Kinetics of Joint Ordering and Decomposition in Binary Alloys

We study phase segregation in a model alloy undergoing both ordering and decomposition, using computer simulations of Kawasaki exchange dynamics on a square lattice. Following a quench into the miscibility gap we observe an early stage in which ordering develops while the composition remains almost uniform. Then decomposition starts with segregation into ordered and disordered phases. The two spherically averaged structure functions, related to decomposition and to ordering, were both observed to obey scaling rules in the late coarsening stage where the time increase of the characteristic lengths was consistent with $a(t^{1/3} + b)$. While $a$ was similar for ordering and decomposition at low concentration of the minority component, it showed an increase (decrease) with concentration for ordering (decomposition). The domain morphology was found to depend on the concentration of the minority component, in a way that suggests a wetting of antiphase boundaries in the ordered domains by the disordered phase.Comment: 23 pages, in TeX, figues available upon reques

Modelling of Phase Separation in Alloys with Coherent Elastic Misfit

Elastic interactions arising from a difference of lattice spacing between two coherent phases can have a strong influence on the phase separation (coarsening) of alloys. If the elastic moduli are different in the two phases, the elastic interactions may accelerate, slow down or even stop the phase separation process. If the material is elastically anisotropic, the precipitates can be shaped like plates or needles instead of spheres and can form regular precipitate superlattices. Tensions or compressions applied externally to the specimen may have a strong effect on the shapes and arrangement of the precipitates. In this paper, we review the main theoretical approaches that have been used to model these effects and we relate them to experimental observations. The theoretical approaches considered are (i) `macroscopic' models treating the two phases as elastic media separated by a sharp interface (ii) `mesoscopic' models in which the concentration varies continuously across the interface (iii) `microscopic' models which use the positions of individual atoms.Comment: 106 pages, in Latex, figures available upon request, e-mail addresses: [email protected], [email protected], [email protected], submitted to the Journal of Statistical Physic

Kawasaki-type Dynamics: Diffusion in the kinetic Gaussian model

In this article, we retain the basic idea and at the same time generalize Kawasaki's dynamics, spin-pair exchange mechanism, to spin-pair redistribution mechanism, and present a normalized redistribution probability. This serves to unite various order-parameter-conserved processes in microscopic, place them under the control of a universal mechanism and provide the basis for further treatment. As an example of the applications, we treated the kinetic Gaussian model and obtained exact diffusion equation. We observed critical slowing down near the critical point and found that, the critical dynamic exponent z=1/nu=2 is independent of space dimensionality and the assumed mechanism, whether Glauber-type or Kawasaki-type.Comment: accepted for publication in PR

The Small World of Osteocytes: Connectomics of the Lacuno-Canalicular Network in Bone

Osteocytes and their cell processes reside in a large, interconnected network of voids pervading the mineralized bone matrix of most vertebrates. This osteocyte lacuno-canalicular network (OLCN) is believed to play important roles in mechanosensing, mineral homeostasis, and for the mechanical properties of bone. While the extracellular matrix structure of bone is extensively studied on ultrastructural and macroscopic scales, there is a lack of quantitative knowledge on how the cellular network is organized. Using a recently introduced imaging and quantification approach, we analyze the OLCN in different bone types from mouse and sheep that exhibit different degrees of structural organization not only of the cell network but also of the fibrous matrix deposited by the cells. We define a number of robust, quantitative measures that are derived from the theory of complex networks. These measures enable us to gain insights into how efficient the network is organized with regard to intercellular transport and communication. Our analysis shows that the cell network in regularly organized, slow-growing bone tissue from sheep is less connected, but more efficiently organized compared to irregular and fast-growing bone tissue from mice. On the level of statistical topological properties (edges per node, edge length and degree distribution), both network types are indistinguishable, highlighting that despite pronounced differences at the tissue level, the topological architecture of the osteocyte canalicular network at the subcellular level may be independent of species and bone type. Our results suggest a universal mechanism underlying the self-organization of individual cells into a large, interconnected network during bone formation and mineralization

Passive and active mechanics of Banksia seed pods

Biological materials consist of only a few basic building blocks, namely sugars, proteins and a few minerals which are assembled into structurally complex materials to ensure (multi)functionality for the particular organism. Prominent examples are bone or nacre, composites of mineral and protein possessing high stiffness and strength. Apart from mineralized materials, protein- and sugar-based materials such as spider dragline silk or plant fibres achieve similarly high values. On the other end of the scale are soft materials with 3 to 4 magnitude lower stiffness and strength values (eg. parenchymatic tissue or artery). Common concept for all biological materials is that a wide range of material properties is achieved by structuring rather than changing their chemical components and frequently materials combine high strength and toughness when needed. A comprehensive understanding of the structure function relationships of biological materials requires measurements of mechanical properties at a range of different length scales, often in combination with other techniques (eg. X-rays, microscopy, spectroscopy). This approach will be illustrated on the example of plants and especially dead but multifunctional tissues such as the seed pods of Banksia attenuata, a native Australian species. The seed storing pods can remain on the plant for up to 15 years without metabolism before they open upon elevated temperatures (eg. caused by bush fires). During the storage period the seed pod material must passively resist weathering, microbial degradation and attacks by bird beaks. Interestingly, the seed pods do not open at uniform temperatures. Instead, opening temperatures change gradually along a climatic South-North gradient increasing towards North. We were able to identify the “temperature sensor” of the seed pods: the inner curvature of the layered follicles gradually increases providing Northern seed pods with a higher flexural rigidity. Opening is activated by a temperature-dependent decrease of the elastic modulus of the inner resistance layer, allowing pre-stresses to be released. However, the initial opening is not sufficient to release the seeds, further opening is fueled by moisture changes which lead to directional swelling and at the same time to changing mechanical properties in different layers of the seed pod. The findings on Banksia seed pods provide inspiration for self-sensing, moving and actuating materials and systems. We expect a comparably easy transfer into technical application because metabolism and biological signaling is not required for functionality. Since the material consists only of a few basic building blocks, namely cellulose, hemicelluloses, lignin, tannins and waxes, recycling and sustainable material use seem to be much easier compared to multi-component composites

Hierarchically-structured metalloprotein composite coatings biofabricated from co-existing condensed liquid phases

Complex hierarchical structure governs emergent properties in biopolymeric materials; yet, the material processing involved remains poorly understood. Here, we investigated the multi-scale structure and composition of the mussel byssus cuticle before, during and after formation to gain insight into the processing of this hard, yet extensible metal cross-linked protein composite. Our findings reveal that the granular substructure crucial to the cuticle’s function as a wear-resistant coating of an extensible polymer fiber is pre-organized in condensed liquid phase secretory vesicles. These are phase-separated into DOPA-rich proto-granules enveloped in a sulfur-rich proto-matrix which fuses during secretion, forming the sub-structure of the cuticle. Metal ions are added subsequently in a site-specific way, with iron contained in the sulfur-rich matrix and vanadium coordinated by DOPA-catechol in the granule. We posit that this hierarchical structure self-organizes via phase separation of specific amphiphilic proteins within secretory vesicles, resulting in a meso-scale structuring that governs cuticle function