A model for the fragmentation kinetics of crumpled thin sheets

As a confined thin sheet crumples, it spontaneously segments into flat facets delimited by a network of ridges. Despite the apparent disorder of this process, statistical properties of crumpled sheets exhibit striking reproducibility. Experiments have shown that the total crease length accrues logarithmically when repeatedly compacting and unfolding a sheet of paper. Here, we offer insight to this unexpected result by exploring the correspondence between crumpling and fragmentation processes. We identify a physical model for the evolution of facet area and ridge length distributions of crumpled sheets, and propose a mechanism for re-fragmentation driven by geometric frustration. This mechanism establishes a feedback loop in which the facet size distribution informs the subsequent rate of fragmentation under repeated confinement, thereby producing a new size distribution. We then demonstrate the capacity of this model to reproduce the characteristic logarithmic scaling of total crease length, thereby supplying a missing physical basis for the observed phenomenon.

Изменение формы наноостровка при селективной эпитаксии

A model is developed for the changes in the nanoisland shape in selective area epitaxy. The model is based on the minimization of the surface energy at a given volume. The geometry considered is an island restricted by the two (101) and (112) side facets and (001) facet at the top, with the facet size changing with the island volume. The calculations are presented for the facet lengths, the corresponding aspect ratios and fractions of the surface areas of the competing facets.

Species and sex differences in eye morphometry and visual responsivity of two crepuscular sweat bee species (Megalopta spp., Hymenoptera: Halictidae)

Visually dependent dim-light foraging has evolved repeatedly, broadening the ecological niches of some species. Many dim-light foraging lineages evolved from diurnal ancestors, requiring immense visual sensitivity increases to compensate for light levels a billion times dimmer than daylight. Some taxa, such as bees, are anatomically constrained by apposition compound eyes, which function well in daylight but not in starlight. Even with this constraint, the bee genus Megalopta has incredibly sensitive eyes, foraging in light levels up to nine orders of magnitude dimmer than diurnal relatives. Despite many behavioural studies, variation in visual sensitivity and eye morphometry has not been investigated within and across Megalopta species. Here we quantify external eye morphology (corneal area and facet size) for sympatric species of Megalopta, M. genalis and M. amoena, which forage during twilight. We use electroretinograms to show that males, despite being smaller than females, have equivalent visual sensitivity and increased retinal responsivity. Although males have relatively larger eyes compared with females, corneal area and facet size were not correlated with retinal responsivity, suggesting that males have additional non-morphological adaptations to increase retinal responsiveness. These findings provide the foundation for future work into the neural and physiological mechanisms that interface with morphology to influence visual sensitivity, with implications for understanding niche exploitation.

Photo-printing of faceted DNA patchy particles

Patchy particles with shape complementarity can serve as building blocks for assembling colloidal superstructures. Alternatively, encoding information on patches using DNA can direct assembly into a variety of crystalline or other preprogrammed structures. Here, we present a tool where DNA is used both to engineer shape and to encode information on colloidal particles. Two reactive oil emulsions with different but complementary DNA (cDNA) brushes are assembled into CsCl-like crystalline lattices. The DNA brushes are recruited to and ultimately localized at the junctions between neighboring droplets, which gives rise to DNA-encoded faceted patches. The emulsions are then solidified by ultraviolet (UV) polymerization, producing faceted patchy particles. The facet size and DNA distribution are determined by the balance between the DNA binding energy and the elastic deformation energy of droplets. This method leads to a variety of new patchy particles with directional interactions in scalable quantities.

About modeling the waves scattering on the complex shape objects

Background: Reducing the detection probability by radar stations for military or civil complex shape objects is very important problem of modern radar theory. The solution to this problem is impossible without numerous estimates of the effective scattering surface (RCS) of the investigated object. Objectives: The purpose of the work was to analyze and improve the efficiency of methods for modeling electromagnetic scattering on objects of complex shape. The process of constructing a facet model of an complex shape object for modeling electromagnetic scattering and an algorithm to simplify the geometric model of an object to reduce the time required for modeling is considered. Materials and methods: As the main method for calculating the RCS, we chose the method of rays that fall and reflect (in the English language literature “Shooting and Bouncing Rays” SBR). Wherein the modeling electromagnetic scattering on an complex shape object is based on the methods of physical and geometric optics. Object is represented in the form of a geometric model which takes into account the shape of the object and its electrodynamic properties (reflection coefficient, dielectric and magnetic permeability of the surface, conductivity, etc.). Results: The general steps of the SBR method are discussed, and the efficient of using the surface mesh method to launch the rays is shown. Effective approaches to simplify the calculation for complex shape objects are proposed, in particular, the use of Raytressing and k-d tree is considered. The using of large facets for modeling surface flat sections of complex shape object is proposed. To assess the effect of the facet size on the accuracy of RCS calculations, we simulated electromagnetic scattering for a triangular plate, the surface of which is represented using different numbers of facets. A comparison with the well-known expressions for a triangular plate RCS is made. Conclusions: The accelerating possibilities of the process of electromagnetic wave diffraction calculation on complex shape objects are offered. The efficiency of the created algorithms is tested numerically.

Modeling of the growth of GaAs–AlGaAs core–shell nanowires

Heterostructured GaAs–AlGaAs core–shell nanowires with have attracted much attention because of their significant advantages and great potential for creating high performance nanophotonics and nanoelectronics. The spontaneous formation of Al-rich stripes along certain crystallographic directions and quantum dots near the apexes of the shell are observed in AlGaAs shells. Controlling the formation of these core–shell heterostructures remains challenging. A two-dimensional model valid on the wire cross section, that accounts for capillarity in the faceted surface limit and deposition has been developed for the evolution of the shell morphology and concentration in Al x Ga1− x As alloys. The model includes a completely faceted shell–vapor interface. The objective is to understand the mechanisms of the formation of the radial heterostructures (Al-rich stripes and Al-poor quantum dots) in the nanowire shell. There are two issues that need to be understood. One is the mechanism responsible for the morphological evolution of the shells. Analysis and simulation results suggest that deposition introduces facets not present on the equilibrium Wulff shapes. A balance between diffusion and deposition yields the small facets with sizes varying slowly over time, which yield stripe structures, whereas deposition-dominated growth can lead to quantum-dot structures observed in experiments. There is no self-limiting facet size in this case. The other issue is the mechanism responsible for the segregation of Al atoms in the shells. It is found that the mobility difference of the atoms on the {112} and {110} facets together determine the non-uniform concentration of the atoms in the shell. In particular, even though the mobility of Al on {110} facets is smaller than that of Ga, Al-rich stripes are predicted to form along the {112} facets when the difference of the mobilities of Al and Ga atoms is sufficiently large on {112} facets. As the size of the shell increases, deposition becomes more important. The Al-poor dots are obtained at the apices of {112} facets, if the attachment rate of Al atoms is smaller there.

USE OF DIGITAL IMAGE CORRELATION TO INVESTIGATE THE BIOMECHANICS OF THE VERTEBRA

Digital image correlation (DIC) is being introduced to the biomechanical field. However, as DIC relies on a number of major assumptions, it requires a careful optimization in order to obtain accurate and precise results. The first step was the preparation of the speckle pattern by an airbrush spray gun following a factorial design to explore the different settings: the different speckle patterns created were analyzed to achieve the optimal speckle size, with minimal dispersion of speckle sizes. A benchmark test, with an aluminum specimen prepared with the speckle pattern, was conducted in which the errors affecting the computed strain were measured in a zero-displacement, zero-strain condition. The software parameters (facet size, step, and local regression) were singularly analyzed in order to understand their behavior on the final output. Moreover, the hardware parameters (camera gain, exposure, lens distortion) were analyzed. The output showed that a careful optimization allowed the reducing the systematic and random errors, respectively, from 150 to 10 microstrain and from 600 to 110 microstrain. Finally, the acquired know-how was applied to a biological specimen (human vertebra).

The Influence of Facet Size and Filtering on the Results of Strain Fields’ Investigation Performed on Small Surfaces Using Digital Image Correlation

The contribution deals with the investigation of the influence of facet size and smoothing on the results obtained by low-speed digital image correlation (DIC) system by strain analysis performed on specimen with a small hole loaded by tension loading. In conclusion the obtained results are verified by a numerical solution using finite element method.

The Influence of Facet Size on the Accuracy of Modal Parameters Determined by Digital Image Correlation Technique

The paper deals with measurements of modal parameters using high-speed digital image correlation and assesses how the size of image elements (facets) influences the accuracy of modal parameters of the analyzed structure. The facet size is an important parameter that significantly affects the correlation process and its results. In the paper the experimental modal analysis of steel circular plate that has been excited by the impact hammer is described. Correlation system Q-450 Dantec Dynamics was used to measure the responses of the analyzed structure.