Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userDiffraction from the beta-sheet crystallites in spider silk
96
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We analyze the wide angle x-ray scattering from oriented spider silk fibers in terms of a quantitative scattering model, including both structural and statistical parameters of the $beta$-sheet crystallites of spider silk in the amorphous matrix. The model is based on kinematic scattering theory and allows for rather general correlations of the positional and orientational degrees of freedom, including the crystallites size, composition and dimension of the unit cell. The model is evaluated numerically and compared to experimental scattering intensities allowing us to extract the geometric and statistical parameters. We show explicitly that for the experimentally found mosaicity (width of the orientational distribution) inter-crystallite effects are negligible and the data can be analyzed in terms of single crystallite scattering, as is usually assumed in the literature.
rate research
Read More
Spider silk possesses unique mechanical properties like large extensibility, high tensile strength, super-contractility, etc. Understanding these mechanical responses require characterization of the rheological properties of silk beyond the simple force-extension relations which are widely reported. Here we study the linear and non-linear viscoelastic properties of dragline silk obtained from social spiders Stegodyphus sarasinorum using a Micro-Extension Rheometer that we have developed. Unlike continuous extension data, our technique allows for the probing of the viscoelastic response by applying small perturbations about sequentially increasing steady-state strain values. In addition, we extend our analysis to obtain the characteristic stress relaxation times and the frequency responses of the viscous and elastic moduli. Using these methods, we show that in a small strain regime (0-4%) dragline silk of social spiders shows strain-softening response followed by strain-stiffening response at higher strains (> 4%). The stress relaxation time, on the other hand, increases monotonically with increasing strain for the entire range. We also show that silk stiffens while ageing within the typical lifetime of a web. Our results demand the inclusion of the kinetics of domain unfolding and refolding in the existing models to account for the relaxation time behaviour.
We report on three launches of ballooning $Erigone$ spiders observed in a 0.9 m$^3$ laboratory chamber, controlled under conditions where no significant air motion was possible. These launches were elicited by vertical, downward-oriented electric fields within the chamber, and the motions indicate clearly that negative electric charge on the ballooning silk, subject to the Coulomb force, produced the lift observed in each launch. We estimate the total charge required under plausible assumptions, and find that at least 1.15 nC is necessary in each case. The charge is likely to be non-uniformly distributed, favoring initial longitudinal mobility of electrons along the fresh silk during extrusion. These results demonstrate for the first time that spiders are able to utilize charge on their silk to attain electrostatic flight even in the absence of any aerodynamic lift.
A theoretical study of photonic bands for one-dimensional (1D) lattices embedded in planar waveguides with strong refractive index contrast is presented. The approach relies on expanding the electromagnetic field on the basis of guided modes of an effective waveguide, and on treating the coupling to radiative modes by perturbation theory. Photonic mode dispersion, gap maps, and intrinsic diffraction losses of quasi-guided modes are calculated for the case of self-standing membranes as well as for Silicon-on-Insulator structures. Photonic band gaps in a waveguide are found to depend strongly on the core thickness and on polarization, so that the gaps for transverse electric and transverse magnetic modes most often do not overlap. Radiative losses of quasi-guided modes above the light line depend in a nontrivial way on structure parameters, mode index and wavevector. The results of this study may be useful for the design of integrated 1D photonic structures with low radiative losses.
The quasiparticle band structure and optical properties of single-walled zigzag and armchair SiC nanotubes (SiC-NTs) as well as single SiC sheet are investigated by ab initio many-body calculations using the GW and the GW plus Bethe-Salpeter equation (GW+BSE) approaches, respectively. Significant GW quasiparticle corrections of more than 1.0 eV to the Kohn-Sham band gaps from the local density approximation (LDA) calculations are found. The GW self-energy corrections transform the SiC sheet from a indirect LDA band gap to a direct band gap material. Furthermore, the quasiparticle band gaps of SiC-NTs with different chiralities behave very differently as a function of tube diameter, and this can be attributed to the difference in the curvature-induced orbital rehybridization between the different chiral nanotubes. The calculated optical absorption spectra are dominated by discrete exciton peaks due to exciton states with large binding energy up to 2.0 eV in the SiC sheet and SiC-NTs. The formation of strongly bound excitons is attributed to the enhanced electron-hole interaction in these low dimensional systems. Remarkably, the excited electron amplitude of the exciton wavefunction is found to peak on the Si atoms near the hole position (which is on the C site) in the zigzag SiC-NTs, indicating a charge transfer from an anion (hole) to its neighboring cations by photoexcitation. In contrast, this pronounced peak structure disappear in the exciton wavefunction in the armchair SiC-NTs. Furthermore, in the armchair SiC-NTs, the bound exciton wavefunctions are more localized and also strongly cylindrically asymmetric.
The distribution of the transport current in a superconducting filament aligned parallel to the flat surface of a semi-infinite bulk magnet is studied theoretically. An integral equation governing the current distribution in the Meissner state of the filament is derived and solved numerically for various filament-magnet distances and different relative permeabilities. This reveals that the current is depressed on the side of the filament adjacent to the surface of the magnet and enhanced on the averted side. Substantial current redistributions in the filament can already occur for low values of the relative permeability of the magnet, when the distance between the filament and the magnet is short, with evidence of saturation at moderately high values of this quantity, similar to the findings for magnetically shielded strips.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
