K.M.Y. P'ng

Control of starch granule numbers in Arabidopsis chloroplasts

The aim of this work was to investigate starch granule numbers in Arabidopsis (Arabidopsis thaliana) leaves. Lack of quantitative information on the extent of genetic, temporal, developmental, and environmental variation in granule numbers is an important limitation in understanding control of starch degradation and the mechanism of granule initiation. Two methods were developed for reliable estimation of numbers of granules per chloroplast. First, direct measurements were made on large series of consecutive sections of mesophyll tissue obtained by focused ion beam-scanning electron microscopy. Second, average numbers were calculated from the starch contents of leaves and chloroplasts and estimates of granule mass based on granule dimensions. Examination of wild-type plants and accumulation and regulation of chloroplast (arc) mutants with few, large chloroplasts provided the following new insights. There is wide variation in chloroplast volumes in cells of wild-type leaves. Granule numbers per chloroplast are correlated with chloroplast volume, i.e. large chloroplasts have more granules than small chloroplasts. Mature leaves of wild-type plants and arc mutants have approximately the same number of granules per unit volume of stroma, regardless of the size and number of chloroplasts per cell. Granule numbers per unit volume of stroma are also relatively constant in immature leaves but are greater than in mature leaves. Granule initiation occurs as chloroplasts divide in immature leaves, but relatively little initiation occurs in mature leaves. Changes in leaf starch content over the diurnal cycle are largely brought about by changes in the volume of a fixed number of granules.

Measurement of the size effect in the yield strength of nickel foils

There is a growing consensus that materials become stronger in small volumes and in the presence of large strain gradients. It has not been clear whether this is due to increased resistance to the motion of dislocations, fewer dislocations, or increased difficulty of multiplying dislocations in these situations. A classic experiment by Stölken and Evans (J.S. Stölken and A.G. Evans, Acta metall. 46 5109 (1998)) showed that thin nickel foils under bending display increased strengthening at large plastic strain values and, correspondingly, large plastic strain gradients. We have adapted their technique to small strains, and report preliminary data for the stress–strain curves of thin nickel foils through the elastic–plastic transition. These data show unambiguously that the yield strength is greater in the thinner foils. The strengthening is additive to the Hall–Petch effect, and is consistent with a size effect at the onset of plastic deformation.

Three-dimensional aspects of matrix assembly by cells in the developing cornea

Significance The cornea is a specialized connective tissue assembled as a remarkably ordered array of superimposed collagenous lamellae, and their component collagen fibrils, essential for optical transparency. Surprisingly, the mechanisms involved in deposition of this unique structure are still not fully understood. Here we have used correlative microscopy techniques, including innovative methods of serial block face scanning electron microscopy, to observe the sequence of corneal matrix formation in three-dimensional reconstructions of embryonic chick cornea. Our data show that corneal cells, keratocytes, exhibit long-range associations with collagen bundles in the developing matrix via an extended network of actin-rich tubular cytoplasmic protrusions, which we term keratopodia. Synchronized alignment of keratopodia and collagen is evident during the course of lamella formation. Cell-directed deposition of aligned collagen fibrils during corneal embryogenesis is poorly understood, despite the fact that it is the basis for the formation of a corneal stroma that must be transparent to visible light and biomechanically stable. Previous studies of the structural development of the specialized matrix in the cornea have been restricted to examinations of tissue sections by conventional light or electron microscopy. Here, we use volume scanning electron microscopy, with sequential removal of ultrathin surface tissue sections achieved either by ablation with a focused ion beam or by serial block face diamond knife microtomy, to examine the microanatomy of the cornea in three dimensions and in large tissue volumes. The results show that corneal keratocytes occupy a significantly greater tissue volume than was previously thought, and there is a clear orthogonality in cell and matrix organization, quantifiable by Fourier analysis. Three-dimensional reconstructions reveal actin-associated tubular cell protrusions, reminiscent of filopodia, but extending more than 30 µm into the extracellular space. The highly extended network of these membrane-bound structures mirrors the alignment of collagen bundles and emergent lamellae and, we propose, plays a fundamental role in dictating the orientation of collagen in the developing cornea.

Effect of coherency strain on the deformation of InxGa1-xAs superlattices under nanoindentation and bending

It has been shown elsewhere that the room temperature yield pressure of In x Ga1− x As superlattices measured by nanoindentation, decreases from a high value as the volume averaged strain modulation is increased, while at 500°C under uniaxial compression or tension the yield stress increases from a low value with increasing strain modulation. We have used cross-sectional transmission electron microscopy to examine the deformation mechanisms in these two loading regimes. At room temperature both twinning and dislocation flow was found with the proportion of twinning decreasing with increasing strain modulation. The coherency strain of the superlattice is retained in a twin but partially relaxed by dislocation flow. The strain energy released by the loss of coherency assists dislocation flow and weakens the superlattice. Twins are only nucleated when a critical elastic shear of about 7° is achieved at the surface. The plastic zone dimensions under the indent are finite at the yield point, with a width and depth of approximately 1.3 µm and 1.1 µm respectively. Under uniaxial compression and tension at 500°C the superlattices deform by dislocation flow along {111} planes. The most highly strained samples also partially relax through the formation of misfit dislocations.

Deformation of small volumes of material using nanostructured strained layered superlattices

Abstract A key aspect of nanostructured materials is that large coherency strains can readily exist between nano-sized phases. This can result in strengthening or in improved ductility. However, in conventional materials, it can be very difficult to separate the effects of coherency strain from other phenomena. Electronic grade single crystal semiconductor structures provide a means to study the effects of coherency strain in isolation. In this work, thin coherently strained InGaAs superlattices grown on thick InP substrates were tested in three-point bending at 500°C. The stress–strain curves of the specimens were measured, and from them, analysis yields the actual stress supported by the thin superlattice. The superlattices display extraordinary strength compared to the corresponding bulk material. This effect can be attributed only to the coherency strain in the superlattices.

Mapping of the Initial Volume at the Onset of Plasticity in Nanoindentation

Understanding the finite volume throughout which plastic deformation begins is necessary to understand the mechanics of small-scale deformation. In indentation using spherical indenters, conventional yield criteria predict that yield starts at a point on the axis and at a depth of half the contact radius. However, Jayaweera et al (Proc. Roy. Soc. 2003)[4] concluded that yield occurs over a finite volume at least 100 nm thick. Semiconductor superlattice structures, in which the stress and thickness of individual layers can be varied and in which known internal stresses can be incorporated, open up new possibilities for investigation that cannot be achieved by varying external stresses on a homogenous specimen. We have designed samples with bands of highly strained InGaAs superlattice, which are essentially bands of low yield-stress material devoid of other metallurgical artifacts. These bands are placed at different depths in a series of samples. Spherical indenters with a range of radii were used to determine the elastic-plastic transition. The stress field from different sized indenters interacts with the low yield-stress material at different depths below the surface to map out the size of the initial yield volume.

Size effect in the initiation of plasticity for ceramics in nanoscale contact loading

In nanoindentation, the plasticity size effect has been observed for several years, where a higher hardness is measured as indenter size decreases. In this paper, we report the size effect on the initiation of plasticity in ceramics by using spherical indenters. Here, we show a clear method that is able to determine the details of the onset of plasticity in nanoindentation. This enables us to measure the yield pressure with a high degree of accuracy and over a large range of indenter radii (hundreds of nanometers to several tens of micrometers). Our data shows clearly that there is a significant yield strength enhancement, which is inversely proportional to the cube root of the indenter radius. Also after normalization by the bulk yield strength, the increase in yield strength with decreasing indenter radius is shown to follow a single relation for all the ceramics studied in agreement with recent results for metals [1], and consistent with critical thickness theory for the initiation of yielding over a finite volume.