The role of microorganisms in the formation of calcitic moonmilk deposits and speleothems in Altamira Cave

Bacteria are able to induce carbonate precipitation although the participation of microbial or chemical processes in speleothem formation remains a matter of debate. In this study, the origin of carbonate depositions such as moonmilk, an unconsolidated microcrystalline formation with high water content, and the consolidation of carbonate precipitates into hard speleothems were analyzed. The utilized methods included measurements of the composition of stable isotopes in these precipitates, fluorimetric determinations of RNA/DNA ratios and respirometric estimations in Altamira Cave. Results from isotope composition showed increases of the δ18O and δ13C ratios from moonmilk in the very first stages of formation toward large speleothems. Estimates of RNA/DNA ratios suggested an inactivation of microorganisms from incipient moonmilk toward consolidated deposits of calcium carbonate. Respiratory activity of microorganisms also showed a significant decrease in samples with accumulated calcite. These results suggest that bacterial activity induces the conditions required for calcium carbonate precipitation, initiating the first stages of deposition. Progressive accumulation of carbonate leads towards a less favorable environment for the development of bacteria. On consolidated speleothems, the importance of bacteria in carbonate deposition decreases and chemical processes gain importance in the deposition of carbonates.

Mechanical annealing of metallic electrodes at the atomic scale

The process of creating an atomically defined and robust metallic tip is described and quantified using measurements of contact conductance between gold electrodes and numerical simulations. Our experiments show how the same conductance behavior can be obtained for hundreds of cycles of formation and rupture of the nanocontact by limiting the indentation depth between the two electrodes up to a conductance value of approximately 5G0 in the case of gold. This phenomenon is rationalized using molecular dynamics simulations together with density functional theory transport calculations which show how, after repeated indentations (mechanical annealing), the two metallic electrodes are shaped into tips of reproducible structure. These results provide a crucial insight into fundamental aspects relevant to nanotribology or scanning probe microscopies.

Understanding the structure of the first atomic contact in gold

We have studied experimentally jump-to-contact (JC) and jump-out-of-contact (JOC) phenomena in gold electrodes. JC can be observed at first contact when two metals approach each other, while JOC occurs in the last contact before breaking. When the indentation depth between the electrodes is limited to a certain value of conductance, a highly reproducible behaviour in the evolution of the conductance can be obtained for hundreds of cycles of formation and rupture. Molecular dynamics simulations of this process show how the two metallic electrodes are shaped into tips of a well-defined crystallographic structure formed through a mechanical annealing mechanism. We report a detailed analysis of the atomic configurations obtained before contact and rupture of these stable structures and obtained their conductance using first-principles quantum transport calculations. These results help us understand the values of conductance obtained experimentally in the JC and JOC phenomena and improve our understanding of atomic-sized contacts and the evolution of their structural characteristics.

Plastisol foaming process. Decomposition of the foaming agent, polymer behavior in the corresponding temperature range and resulting foam properties

The decomposition of azodicarbonamide, used as foaming agent in PVC—plasticizer (1/1) plastisols was studied by DSC. Nineteen different plasticizers, all belonging to the ester family, two being polymeric (polyadipates), were compared. The temperature of maximum decomposition rate (in anisothermal regime at 5 K min−1 scanning rate), ranges between 434 and 452 K. The heat of decomposition ranges between 8.7 and 12.5 J g−1. Some trends of variation of these parameters appear significant and are discussed in terms of solvent (matrix) and viscosity effects on the decomposition reactions. The shear modulus at 1 Hz frequency was determined at the temperature of maximum rate of foaming agent decomposition, and differs significantly from a sample to another. The foam density was determined at ambient temperature and the volume fraction of bubbles was used as criterion to judge the efficiency of the foaming process. The results reveal the existence of an optimal shear modulus of the order of 2 kPa that corresponds roughly to plasticizer molar masses of the order of 450 ± 50 g mol−1. Heavier plasticizers, especially polymeric ones are too difficult to deform. Lighter plasticizers such as diethyl phthalate (DEP) deform too easily and presumably facilitate bubble collapse.