Revisiting sticking property of submillimetre-sized aggregates

ABSTRACT Understanding the physical properties of dust aggregates is of great importance in planetary science. In this study, we revisited the sticking property of submillimetre-sized aggregates. We revealed that the ‘effective surface energy’ model used in previous studies underestimates the critical pulling force needed to separate two sticking aggregates. We also derived a new and simple model of the critical pulling force based on the canonical theory of two contacting spheres. Our findings indicate that we do not need to consider the ‘effective surface energy’ of dust aggregates when discussing the physical properties of loose agglomerates of submillimetre-sized aggregates.

Drifting inwards in protoplanetary discs I Sticking of chondritic dust at increasing temperatures

Sticking properties rule the early phases of pebble growth in protoplanetary discs in which grains regularly travel from cold, water-rich regions to the warm inner part. This drift affects composition, grain size, morphology, and water content as grains experience ever higher temperatures. In this study we tempered chondritic dust under vacuum up to 1400 K. Afterwards, we measured the splitting tensile strength of millimetre-sized dust aggregates. The deduced effective surface energy starts out as γe = 0.07 J m−2. This value is dominated by abundant iron-oxides as measured by Mössbauer spectroscopy. Up to 1250 K, γe continuously decreases by up to a factor five. Olivines dominate at higher temperature. Beyond 1300 K dust grains significantly grow in size. The γe no longer decreases but the large grain size restricts the capability of growing aggregates. Beyond 1400 K aggregation is no longer possible. Overall, under the conditions probed, the stability of dust pebbles would decrease towards the star. In view of a minimum aggregate size required to trigger drag instabilities it becomes increasingly harder to seed planetesimal formation closer to a star.

Mechanism of preferential nucleation of [\bf 1{\overline 1}0{\overline 3}]-oriented GaN twins on an SiO2-patternedm-plane sapphire substrate

The mechanism of preferential nucleation of [1{\overline 1}0{\overline 3}]-oriented GaN faceted twins on an SiO2-patternedm-plane sapphire substrate was investigated. Each variant of twins, which were enclosed byc- andm-facets, was observed to be preferentially nucleated over the opposite sides of an SiO2pattern. It was hypothesized, from the fact that the same method of Legendre transformation is applied to a Wulff plot and a kinetic Wulff plot to determine the growth morphology of a crystalline domain, that theeffectivesurface energy ofc- andm-facets would be proportional to the growth rate of the respective facet. On the basis of this hypothesis, minimization of the effective surface energy of a nucleated domain was proposed as a mechanism of preferential nucleation. This proposed mechanism successfully explained the preferential nucleation behaviour.

Microstructural Unit Controlling Cleavage Crack Propagation in High Strength Bainitic Steels

The strengthening mechanisms which are operative in bainite are very well known: small bainite packet, small width of the laths, dislocation density and size and number of carbide particles (Fe3C), among others. Bainite packet size has been traditionally considered as the value measured by optical microscopy (OM), as electron back scattered diffraction (EBSD) technique is relatively recent. In a V-microalloyed steel with bainitic microstructure of C=0.38%, V=0.12% and N= 0.0214% the average length and width of ferrite laths and of cementite carbides were measured. On the other hand, the bainite packet size was measured by OM and EBSD with a misorientation of 15o. These values of the microstructural units have been taken in account to calculate the effective surface energy γp given by Griffith’s model for cleavage fracture. It was concluded that bainite packet size determined by EBSD with a misorientation angle criterion of 15o was the microstructural parameter that controls cleavage crack propagation. Given the relationship between the average unit crack path (UCP) and the bainite packet size, it was concluded that the effective surface energy of cleavage fracture (γp) would be between 71.6 and 82.6 J m-2.

Interfacial energy of calcite-MHA/MUA self-assembled monolayers during nucleation

ABSTRACTThe interfacial energies between template-directed calcite nuclei and self-assembled monolayers of 16-mercaptohexadecanoic acid (MHA) and 11-mercaptoundecanoic acid (MUA) were measured. The results reveal that (1) the MHA and MUA significantly reduce the effective surface energy of calcite from about 97 mJ/m2 in solution to about 45.3 and 52.7 mJ/m2 on MHA and MUA respectively, providing a thermodynamic basis for the strong capacity of MHA and MUA to promote calcite nuclei; and (2) the barrier to nucleation is dominated by the free energy barrier which enhances the rate of surface nucleation on MHA and MUA.