewis acid and aerobic oxidation catalysis performed by a microporous Co-BTT (BTT3-=1,3,5-benzenetristetrazolate) framework having coordinatively unsaturated sites

[EN] Two isostructural microporous metal-organic frameworks [Co(DMA)(6)](3)[(Co4Cl)(3-)(BTT)(8)(H2O)(12)](2)center dot 12H2O (BTT3- = 1,3,5-benzenetristetrazolate; DMA N,N'-dimethylacetamide) (1) and [Cd(DMF)(6)](3)[(Cd4Cl)(3)(BTT)(8)(H2O)(12)](2)center dot 14H(2)O center dot 4DMF (DMF = N,N'-dimethylformamide) (2) were synthesized under solvothermal conditions. The structures of both compounds were determined by single-crystal X-ray diffraction data. Each compound adopts a porous three-dimensional framework consisting of square-planar [M4Cl](7+) (M2+ = Co, 1; Cd, 2) units interconnected by triangular tritopic BTT3- bridging ligands to give an anionic (3,8)-connected "Moravia" net. Phase purity of the compounds was confirmed by X-ray powder diffraction (XRPD), IR spectroscopy, thermogravimetric (TG) and elemental analysis. TGA and temperature-dependent XRPD (TDXRPD) experiments indicate a moderate thermal stability up to 350 and 300 degrees C, respectively. Guest exchange followed by heating led to microporous solids with coordinatively unsaturated metal sites. These unsaturated metal sites create opportunities in adsorptive and catalytic applications. These have been probed by the selective removal of sulfur compounds from fuel feeds as well as the catalytic ring opening of styrene oxide and the oxidation of several cycloalkanes and benzyl compounds.The Deutsche Forschungsgemeinschaft (DFG, SPP 1362 "Porous Metal-Organic Frameworks" under the grant STO 643/5-2) is gratefully acknowledged for the financial support. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 228862.Biswas, S.; Maes, M.; Amarajothi, D.; Feyand, M.; De Vos, DE.; García Gómez, H.; Stock, N. (2012). Fuel purification, Lewis acid and aerobic oxidation catalysis performed by a microporous Co-BTT (BTT3-=1,3,5-benzenetristetrazolate) framework having coordinatively unsaturated sites. Journal of Materials Chemistry. 22(20):10200-10209. https://doi.org/10.1039/c2jm15592cS1020010209222

Experimental diagenesis: insights into aragonite to calcite transformation of Arctica islandica shells by hydrothermal treatment

Biomineralised hard parts form the most important physical fossil record of past environmental conditions. However, living organisms are not in thermodynamic equilibrium with their environment and create local chemical compartments within their bodies where physiologic processes such as biomineralisation take place. In generating their mineralised hard parts, most marine invertebrates produce metastable aragonite rather than the stable polymorph of CaCO3, calcite. After death of the organism the physiological conditions, which were present during biomineralisation, are not sustained any further and the system moves toward inorganic equilibrium with the surrounding inorganic geological system. Thus, during diagenesis the original biogenic structure of aragonitic tissue disappears and is replaced by inorganic structural features. In order to understand the diagenetic replacement of biogenic aragonite to non-biogenic calcite, we subjected Arctica islandica mollusc shells to hydrothermal alteration experiments. Experimental conditions were between 100 and 175 °C, with the main focus on 100 and 175 °C, reaction durations between 1 and 84 days, and alteration fluids simulating meteoric and burial waters, respectively. Detailed microstructural and geochemical data were collected for samples altered at 100 °C (and at 0.1 MPa pressure) for 28 days and for samples altered at 175 °C (and at 0.9 MPa pressure) for 7 and 84 days. During hydrothermal alteration at 100 °C for 28 days most but not the entire biopolymer matrix was destroyed, while shell aragonite and its characteristic microstructure was largely preserved. In all experiments up to 174 °C, there are no signs of a replacement reaction of shell aragonite to calcite in X-ray diffraction bulk analysis. At 175 °C the replacement reaction started after a dormant time of 4 days, and the original shell microstructure was almost completely overprinted by the aragonite to calcite replacement reaction after 10 days. Newly formed calcite nucleated at locations which were in contact with the fluid, at the shell surface, in the open pore system, and along growth lines. In the experiments with fluids simulating meteoric water, calcite crystals reached sizes up to 200 µm, while in the experiments with Mg-containing fluids the calcite crystals reached sizes up to 1 mm after 7 days of alteration. Aragonite is metastable at all applied conditions. Only a small bulk thermodynamic driving force exists for the transition to calcite. We attribute the sluggish replacement reaction to the inhibition of calcite nucleation in the temperature window from ca. 50 to ca. 170 °C or, additionally, to the presence of magnesium. Correspondingly, in Mg2+-bearing solutions the newly formed calcite crystals are larger than in Mg2+-free solutions. Overall, the aragonite–calcite transition occurs via an interface-coupled dissolution–reprecipitation mechanism, which preserves morphologies down to the sub-micrometre scale and induces porosity in the newly formed phase. The absence of aragonite replacement by calcite at temperatures lower than 175 °C contributes to explaining why aragonitic or bimineralic shells and skeletons have a good potential of preservation and a complete fossil record