U.S. Air Force Research Laboratory Materials and Manufacturing Directorate

Films subsumed with topological defects are transformed into complex, topographical surface features with light irradiation of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs). Using a specially designed optical setup and photoalignment materials, azo-LCN films containing either singular or multiple defects with strengths ranging from |½| to as much as |10| are examined. The local order of an azo-LCN material for a given defect strength dictates a complex, mechanical response observed as topographical surface features. The ability of biological systems to self-assemble materials into pre-programmed shapes in response to external stimuli is inspirational. Emulating the capability of self-assembling, stimuli-responsive materials with tailored functionality (actuation, shape change, surface manipulation, or other property changes) is of paramount interest to fields ranging from biomedical engineering1 to robotics.2 Liquid crystalline phases and defects play a major role in the self-assembly of biological materials, including the plasma membrane, wood, silk, and the insect cuticle.3 Here, we examine the response of complexly patterned photoresponsive liquid crystal polymer networks (LCN). The rich and diverse topographical features reported here are retiscent to shape adaptations and topographical surface manipulation observable in nature and could be useful in a range of applications including haptic displays, optics/photonics, flow control, or even catalysis. Liquid crystalline materials have repeatedly been demonstrated to exhibit a diverse range of stimuli-responsive behavior, such as negative thermal expansion and multi-stage phase transitions.7, 8 The responsive nature of liquid crystal polymers are enabled and controlled by the anisotropic orientation of the mesogenic moieties. Azobenzene-functionalized LCN (azo-LCN) materials have been the subject of considerable recent examinations as adaptive materials9 and shape memory polymers.19 Importantly for the work presented here, the directionality of the strain generated with a stimulus in aligned LCN materials (monodomain, twisted nematic, splay) is dictated by the director orientation of the material. For example, strain generated in a monodomain (nematic) LCN is primarily observed parallel to the alignment of the mesogens.16, 22 Offsetting the orientation of the director profile to the sample geometry (film or cantilever) results in out-of-plane deformation observable as twisting.10, 18, 20, 23 The work presented here was motivated by a series of recent papers24 in which Modes and Warner predict that defect-containing LCNs should exhibit mechanically adaptive responses highly distinguishable to any reports to date.24 The authors clearly show that sheets composed of glassy LCNs subsumed with a central topological defect and resulting director profile spanning the entirety of the film will result in a complex distribution of strain that concentrates at the defect. The clearest depiction of this is the predictions that subjecting glassy LCN materials patterned with a single +1 topological defect to an appropriate stimulus will cause the sheet to spontaneously morph into a cone with the center of the defect as the apex.25 The subsequent papers extended upon these results by introducing the design framework (so-called “grammar and vocabulary”28) surrounding the piece-wise use of topological defects (strengths |m| ≤1) as localized Gaussian curvature building blocks that could be assembled to create pre-programmed, shape-reconfigurable, non-developable surfaces from flat sheets. One of the salient features of liquid crystalline materials is the ability to readily organize the director profile into complex patterns. Most commonly, director patterning is facilitated by the use of photoalignment surfaces based on azobenzene materials. Domain profile patterning with photoalignment has been recently reported, such as in the fabrication of arrays of axial waveplates.36 Recently, Broer and coworkers employed photoalignment to pattern defects into glassy, LCN materials containing a heat transfer dye and demonstrated photothermal formation of cone and anti-cone surface topographies as predicted by Modes and Warner.37 Here, we present the systematic examination of topographical transformation resulting from photomechanical response of azo-LCN films with defects ranging from -5/2 to +5/2, the response of high-strength defects (|m| = 10), as well as explore the coordinated response of periodic arrays of topological defects. Allowable, naturally occurring defects in liquid crystalline materials have strengths of -1/2, +1/2, -1, and +1. Defects with strength |m| > 1 are unstable due to the high elastic energy, F, which scales with defect strength as F ∼ m2.34, 35 Due to the fact that we are dealing with lateral dimensions of thin films many orders of magnitude larger than the core size and film thickness, we will neglect discussion of defects that “escape in the third dimension” to form nonsingular configurations with complex structure and assume that the director field is two-dimensional, n(x, y). Our approach to dictating the topological defects examined here utilize photoalignment materials to generate the desired surface alignment patterns, similar to techniques employed to create axial waveplates36, 38 and polarization independent LC lenses.39 The optical patterning setup passes a blue laser (445 nm) through a half-wave (λ/2) plate and a cylindrical lens projected onto the LC cell prepared from substrates coated with photoalignment surface material (PAAD-22, purchased from BEAM Engineering for Advanced Measurements). As illustrated in Figure 1, both the λ/2 plate and the sample cell were mounted on computer-controlled, actuated rotation stages. The λ/2 plate allows the orientation of the linear polarization of the light to the sample to be rotated. Importantly, a cylindrical lens was used to focus the expanded laser beam into a sharp line (<50 μm width, 1.6 cm length). By rotating the sample with respect to this line, a circular area of the cell was exposed to 445 nm light. Both the orientation of the polarization and the sample were rotated during irradiation. The resulting defect strength was regulated by varying the ratio of polarization rotation rate, R1, to sample rotation rate, R2, via the following equation, m = 1 − R1/R2, which is valid for allowable defect strengths (i.e. strength must be an integer or half- integer value). For example, a +1 strength topological defect is produced by simply rotating the sample at any angular velocity without rotating the polarization (R1 = 0). A +2 strength defect is produced by rotating the polarization and the sample at the same rate but opposite directions (R1/R2 = −1). A schematic representation of the sample preparation and harvesting procedure is presented in Figure 1. After patterning the orientation of the surface alignment layer the cells were subsequently filled with the mixture of liquid crystalline monomers and photopolymerized with green light (l = 532 nm, 35 mW/cm2 intensity for 30 min) at 75 °C (nematic phase). Green light was used to avoid disruption of the photoalignment patterns. After polymerization, the cells were split open and the film was harvested. Polarized optical microscopy (POM) was used to confirm that the polymerized films arrest the defect written into the alignment cell. Building upon the work of Modes et al.,24 we report on the photochemically initiated mechanical responses of azo-LCN films subsumed with the director field of a large range of topological defects. Notably, we also extend upon the theoretical predictions as well as the recent experimental examination37 by preparing higher order defect strengths not previously considered (m > |1|, up to |10|). Figure 2 is composed of an illustration of the director field, confirmation of the defect in the circular azo-LCN film by POM (1 cm diameter), a photograph of the observed photomechanical response, as well as an illustration of the observed photomechanical response. Each column is labeled with the defect strength and ratio of rotation rates employed. As is apparent from the polarized optical microscopy images, the generated defects have N = |m|*4 brushes. Notably, although each director profile is distinctive–there are observable similarities, for example in the three-fold symmetry of the director orientation of the +5/2 defect and the -1/2 defect. The origin of the symmetry is more apparent when noting that the R1/R2 ratio is equal and opposite between the n-fold pairs. By exposing the photoresponsive, defect-patterned azo-LCN films to unpolarized blue light the materials adopt a complex topography specific to the defect. The underlying photochemical mechanism of transduction of light into mechanical response has been detailed in a number of recent reports40 and attributable to local strain generation in the glassy polymer caused by the reorientation and isomerization of the azobenzene chromophores. Due to the complex alignment of the azobenzene mesogenic units in the azo-LCN, we employed unpolarized light as an isotropic stimulus. The photomechanical response of these films is also illustrated in Figure 2. A description of the photomechanical response of each of these films is included in the Supporting Information. From the images presented in Figure 2 the topographical features formed upon irradiation with 445 nm light are rich and diversified. Close examination of Figure 2 reveals two general trends. First, the deformations of the negatively charged defects follow a clear pattern where the number of equally spaced valleys, V, radiating from the center of the defect is correlated to double the absolute strength of the defect plus two (V = 2*|m| + 2 if m < 0). Several times the director orientation was tracked throughout the fabrication and deformation process (illustrated in Figure 1). The general trend is the radiating valleys were found to be localized to regions where the director orientation points towards the center of the defect. Second, the deformation of the positively signed defects of strength m > 2 where the number of tear shaped dimples, T is related to the defect strength through T = 2*|m| − 2 (if m > 2). These relationships emphasize that the response of the films is strongly underpinned by the local anisotropy of the director profiles radiating from the topological defect. Motivated by the complex patterns apparent in the higher-order defects examined in Figure 2, we applied the defect patterning method to form azo-LCN films that retain high-strength defects of m = ±10. The polarized optical microscopy images presented in Figure 3 confirm that indeed, ±10 strength defects were retained in the films. As previously mentioned, the elastic energy scales with defect strength as F ∼ m2. Therefore, the nematic Frank elastic energy for the m = +10 and m = -10 defects is roughly two orders of magnitude larger than the other defects examined here. Because of this high energetic cost, these high-strength defects would be unstable in conventional nematic liquid crystal fluids with tangentially degenerate boundary conditions (e.g., in the case of the so-called “Schlieren texture”), however, they are stabilized by the deliberate spatial patterning of the easy axis orientation in the photopatterned alignment cells. Evident in Figure 3, the m = +10 defects has eighteen tear shaped nodes, confirming that the above relationship for T holds true despite the considerable increase in elastic energy. The tear shaped nodes are very elongated due to the fact that the director makes a full rotation as one navigates the core every 36°. The -10 strength defect has 22 valleys as shown in Figure 3, also confirming the above relationship for V holds true despite the considerable increase in elastic energy. Notably, the response of the azo-LCN film with a -10 defect has numerous rows of “struts” connecting the expected valleys that radiate from the center of the topological defect. The circumference of the struts spans the entire film and increase in diameter as the distance increases from the center of the defect. Close examination of lower strength defects in the images presented in Figure 2 confirms these struts are also apparent in the -1, -3/2, -2 and -5/2 defects (see Supporting Information for further discussion of these features). The scale invariance and self-similar pattern formation apparent in Figures 2 and 3 are also common features of biological systems, especially in response to external stimulus. An active area of biological research is the role that external mechanical stress plays in mediating scale. We extend this examination to characterize the response of arrays of “printed” defects. Defect printing was enabled by projecting the 445 nm irradiation through either a +2 or +4 axial waveplate36 during photoalignment. The adjacent defects spacing was 3 mm and the diagonal defect spacing was 2.1 mm. The printed defect patterns had a diameter of ∼3.1 mm such that the far field of the director profiles overlapped one another. The sample was placed on a motorized stage that allowed for x-y control of the repeated transcribing of the defect pattern onto the photoalignment material. The arrays were printed in a hexagonal pattern and the defects were oriented in the same direction with respect to each other. The resulting complex director field can be interpreted from the photograph imaged of the sample between crossed polarizers presented in Figure 4. Upon exposure to unpolarized blue light both the samples demonstrated the expected N-fold topography, with the +2 defects having 2-fold topography and the +4 defect having 6-fold topography. Notably, the hexagonal array of +2 defects results in a square array of 2-fold dimpled geometry. Each defect results in a pair of dimples, thereby effectively cutting the periodic length scale in half along one axis and thus resulting in a square pattern instead of a hexagonal pattern. On the other hand, the hexagonal array of +4 defects, with 6-fold resulting topography, cut the periodic length scale in half along the 3 periodic length scales and thus resulted in a hexagonal pattern. In addition to the patterned positively charged defects, a series of defects having equal net negative charge appear in order to satisfy topological charge/strength conservation, further enriching the photoresponsive topography of these films. The contrast between the resulting topography of these samples serves as an illustrative example of the complexity and richness in transforming the topological patterns in to topographical features. In conclusion, we show that light can induce rich and diverse topographical transformations of initially flat films composed of photoresponsive, azobenzene-functionalized liquid crystalline polymer networks (azo-LCN). The topographical response of the defect-containing azo-LCN films was further confirmed experimentally and expanded to examine the response of higher order topological defects that have yet to be considered theoretically or experimentally. Ultimately, the N-fold symmetry of the topography of the photoinduced response of the defect-containing azo-LCN films was inherently coupled to the strength of the defect confirming that the director profile emanating from the topological defect is governing the behavior. The topographical transformation of flat films composed of hexagonal arrays of +2 and +4 topological defects were also presented. This work stands as an experimental demonstration of a ubiquitous approach to forming stimuli-responsive surface topographies and may provide insight into self-assembly processes in nature. Optically patternable alignment cells were fabricated by spin coating freshly cleaned pieces of glass with PAAD-22 (purchased from BEAM Co.) at 3000 rpm for 60 seconds. The coated glass substrates were subsequently baked on a hot plate at 100 °C for 10 minutes to evaporate any excess solvent. The two pieces of glass were spaced and glued together by mixing 12 μm cylindrical spacers into an adhesive. Due to slight differences in flatness of the glass and other factors the final cell thickness varied by approximately 1 μm. This small variation in cell thickness was the source of color changes in the cross-polarized images in Figure 4, for example. Optical patterning of the director orientation of the alignment cells was accomplished in a setup employing a blue DPSS laser (445 nm) that was first passed through a λ/2 plate mounted on an actuated, rotation stage to allow for computer-control of the orientation of the linear polarization of the 445 nm laser to the alignment cell. Subsequent to passing through the λ/2 plate, the beam was expanded, colliminated and focused into a line with a cylindrical lens. The alignment cell was also mounted on an actuated, rotation stage which was rotated at 2°/sec for 180 seconds in all cases. The angular velocity of the rotation of the λ/2 plate was adjusted to achieve the desired defect strength (governed by m = 1 − R1/R2,). Before the cylindrical lens, the light intensity was 6.5 mW/cm2 (beam diameter of 1.6 cm). After passing through the cylindrical lens, the focused line had dimensions of approximately 50 μm by 1.6 cm. Because of the Gaussian profile of the laser beam, the intensity near the end points of the line was smaller than in the center of the line. Furthermore, in this rotation method the exposure time decreases with increasing distance from the center of the topological defect. Fortunately, the photoalignment material employed here has a large range of exposure conditions in which excellent planar alignment can be achieved. Before and after photoalignment the cell was kept in the dark (hours to days) to avoid room lighting erasing the photoalginment pattern before the cell was filled and polymerized. The monomer mixture was prepared by mixing 20% A6ZA6 diacrylate azobenzene monomer (BEAM Co.), 78.2% RM 257 diacrylate monomer (Merck), and 1.8 wt% of the visible light photoinitiator Irgacure 784 (Ciba). The mixture of these materials was solid at room temperature. Special care was taken to mix thoroughly. This mixture was heated into the isotropic phase. The isotropic liquid was placed on the lip of the alignment cell. The monomer mixture filled the photoaligned cells by capillary action in dark room conditions on a hotplate heated to 85 °C. After filling the cell the alignment was checked under cross-polarizers with red lighting. If the alignment of the cell was deemed imperfect the cell was heated to isotropic with a heat gun on the hot plate and allowed to slowly cool to 85 °C. After proper alignment the cell was transferred to a separate hotplate set to 75 °C, allowing the material to cool to the nematic phase. The sample was left for at least 3 minutes to reach this lower temperature. The cell was subsequently exposed to green light (λ = 532 nm, 35 mW/cm2) for 30 minutes to initiate photopolymerization. Green light was used so as to not impact the photoalignment of the cell. After photopolymerization, the cells were opened with a razor blade to harvest the films. The films were cut into a circle with the topological defect centered within the circle. To assure that the defect was centered, the film was mounted (still on a piece of glass) at the center of a manual rotation stage. A stationary exacto knife was brought into contact with the film and subsequently the sample was rotated to the The samples were cut into with a mm substrates were by diameter of diameter of onto a piece of After the was the diameter of the and the sample was placed onto the was taken to that the between the and the film was The boundary conditions by this procedure allowed for and repeated of the topographical due to the of and allowing the film to be a and small This sample allowed for even capillary on the of the film and the allowed for to photoinduced The mounted film was exposed to mW/cm2 of blue (λ = unpolarized light from an for at least 30 The shape was and after irradiation. the of the deformation but without the film to the flat mechanical in these materials have been reported to exhibit shape memory This work was at the and of the We BEAM Engineering for Advanced for the +2 and +4 axial We with This work was by the of and the and the of the As a to authors and this by the materials are and may be for but are not or from than should be to the The is not for the or functionality of any by the than should be to the for the

Sulfur is a very promising cathode material for rechargeable energy storage devices. However, sulfur cathodes undergo a noticeable volume variation upon cycling, which induces mechanical stress. In spite of intensive investigation of the electrochemical behavior of the lithiated sulfur compounds, their mechanical properties are not very well understood. In order to fill this gap, we developed a ReaxFF interatomic potential to describe Li-S interactions and performed molecular dynamics (MD) simulations to study the structural, mechanical, and kinetic behavior of the amorphous lithiated sulfur (a-LixS) compounds. We examined the effect of lithiation on material properties such as ultimate strength, yield strength, and Young's modulus. Our results suggest that with increasing lithium content, the strength of lithiated sulfur compounds improves, although this increment is not linear with lithiation. The diffusion coefficients of both lithium and sulfur were computed for the a-LixS system at various stages of Li-loading. A grand canonical Monte Carlo (GCMC) scheme was used to calculate the open circuit voltage profile during cell discharge. The Li-S binary phase diagram was constructed using genetic algorithm based tools. Overall, these simulation results provide insight into the behavior of sulfur based cathode materials that are needed for developing lithium-sulfur batteries.

Abstract Phototuning of more than 2000 nm is demonstrated in an azobenzene‐based cholesteric liquid crystal (azo‐CLC) consisting of a high‐helical‐twisting‐power, axially chiral bis(azo) molecule (QL76). Phototuning range and rate are compared as a function of chiral dopant concentration, light intensity, and thickness. CLCs composed of QL76 maintain the CLC phase regardless of intensity or duration of exposure. The time necessary for the complete restoration of the original spectral properties (position, bandwidth, baseline transmission, and reflectivity) of QL76‐based CLC is dramatically reduced from days to a few minutes by polymer stabilization of the CLC helix.

Abstract The facile synthesis of well‐aligned, main‐chain liquid crystalline elastomers that retain the cholesteric phase (CLCEs) is reported. The selective reflection inherent to this phase is thermally tuned more than 200 nm in these solid films, across the visible spectrum. The optical response is directly correlated to thermomechanical expansion of the CLCE film thickness. The bandwidth of the selective reflection of the CLCEs is increased to more than 200 nm by the incorporation of photosensitive chiral dopants that introduce heterogeneity in the pitch distribution. The mirror‐like reflection of this CLCE film is also thermochromic, shifting from the visible to infrared. Reminiscent of cephalopods, when combined with the mechanical deformation of voxelated nematic liquid crystal elastomer, the thermochromic response of the CLCE produces solid‐state elements with concurrent variation of specular and diffuse reflectance. These results demonstrate distinctive potential opportunities for liquid crystal elastomers to control light enabling new application in textiles, optics, and architecture.

Marine-based deoxyribonucleic acid (DNA), purified from waste products of the Japanese fishing industry, has recently become a material of interest in photonics applications. Using highly purified DNA, unique processing techniques developed specifically to transform the purified DNA into a biopolymer suitable for optical device fabrication are reported.

Microsphere‐assisted imaging has emerged as an extraordinary simple technique of obtaining optical super‐resolution. This work addresses two central problems in developing this technology: i) methodology of the resolution measurements and ii) limited field‐of‐view provided by each sphere. It is suggested that a standard method of resolution analysis in far‐field microscopy based on convolution with the point‐spread function can be extended into the super‐resolution area. This allows developing a unified approach to resolution measurements, which can be used for comparing results obtained by different techniques. To develop the surface scanning functionality, the high‐index ( n ∼ 2) barium titanate glass microspheres were embedded in polydimethylsiloxane (PDMS) thin‐films. It is shown that such films adhere to the surface of nanoplasmonic structures so that the tips of embedded spheres experience the objects’ optical near‐fields. Based on rigorous criteria, the resolution ∼ λ /6‐ λ /7 (where λ is the illumination wavelength) is demonstrated for arrays of Au dimers and bowties. Such films can be translated along the surface of investigated samples after liquid lubrication. It is shown that just after lubrication the resolution is diffraction limited, however the super‐resolution gradually recovers as the lubricant evaporates. image

A universal approach to develop 3D printable, free-standing, and current collector-embedded electrode inks has been established.

Interactions between two layered silicate sheets, as found in various nanoscale materials, are investigated as a function of sheet separation using molecular dynamics simulation. The model systems are periodic in the xy plane, open in the z direction, and subjected to stepwise separation of the two silicate sheets starting at equilibrium. Computed cleavage energies are 383 mJ /m(2) for K-mica, 133 mJ /m(2) for K-montmorillonite (cation exchange capacity=91), 45 mJ /m(2) for octadecylammonium (C(18))-mica, and 40 mJ /m(2) for C(18)-montmorillonite. These values are in quantitative agreement with experimental data and aid in the molecular-level interpretation. When alkali ions are present at the interface between the silicate sheets, partitioning of the cations between the surfaces is observed at 0.25 nm separation (mica) and 0.30 nm separation (montmorillonite). Originally strong electrostatic attraction between the two silicate sheets is then reduced to 5% (mica) and 15% (montmorillonite). Weaker van der Waals interactions decay within 1.0 nm separation. The total interaction energy between sheets of alkali clay is less than 1 mJ /m(2) after 1.5 nm separation. When C(18) surfactants are present on the surfaces, the organic layer (>0.8 nm) acts as a spacer between the silicate sheets so that positively charged ammonium head groups remain essentially in the same position on the surfaces of the two sheets at any separation. As a result, electrostatic interactions are efficiently shielded and dispersive interactions account for the interfacial energy. The flexibility of the hydrocarbon chains leads to stretching, disorder, and occasional rearrangements of ammonium head groups to neighbor cavities on the silicate surface at medium separation (1.0-2.0 nm). The total interaction energy amounts to less than 1 mJ /m(2) after 3 nm separation.

Experimental data on the oxidation kinetics of SiC ‐containing diborides of Zr and Hf in the temperature regime of 1473–2273 K are interpreted using a mechanistic model. The model encompasses counter‐current gas diffusion in the internal SiC depleted zone, oxygen permeation through borosilicate glass channels in the oxide scale, and boundary layer evaporation at the surface. The model uses available viscosity, thermodynamic and kinetic data for boria, silica, and borosilicate glasses, and a logarithmic mean approximation for compositional variations. The internal depletion region of SiC is modeled with CO/CO 2 counter diffusion as the oxygen transport mechanism. Data reported for pure SiC in air/oxygen, for ZrB 2 containing varying volume fractions of SiC , and for SiC–HfB 2 ultra‐high temperature ceramics ( UHTC s) by different investigations were compared with quantitative predictions of the model. The model is found to provide good correspondence with laboratory‐furnace‐based experimental data for weight gain, scale thicknesses, and depletion layer thicknesses. Experimental data obtained from arc‐jet tests at high enthalpies are found to fall well outside the model predictions, whereas lower enthalpy data were closer to model predictions, suggesting a transition in mechanism in the arc‐jet environment.

Liquid crystal elastomers with tunable actuation strain are synthesized with simple techniques that enable complexly patterned actuation.

Polar polyimides with added CN dipoles exhibit higher discharged energy density than nonpolar polyimides such as Ultem and Kapton.

Artificial hair sensors consisting of a piezoresistive carbon-nanotube-coated glass fiber embedded in a microcapillary are assembled and characterized. Individual sensors resemble a hair plug that may be integrated in a wide range of host materials. The sensors demonstrate an air-flow detection threshold of less than 1 m/s with a piezoresistive sensitivity of 1.3% per m/s air-flow change.

Cu-rich surface defects on MOF crystallites lead to enhanced polysulfide uptake and improved capacity retention in lithium sulfur batteries.

Self-organized 3D photonic superstructures loaded with plasmonic hybrid nanorods were found to undergo structural transformation from body-centered cubic to simple cubic upon NIR-light irradiation resulting from the "photothermal effect" of gold nanorods. Furthermore, dynamic NIR light-directed red, green and blue reflections of the nanocomposites were demonstrated.

Research on fatigue crack formation from a corroded 7075-T651 surface provides insight into the governing mechanical driving forces at microstructure-scale lengths that are intermediate between safe life and damage tolerant feature sizes. Crack surface marker-bands accurately quantify cycles (Ni) to form a 10–20 μm fatigue crack emanating from both an isolated pit perimeter and EXCO corroded surface. The Ni decreases with increasing-applied stress. Fatigue crack formation involves a complex interaction of elastic stress concentration due to three-dimensional pit macro-topography coupled with local micro-topographic plastic strain concentration, further enhanced by microstructure (particularly sub-surface constituents). These driving force interactions lead to high variability in cycles to form a fatigue crack, but from an engineering perspective, a broadly corroded surface should contain an extreme group of features that are likely to drive the portion of life to form a crack to near 0. At low-applied stresses, crack formation can constitute a significant portion of life, which is predicted by coupling macro-pit and micro-feature elastic–plastic stress/strain concentrations from finite element analysis with empirical low-cycle fatigue life models. The presented experimental results provide a foundation to validate next-generation crack formation models and prognosis methods.

Three-dimensional electron backscatter diffraction data, obtained by serial sectioning a nickel–base superalloy, has been analysed to measure the geometric arrangement of grain boundary planes at triple junctions. This information has been used to calculate the grain boundary character distribution (GBCD) and the grain boundary energy distribution (GBED). The twin content from the three-dimensional GBCD calculation compares favourably with the twin content estimated by stereology. Important factors in the analysis are the alignment of the parallel layers, the ratio of the out-of-plane to in-plane spacing of the discrete orientation data and the discretisation of the domain of grain boundary types. The results show that grain boundaries comprised of (111) planes occur most frequently and that these grain boundaries have a relatively low energy. The GBCD and GBED are inversely correlated.

Sharp leading edge (LE) samples of UHTC (20 vol% SiC – HfB 2 ) and SiC were exposed to simulated hypersonic flight conditions using a direct‐connect scramjet rig and their thermal and oxidation responses measured. The measured back‐wall temperatures and scale thicknesses were significantly smaller than might be expected from stagnation temperatures at the LE. Furthermore, the scale that formed around the LE was more uniform than expected from the steep drop in cold wall heat flux with distance from the tip. These results were interpreted and rationalized using physics‐based models. An aerothermal model in combination with an oxidation model accounted for the observed scale thicknesses at the tip and their slight variation with distance. The scale thicknesses were similar to values reported for exposures in furnaces at temperatures calculated for the tip, but less than those reported in arc jet tests. The formation of hafnon (HfSiO 4 ) and the absence of external glassy layer and of silica in the outer portions of the oxide region are unique to scramjet tested samples, presumably due to the high fluid flow (high shear and evaporation) rates.

Optically reconfigurable monodisperse chiral microspheres of self-organized helical superstructures with dynamic chirality were fabricated via a capillary-based microfluidic technique. Light-driven handedness-invertible transformations between different configurations of microspheres were vividly observed and optically tunable RGB photonic cross-communications among the microspheres were demonstrated.

Laser acceleration of ions to ≳MeV energies has been achieved on a variety of Petawatt laser systems, raising the prospect of ion beam applications using compact ultra-intense laser technology. However, translation from proof-of-concept laser experiment into real-world application requires MeV-scale ion energies and an appreciable repetition rate (>Hz). We demonstrate, for the first time, proton acceleration up to 2 MeV energies at a kHz repetition rate using a milli-joule-class short-pulse laser system. In these experiments, 5 mJ of ultrashort-pulse laser energy is delivered at an intensity near $5\times {10}^{18}\,{\rm{W}}\,{\mathrm{cm}}^{-2}$ onto a thin-sheet, liquid-density target. Key to this effort is a flowing liquid ethylene glycol target formed in vacuum with thicknesses down to 400 nm and full recovery at 70 μ s, suggesting its potential use at ≫kHz rate. Novel detectors and experimental methods tailored to high-repetition-rate ion acceleration by lasers were essential to this study and are described. In addition, particle-in-cell simulations of the laser–plasma interaction show good agreement with experimental observations.

Abstract A novel type of all‐natural, biocompatible, and very robust nanoscale free‐standing biohybrids are reported. They are obtained by integrating a silk fibroin matrix with functional inorganic nanoplatelets using a spin‐assisted layer‐by‐layer assembly. The organized assembly of the silk fibroin with clay (montmorillonite) nanosheets results in highly transparent nanoscale films with significantly enhanced mechanical properties, including strength, toughness, and elastic modulus, as compared to those for the pristine silk nanomaterials. Moreover, replacing clay nanoplatelets with a highly reflective Langmuir monolayer of densely packed silver nanoplates causes a similar enhancement of the mechanical properties, but in contrast to the materials above, highly reflective, mirror‐like, nanoscale flexible films are created. This strategy offers a new perspective for the fabrication of robust all‐natural flexible nanocomposites with exceptional mechanical properties important for biomedical applications, such as reinforced tissue engineering. On the other hand, the ability to convert silk‐based nanoscale films into mirror‐like biocompatible flexible films can be intriguing for prospective photonics and optical exploitation of these nanobiohybrids.