Structure of synthetic K-rich birnessite obtained by high-temperature decomposition of KMnO4. I. Two-layer polytype from 800°C experiment.

International audienceThe structure of a synthetic potassium birnessite (KBi) obtained as a finely dispersed powder by thermal decomposition of KMnO4 at 800°C was for the first time studied by single crystal X-ray diffraction (XRD). It is shown that KBi has a two-layer cell with a = 2.840(1) Ä, and c = 14.03(1) Ä, and space group P63/mmc. In contrast to the structure model proposed by Kim et al., 1 the refined model demonstrates the sole presence of Mn4+ in the octahedral layers, the presence of 0.12 vacant layer sites per octahedron being responsible for the layer charge deficit. This layer charge deficit is compensated for 1) by the presence of interlayer Mn 3+ above or below vacant layer octahedra sharing three O layer with neighboring Mnlayer octahedra to form a triple-corner surface complex ( VITC sites), and 2) by the presence of interlayer K in prismatic cavities located above or below empty tridentate cavities, sharing three edges with neighboring Mnlayer octahedra ( VITE sites). As compared to the structure model proposed by Kim et al., 1 this VITE site is shifted from the center of the prismatic cavity towards its edges. A complementary powder XRD study confirmed the structure model of the main defect-free KBi phase and allowed to determine the nature of stacking disorder in a defective accessory KBi phase admixed to the defect-free KBi

Effects on collagen orientation in the cornea after trephine injury

Purpose: Structural changes are well known to occur in the cornea after injury. The aim of this study was to investigate collagen orientation changes in the cornea during a short-term wound healing process. Methods: Seven bovine corneas were injured using a penetrating 5 mm biopsy punch and were subsequently organ cultured for up to two weeks. Six uninjured corneas acted as controls. The trephine wounded samples were snap frozen in liquid nitrogen either immediately after injury (0 h) or after 1 or 2 weeks in culture. Control/uninjured samples were snap frozen on arrival (0 h) or after 1 or 2 weeks in culture. Wide angle X-ray diffraction data were collected from each cornea at the UK Synchrotron Radiation Source or at the European Synchrotron Radiation Facility. Data analysis revealed information about collagen orientation and distribution in the corneal stroma during wound healing. For histology, two trephine wounded corneas at 0 h and 1 week and one control/uninjured cornea at 0 h were fixed in 10% neutral buffered formalin and processed for wax embedding. Wax sections were subsequently counterstained with haematoxylin and eosin to observe tissue morphology and the time course of complete re-epithelialization. Results: Immediately after injury, collagen organization was altered in a small area inside the wound but remained similar to the control/uninjured sample in the remainder of the tissue. After one week, the trephine wounded corneas showed complete re-epithelialization and evidence of swelling while collagen adopted a radial arrangement inside and outside the wound. Conclusions: Remarkable changes in collagen fibril orientation were observed in trephine wounded corneas. Orientation changes immediately after wounding are likely to be due to the mechanical deformation of the tissue during the wounding process. However, tissue swelling and changes in collagen orientation at later stages probably reflect the processes of tissue repair. These differences will determine corneal stability and strength following trauma and possibly refractive surgery

Determination of the full deformation tensor by multi-Bragg fast scanning nano X-ray diffraction

International audienceThis work showcases a method to map the full deformation tensor in a single micro-sized crystal. It is shown that measuring the position of two Bragg reflections in reciprocal space is sufficient to obtain the full deformation tensor, if the condition of incompressibility of the material is imposed. This method is used to reveal the surface tension induced deformation at the edges of an as-grown single-crystal VO 2 microwire. All components of the deformation tensor of the microwire were measured down to an absolute value of 10 À4 in an 8 Â 14 mm projected area of the wire. With a beam-defined spatial resolution of 150 Â 150 nm, the measurement time was merely 2.5 h

Multiple Mechanical Gradients are Responsible for the Strong Adhesion of Spider Attachment Hair

Wandering spiders climb vertically and walk upside-down on rough and smooth surfaces using a nanostructured attachment system on their feet. The spiders are assumed to adhere by intermolecular van der Waals forces between the adhesive structures and the substrate. The adhesive elements are arranged highly ordered on the hierarchically structured attachment hair (setae). While walking, it has been suggested that the spiders apply a shear force on their legs to increase friction. However, the detailed mechanical behavior of the hair's structures during attachment and detachment remains unknown. Here, gradients of the mechanical properties of the attachment hair on different length scales that have evolved to support attachment, stabilize adhesion in contact, and withstand high stress at detachment, examined by in situ experiments, are shown. Shearing helps to self-align the adhesive elements with the substrate. The study is anticipated to contribute to the development of optimized artificial dry adhesives

Shear bands and the evolving microstructure in a drying colloidal film studied with scanning μ-SAXS

Shear localisation in thin bands is an important process involved in the plastic deformation of materials subject to stress. This process is often sensitive to the sample microstructure (amorphous / crystalline). Here we show using the scanning μ-SAXS technique, how these different microstructures influence the plastic deformations in a drying colloidal film. In crystalline samples, the presence of an ordering transition at the compaction front was directly identified through the development of a six-fold symmetry in the scattering pattern in 20wt% samples. It is shown that plastic deformations in individual groups of particles during the compaction process can be tracked and measured in real time. Higher concentration suspensions were found to result in amorphous structures. The transition between crystalline and amorphous microstructures with initial particle concentration was also found to correlate with the appearance of shear bands. Through 2D spatial mapping of the local film structure, the presence of shear bands in the films was directly related to the microscale spatial variations in strain magnitude and compression direction. Our measurements also showed that shear bands lead to a reduction in the local particle volume fraction ~ 1-2%, indicating significant dilatancy