Swantje Bargmann

Limpet teeth contain a desirable combination of material properties: they are auxetic (i.e., they have a negative Poisson’s ratio) and have high stiffness and strength. In contrast, synthetic auxetic structures presented in the literature to date are typically accompanied by low stiffness. Hence, limpet teeth microstructures are an attractive candidate for bio-inspired synthetic structures that have both auxeticity and high stiffness. The microstructure consists of iron oxide hydroxide crystal nanorods embedded in an amorphous hydrated silica matrix. Moreover, a portion of the nanorods are arranged into oriented bundles which are surrounded by a chiral arrangement of the remaining nanorods. Many synthetic structures achieve auxeticity by means of a similar chiral arrangement of rods. However, such structures are typically continuous, whereas the limpet teeth structures are not. In this work, a mechanistic description of such microstructural arrangements is provided by means of a computational parametric study of idealized microstructures comprising bimodal particle reinforcements with chiral confinement. The influence of microstructural geometry on the effective mechanical properties is investigated by varying parameters such as particle shape, bundle design, rod orientation, and others. It is shown that chiral rods that impinge on the central bundle are crucial for auxeticity and high material stiffness.

We study 2D RVEs based on microstructures inspired by limpet teeth with the objective of efficiently identifying auxetic designs and building surrogates for effective elastic response. The starting point is an unbalanced database; thus, we run a weighted random forest classifier and a neural network classifier to balance it. The resulting feature importances provide an interpretable ranking of 18 geometric and material variables and guide importance-biased Monte Carlo sampling. Random forest and FCNN classifiers are used to prioritize candidates. Dataset rebalancing is achieved by adding newly FEM-confirmed auxetic samples and applying clustering-guided downsampling to the non-auxetic majority. On this final set, a multi-output FCNN regressor predicts nine targets: inclusion volume fractions and minima/means/maxima of Young’s modulus and Poisson’s ratio. Overall, the framework supports rapid, interpretable screening and property prediction for auxetic composite designs while reducing the need for repeated FEM evaluations.

We present the design, implementation, and validation of a cryo-compatible uniaxial strain cell for strain-dependent investigations in quantum materials. The cell is actuated by three large-format piezoelectric stacks and achieves maximum sample displacements of about 38 μm at room temperature and 1.7 μm at 5 K. The compact geometry provides an extended mounting area that facilitates the investigation of comparatively large samples and allows flexible contact configurations. The actuators, with a cross-sectional area of 7 × 7 mm2, ensure high mechanical stability and enable the application of substantial forces. A key advantage of the design is the high-resolution capacitive displacement sensor, which enables precise in situ strain control with excellent temporal resolution, allowing for dynamic strain tracking beyond the capabilities of conventional setups. To operate the strain cell over a broad temperature range from 4.2 to 300 K, a modular measurement probe was developed, allowing efficient thermal coupling and integration into cryogenic environments. The functionality and precision of the setup were demonstrated by elastoresistance measurements on the iron-based superconductor BaFe2As2 along the [110] direction. The results were compared to those obtained using the conventional approach, in which the sample is directly glued onto a single piezoelectric actuator, showing excellent agreement in amplitude and temperature dependence of the elastoresistance coefficient. Additional tests revealed that the system remains robust under mechanical vibrations introduced by vacuum pumps, enabling stable operation even with liquid nitrogen cooling. Beyond elastoresistivity, the combination of high mechanical stability and time-resolved strain detection makes the presented setup a versatile platform for a broad range of strain-dependent experiments, including future implementations of thermodynamic techniques such as elastocaloric measurements.

Stretchable electronics rely on sophisticated structural designs to allow brittle metallic conductors to adapt to curved or moving substrates. Patterns of soft islands and stable cracks in layered silver-PDMS composites provide exceptional stretchability by means of strain localization as the cracks open and the islands strain. To investigate the reliability and potential failure modes, we study the initiation and propagation of delamination in dependence of structure geometry and quality of the metal-polymer bonding. Our numerical experiments show a well-bonded metal film to be under no risk of delamination. Even weakly bonded metal films sustain moderate strains well above the limits of classical electronic materials before the onset of delamination in the soft islands structures. If delamination occurs, it does so in predictable patterns that retain functionality over a remarkable strain range in the double-digit percent range before failure, thus, providing safety margins in applications.