Thin Planar Sheets Research Articles

A Study of Deployable Structures Based on Nature Inspired Curved-Crease Folding.

Fascinating 3D shapes arise when a thin planar sheet is folded without stretching, tearing or cutting. The elegance amplifies when the fold/crease is changed from a straight line to a curve, due to the association of plastic deformation via folding and elastic deformation via bending. This results in the curved crease working as a hinge support providing deployability to the surface which is of significant interest in industrial engineering and architectural design. Consequently, finding a stable form of curved crease becomes pivotal in the development of deployable structures. This work proposes a novel way to evaluate such curves by taking inspiration from biomimicry. For this purpose, growth mechanism in plants was observed and an analogous model was developed to create a discrete curve of fold. A parametric model was developed for digital construction of the folded models. Test cases were formulated to compare the behavior of different folded models under various loading conditions. A simplified way to visualize the obtained results is proposed using visual programming tools. The models were further translated into physical prototypes with the aid of 3D printing, hybrid and cured-composite systems, where different mechanisms were adopted to achieve the folds. The prototypes were further tested under constrained boundary and compressive loading conditions, with results validating the analytical model.

Geometric mechanics of ordered and disordered kirigami

The presence of incomplete cuts in a thin planar sheet can dramatically alter its mechanical and geometrical response to loading, as the cuts allow the sheet to deform strongly in the third dimension, most beautifully demonstrated in kirigami art-forms. We use numerical experiments to characterize the geometric mechanics of kirigamized sheets as a function of the number, size and orientation of cuts. We show that the geometry of mechanically loaded sheets can be approximated as a composition of simple developable units: flats, cylinders, cones and compressed Elasticae. This geometric construction yields scaling laws for the mechanical response of the sheet in both the weak and strongly deformed limit. In the ultimately stretched limit, this further leads to a theorem on the nature and form of geodesics in an arbitrary kirigami pattern, consistent with observations and simulations. Finally, we show that by varying the shape and size of the geodesic in a kirigamized sheet, we can control the deployment trajectory of the sheet, and thence its functional properties as an exemplar of a tunable structure that can serve as a robotic gripper, a soft light window or the basis for a physically unclonable device. Overall our study of disordered kirigami sets the stage for controlling the shape and shielding the stresses in thin sheets using cuts.

Recent advances in the development of graphene-based surface plasmon resonance (SPR) interfaces

Surface plasmon resonance (SPR) is a powerful technique for measurement of biomolecular interactions in real-time in a label-free environment. One of the most common techniques for plasmon excitation is the Kretschmann configuration, and numerous studies of ligand-analyte interactions have been performed on surfaces functionalized with a variety of biomolecules, for example DNA, RNA, glycans, proteins, and peptides. A significant limitation of SPR is that the substrate must be a thin metal film. Post-coating of the metal thin film with a thin dielectric top layer has been reported to enhance the performance of the SPR sensor, but is highly dependent on the thickness of the upper layer and its dielectric constant. Graphene is a single-atom thin planar sheet of sp2 carbon atoms perfectly arranged in a honeycomb lattice. Graphene and graphene oxide are good supports for biomolecules because of their large surface area and rich π conjugation structure, making them suitable dielectric top layers for SPR sensing. In this paper, we review some of the key issues in the development of graphene-based SPR chips. The actual challenges of using these interfaces for studying biomolecular interactions will be discussed and the first examples of the use of graphene-on-metal SPR interfaces for biological sensing will be presented.

Integration of Raman microscopy, differential interference contrast microscopy, and attenuated total reflection Fourier transform infrared spectroscopy to investigate chlorhexidine spatial and temporal distribution in Candida albicans biofilms

Two spectroscopic techniques, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and Raman microscopy (RM), were used to characterize transport of chlorhexidine digluconate (CHG) in Candida albicans (CA) biofilms. Different (volumetric) regions of the biofilm are sampled by these two vibrational spectroscopies making them complementary techniques. Simple mathematical models were developed to analyze ATR-FTIR and RM data to obtain an effective diffusion coefficient describing transport through CA biofilms. CA biofilms were composed primarily of yeast and hyphal forms, with some pseudohyphae. Upper regions of biofilms that had become confluent, (i.e., biofilms that completely covered the germanium (Ge) substratum) were composed primarily of a tangled mass of hyphae with openings between germtubes about 10 to 50 μm across. Quantitative analysis of ATR-FTIR kinetic data curves indicated that the effective diffusion coefficient for transport of CHG through confluent biofilms about 200-μm thick was reduced 0.1 to 0.3 times compared to the diffusion coefficient for CHG in water. Effective diffusion coefficients obtained from analysis of RM data were consistently higher than those indicated by ATR-FTIR data suggesting that transport is more hindered in regions near the base of the biofilm than in the outer layers. Analysis of both ATR-FTIR and RM data obtained from thicker films indicated that adsorption of CHG to biofilm components was responsible for a substantial portion of the transport limitation imposed by the biofilm. Comparison of ATR-FTIR and RM data for both types of biofilms indicated that sites of CHG adsorption were more concentrated in the interfacial region than in the bulk biofilm. Comparison of results for ATR-FTIR and RM measurements suggests that these relatively thick CA biofilms can be modeled, for purposes of predicting transport, approximately as a homogeneous thin planar sheet. Thus, these biofilms offer a relatively tractable model system for initial investigations of the relation between antimicrobial transport and kinetics of antimicrobial action.

Planar fibre reinforced carbon aerogels for application in PEM fuel cells

Organic precursor aerogels have been prepared from resorcinol-formaldehyde (RF) with a very high molar ratio of resorcinol/catalyst of 1500, (RF/total) mass ratios between 30 and 25 wt. % (target density ≈ 300 kg m −3) and an F/ R-ratio between 1.5 and 3.0. These compositions show minimal shrinkage during ambient pressure drying. Subsequently, the precursors were pyrolysed at 1050°C to form carbon aerogels. The use of fibre additives from ceramic, carbon or polymeric fibres (novoloid, PAN, aramide) increase the stability of the carbon aerogel matrix drastically and allow to prepare thin planar sheets with thicknesses between 50 and 300 μm. By using untreated cellulose as the additive, a bimodal pore structure is generated. During pyrolysis the fibre–aerogel compounds were transformed into carbon aerogels exhibiting the morphology and planarity of the RF precursors. The carbon aerogels were investigated with respect to their structure, using scanning electron microscopy (SEM), small angle X-ray scattering (SAXS) and gas permeability measurements. The prepared aerogel sheets show a rather smooth surface and a somewhat coarse interior structure, which is advantageous for application in fuel cells. For the fibre reinforced gels, the structural changes due to a variation of the composition parameters is less distinct than for the non-reinforced gels. The structure of the reinforced gel depends strongly on the time, the sol was allowed to age prior to impregnation. In addition, it was found that the surface of certain fibre materials (e.g. cellulose) influence the gelation catalytically, while novoloid rather ‘slows’ the gelation down. The catalytic effect can be described by a semi-empirical model, involving a catalytic penetration depth.

Three-dimensional wave distortion and disintegration of thin planar liquid sheets

Three-dimensional dilational and sinuous wave propagation on infinite or semi-infinite thin planar sheets flowing into a gas of negligible density is investigated. The assumption of thin sheets allows the reduction of the problem dimensionality by integration across the sheet thickness. For finite-amplitude disturbances, the strongest nonlinear effects occur when the cross-sectional wavenumber (l) is close to the streamwise wavenumber (k). First, dilational wave propagation is considered. When l is close to k for infinite sheets, higher harmonics are generated in the streamwise direction, and the standing wave with finite amplitude in the cross-sectional plane becomes flat. As time passes, the waves return to the initial wave shape. This process is repeated in a cycle

Reticular astrocytes in the fish optic nerve: macroglia with epithelial characteristics form an axially repeated lacework pattern, to which nodes of Ranvier are apposed

Astroglia in lower vertebrate optic nerves are unusual: as shown recently (Giordano et al., 1989; Rungger-Brändle et al., 1989), they express abundant Type II cytokeratin, not glial fibrillary acidic protein (GFAP), the cytoskeletal marker for astrocytes elsewhere. To determine the implications for the glial cells of these epithelial-type cytoskeletons, which are linked up by desmosomal junctions, we analyzed the tissue patterning of fish optic nerve astroglia, which we term reticular astrocytes on account of their uniquely specialized arrangement. The processes of the reticular astrocytes fasciculate extensively with one another in a pattern stabilized by the desmosomes, forming a network laid out in thin planar sheets, or partitions. These are arranged transversely, that is at right angles to the optic fibers and are repeated at regular intervals of about 15 microns longitudinally throughout the optic nerve. They merge periodically forming a 3-dimensional framework whose pattern we speculate provides a flexible tissue skeleton for the optic nerve, capable of accommodating eye movements. Virtually all fibers in mature regions of the optic nerve are myelinated, and we show that nodes of Ranvier mostly occur in register with the partitions, displaying perinodal astrocytic associations resembling those found at CNS nodes in mammals. This clustering may account for the unexpectedly high observed incidence of neighbor pairs of nodes. Among other peculiarities associated with the reticular astrocytic network, one is that up to 20% of all cells comprise foamy macrophages not found elsewhere in the CNS.