The nanoscale elastic properties of moist clay minerals are not sufficiently understood. The aim of the present study was to understand the fundamental mechanism for the effects of water and pore size on clay mineral (K+-smectite) elastic properties using the General Utility Lattice Program (GULP) with the minimum energy configurations obtained from molecular dynamics (MD) simulations. The simulation results were compared to an ideal configuration with transversely isotropic symmetry and were found to be reasonably close. The pressures computed from the MD simulations indicated that the changes due to water in comparison to the dry state varied with the water content and pore size. For pore sizes of around 0.8–1.0 nm, the system goes through a process where the normal pressure is decreased and reaches a minimum as the water content is increased. The minimum normal pressure occurs at water contents of 8 wt.% and 15 wt.% for pore sizes of around 0.8 nm and 1 nm, respectively. Further analyses of the interaction energies between water and K+-smectite and between water and water revealed that the minimum normal pressure corresponded to the maximum rate of slope change of the interaction energies (the second derivative of the interaction energies with respect to the water content). The results indicated that in the presence of water the in-plane stiffness parameters were more correlated to the pressure change that resulted from the interplay between the interactions of water with K+-smectite and the interactions of water with water rather than the water content. The in-plane stiffness parameters were much higher than the out-of-plane parameters. Elastic wave velocities for the P and S waves (VP and VS) in the dry K+-smectite with a pore size of ~1 nm were calculated to be 7.5 and 4.1 km/s, respectively. The P and S wave velocity ratio is key in the interpretation of seismic behavior and revealed that VP/VS = 1.64–1.83, which were values in favorable agreement with the experimental data. The results might offer insight into seismic research to predict the mechanical properties of minerals that are difficult to obtain experimentally and can provide complimentary information to interpret seismic surveys that can assist gas and oil exploration.

Mechanical properties of hydrogels are of considerable interest for applications including tissue engineering and drug delivery. However, mechanical characterization of hydrogels is inherently challenging due to their multiphasic construction. Under mechanical loading, internal fluid redistribution affects the gel response, leading to a time- and length-scale-dependent material behavior, known as poroelasticity. Traditional mechanical tests are effective for determining instantaneous flow-independent gel response, and they are limited in characterizing poroelastic behavior as a function of loading time- and length-scales. Here, micro- and nanoindentation experiments are combined to characterize the full range of poroelastic behavior of a hydrogel. A master curve is presented to demonstrate that the relative competition of poroelastic relaxation time with ramp loading time determines gel response across different time- and length-scales. The master curve provides a novel mechanism to establish the instantaneous and equilibrium limits on the elastic modulus for a material, useful for designing hydrogel biomaterials.

Soyarslan et al. [J. Mater. Res. 33(20), 3371 (2018)] proposed a beam-finite element model for the computation of effective elastic properties of nanoporous materials, where the ligament diameter along the skeleton is determined with the biggest sphere algorithm. Although this algorithm is often used in the literature, it is known that it systematically overestimates the diameter in network structures. Thus, the need for further stiffening of the junction zones as proposed by the authors is in contradiction to the literature. Furthermore, the factor 40 appears to be one order of magnitude too high. We show that the 3D microstructures generated from random Gaussian fields contain features that are violating the assumption of circular cross-sections and, therefore, cannot be captured by the biggest sphere algorithm. Consequently, the authors required an unphysically high value of 40 to compensate this hidden effect.

Hydrogels have gained recent attention for biomedical applications because of their large water content, which imparts biocompatibility. However, their mechanical properties can be limiting. There has been significant recent interest in the strength and fracture toughness of hydrogel materials in addition to their stiffness and time-dependent behavior. Hydrogels can fail in a brittle manner, although they are extremely compliant. In this work, the failure and fracture of hydrogels are examined using a compression test of spherical hydrogel particles. Spheres of commercially available polyacrylamide–potassium polyacrylate were hydrated and tested to failure in compression as a function of loading rate. The spheres exhibited little relaxation when compressed to small fixed displacements. The distributions of strength values obtained were examined in a particle fracture framework previously used for brittle ceramics. There was loading rate dependence apparent in the measured peak force and calculated peak strength values, but the data fell on a single empirical distribution function of strength for the hydrogels regardless of loading rate. Strength values for these hydrogels were mostly in the range of 0.05–0.3 MPa, illustrating the challenges using hydrogels for mechanically demanding applications such as tissue engineering.

The measurement of thin film mechanical properties free from substrate influence remains one of the outstanding challenges in nanomechanics. Here, a technique based on indentation of a supported film with a flat punch whose diameter is many times the initial film thickness is introduced. This geometry generates a state of confined uniaxial strain for material beneath the punch, allowing direct access to intrinsic stress versus strain response. For simple elastic–plastic materials, this enables material parameters such as elastic modulus, bulk modulus, Poisson's ratio, and yield stress to be simultaneously determined from a single loading curve. The phenomenon of confined plastic yield has not been previously observed in thin films or homogeneous materials, which we demonstrate here for 170 -470 nm thick polystyrene (PS), polymethyl-methacrylate (PMMA) and amorphous Selenium films on silicon. As well as performing full elastic -plastic parameter extraction for these materials at room temperature, we used the technique to study the variation of yield stress in PS to temperatures above the nominal glass transition of 100 °C.

Atomically thin 2D materials exhibit strong intralayer covalent bonding and weak interlayer van der Waals interactions, offering unique high in-plane strength and out-of-plane flexibility. While atom-thick nature of 2D materials may cause uncontrolled intrinsic/extrinsic deformation in multiple length scales, it also provides new opportunities for exploring coupling between heterogeneous deformations and emerging functionalities in controllable and scalable ways for electronic, optical, and optoelectronic applications. In this review, we discuss (i) the mechanical characteristics of 2D materials, (ii) uncontrolled inherent deformation and extrinsic heterogeneity present in 2D materials, (iii) experimental strategies for controlled heterogeneous deformation of 2D materials, (iv) 3D structure-induced novel functionalities via crumple/wrinkle structure or kirigami structures, and (v) heterogeneous strain-induced emerging functionalities in exciton and phase engineering. Overall, heterogeneous deformation offers unique advantages for 2D materials research by enabling spatial tunability of 2D materials' interactions with photons, electrons, and molecules in a programmable and controlled manner.

This paper discusses new components and approaches to make stretchable optical fiber sensors better meet the power and washability requirements of wearables. First, an all-polymer quick connector allows the light source and photosensor to be quickly detached for washing. Second, the paper investigates the possibility of driving the sensors using ambient light instead of an onboard light source. While optical strain sensors and touch sensors have advantages over electronic ones in wet environments, and the intrinsic stretchability of the fibers is useful for soft robotics and highly conformal wearables, the typical light-emitting diode (LED) light source consumes more power than an electronic resistive or capacitive strain sensor. In this work, ambient light of uniform but unknown intensity is demonstrated to drive an elastomeric optical touch sensor in a differential configuration.

Iron aluminide intermetallic alloys are of great importance in many industries due to their excellent oxidation resistance, low cost, low density, resistance to corrosion and good ductility at room temperature. However, these alloys suffer limited room temperature ductility above 873 K. In this paper, a molecular dynamics-based LAMMPS-EAM was used to model Fe50-XMXAl doped systems with either Ag, Pt or Pd. The lattice side preferences of the dopant were deduced from their energy landscape, and Fe sub-lattices showed promising properties. It was found that the addition of Ag, Pt and Pd enhances the stability of Fe50-XMXAl composition. More importantly, Ag and Pd doped systems gave comparable transition temperatures to experimental findings of 1273 K and 1073 K, respectively. Their thermodynamic and the mechanical stability trends showed promising properties for industrial applications, displaying stability at a high temperature below 1300 K.

New forms of carbon-based materials have received great attention, and the developed materials have found many applications in nanotechnology. Interesting novel carbon structures include the carbon peapods, which are comprised of fullerenes encapsulated within carbon nanotubes. Peapod-like nanostructures have been successfully synthesized, and have been used in optical modulation devices, transistors, solar cells, and in other devices. However, the mechanical properties of these structures are not completely elucidated. In this work, we investigated, using fully atomistic molecular dynamics simulations, the deformation of carbon peapods under high-strain rate conditions, which are achieved by shooting the peapods at ultrasonic velocities against a rigid substrate. Our results show that carbon peapods experience large deformation at impact, and undergo multiple fracture pathways, depending primarily on the relative orientation between the peapod and the substrate, and the impact velocity. Observed outcomes include fullerene ejection, carbon nanotube fracture, fullerene, and nanotube coalescence, as well as the formation of amorphous carbon structures.

A simple model system towards an impedance-probing strain sensor based on conducting tough hydrogels is demonstrated. A poly(acrylamide) hydrogel, cross-linked with N,N’-methylene-bis(acrylamide) was contacted with carbon fibres for electrical impedance analysis. The conductivity of the salt-containing hydrogel was determined to be 114 ± 10 mS/cm. Upon stretching the hydrogel samples to their fourfold initial length, the impedance response increased according to a power law. This was used to establish a sensing equation for the relation between the resistive component of the impedance signal and the applied mechanical strain under tension. This work contributes to the development of highly stretchable and soft strain sensors for applications in soft robotics.

Dielectric elastomer actuators (DEAs), which are inherently complaint capacitors, are emerging as pseudo-muscular actuators with a wide range of applications. In order to achieve high stretchability for large DEA actuation, carbon nanotube (CNT) and other 1D materials-based electrodes are used to maintain conductance at large strains. These electrodes are typically fabricated using spray coating or filter transfer method and resemble a perforated electrode under high magnification. Hence, there can be a loss of field and stray capacitance when multiple layers of carbon nanotubes (CNTs)-based electrodes are used. This study investigates the effect of microscopic perforations on the nature of electric fields and on the capacitance of multi-layered CNT-based DEA structures with various dimensions and geometric properties of the electrodes. It has been found that the capacitance decreases with increase in the perforations however its effect is limited for a reasonable coverage. The change in normalized is found to be negligible (∼5%) for an electrode coverage area of over 90%, however, the maximum output work reduces by 18.2%. This analysis is important to develop robust and reliable CNT-based DEA structures, without using excessive CNTs which can lead to increased mechanical stiffness of the electrodes.

Ionic hydrogels are an abundant class of materials with applications ranging from drug delivery devices to high performance concrete to baby diapers. A more thorough understanding of interactions between polyelectrolyte networks and ionic solutes is critical as these materials are further tailored for performance applications in highly targeted ionic environments. In this work, we seek to develop structure-property relationships between polyelectrolyte gels and environments containing high concentrations of multivalent ions. Specifically, this work seeks to elucidate the causes behind differences in hydrogel response to divalent ions of main group metals versus transition metals. PANa-co-PAM hydrogels containing low and high fractions of ionic groups are investigated in solutions of DI water, NaCl, CaCl2, and CuSO4 at concentrations ranging from 5 to 100 mM in order to understand 1) the transient or permanent nature of crosslinks produced in these networks by divalent counter-ions, 2) the role of polymer ionic content in these interactions, and 3) how these interactions scale with salt concentration. Gravimetric swelling and mechanical compression testing are employed to characterize water and salt-swollen hydrogels in order to develop guiding principles to control and manipulate material properties through polymer-counter-ion interactions. The work presented here confirms the formation of permanent crosslinks by transition metal ions, offers explanation for the behavioral discrepancy observed between ionic hydrogels and main group versus transition metal ions, and illustrates how such hydrogel properties scale with counter-ion concentration.

‘Negative mechanical materials / metamaterials’ refer to materials and/or engineered systems that exhibit anomalous macroscopic thermo-mechanical properties that emerge due to the structure of their subunits, rather than the specific chemical composition. As a result of their design/construction, they may exhibit anomalous macroscopic properties such as zero or negative Poisson’s ratios (auxetic), moduli and/or indices. Such zero/negative properties are not normally manifested by their conventional counterparts and may thus potentially be used in applications where typical materials cannot. This work will look into some of the more recent developments made in this field, focusing on how existing materials (e.g. crystals) are providing the blueprint for the design and manufacture of novel ’negative materials’. In particular, this work looks at how wine-rack like crystalline materials which are typically studied for their negative thermal expansion and/or negative compressibility properties can be modified so as to generate negative Poisson’s ratio through a novel mechanism involving forcing elements to move out-of-plane to generate giant out-of-plane auxeticity.

The dynamic mechanical force transmitted through microenvironments during tissue formation and regeneration continuously impacts the mechanics of cells and thereby regulates gene and protein expression. The mechanical properties are altered during the process of stem cells differentiating into different lineages. At different stages of differentiation, stem cells display different mechanical properties in response to surrounding microenvironments, which depend on the subcellular structures, especially the cytoskeleton and nucleus. The mechanical properties of the cell nucleus affect protein folding and transport as well as the condensation of chromatin, through which the cell fate is regulated. These findings raise the question as to how cell mechanics change during differentiation. In this study, the mechanical properties of human bone marrow mesenchymal stem cells (hBMSCs) were determined during adipogenic and osteogenic differentiation by atomic force microscopy (AFM). The cytoskeletal structure and the modification of histone were investigated using laser confocal microscope and flow cytometry. The mechanical properties of cell nuclei at different stages of cell differentiation were compared. The stiffness of nuclei increased with time as osteogenesis was induced in hBMSCs. The H3K27me3 level increased during osteogenesis and adipogenesis according to flow cytometry analysis. Our results show conclusively that AFM is a facile and effective method to monitor stem cell differentiation. The measurement of cell mechanical properties by AFM improves our understanding on the connection between mechanics and stem cell fate.

The improvement of hydrogen embrittlement (HE) is a key problem for transition-metal silicides. Although C40 TMSi2 disilicides are attracted candidates for ultrahigh-temperature applications, the HE mechanism of TMSi2 is unclear. Importantly, the role of hydrogen on the structural configuration, elastic modulus, and hardness of TMSi2 is entirely unknown. To reveal the HE, we study the role of hydrogen in TMSi2 (TM = Nb, Mo, and W) based on the first-principles calculations. Four H-doped sites are considered in detail. The calculated results show that hydrogen is favorable to occupy the octahedral interstitial site because the C40 TMSi2 layered structure is favorable to absorb hydrogen. H-doping results in lattice expansion of c-axis compared with the a-axis and b-axis. H-doping obviously reduces the elastic modulus and hardness of TMSi2 due to the interaction between hydrogen and TMSi2. In addition, H-doping changes the electronic properties of MoSi2 and WSi2.

A new protocol has been devised for determining elastic properties of natural biocomposites in the form of bivalve shells under wet and dry conditions. Four-point bending on shell slices of Mytilus edulis, Ensis siliqua, and Pecten maximus give generally lower and more reliable values of Young’s modulus, E, than those in the literature from three-point bending, due to the more even distribution of strain. Finite element analysis of the prismatic microstructure of Pinna nobilis, obtained by X-ray tomography, shows that values of E ≈ 20 GPa can be understood in terms of the real microstructure containing a small proportion of organic matrix phase with E ≈ 1 GPa and a dominant proportion of calcite with E ≈ 90 GPa. Higher values of E obtained by nanoindentation give results which are biased toward the properties of the carbonate phase rather than of the biocomposite as a whole.

Structure–property relationships are the foundation of materials science and are essential for predicting material response to driving forces, managing in-service material degradation, and engineering materials for optimal performance. Elastic, thermal, and acoustic properties provide a convenient gateway to directly or indirectly probe materials structure across multiple length scales. This article will review how using the laser-induced transient grating spectroscopy (TGS) technique, which uses a transient diffraction grating to generate surface acoustic waves and temperature gratings on a material surface, nondestructively reveals the material’s elasticity, thermal diffusivity, and energy dissipation on the sub-microsecond time scale, within a tunable subsurface depth. This technique has already been applied to many challenging problems in materials characterization, from analysis of radiation damage, to colloidal crystals, to phonon-mediated thermal transport in nanostructured systems, to crystal orientation and lattice parameter determination. Examples of these applications, as well as inferring aspects of microstructural evolution, illustrate the wide potential reach of TGS to solve old materials challenges and to uncover new science. We conclude by looking ahead at the tremendous potential of TGS for materials discovery and optimization when applied in situ to dynamically evolving systems.

The influence of repeated thermal cycling on mechanical properties, structural relaxation, and evolution of the potential energy in binary glasses is investigated using molecular dynamics simulations. The authors consider a binary mixture annealed with different cooling rates and then exposed to one thousand thermal cycles at constant pressure. It is found that during the first few hundred transient cycles, the potential energy minima after each cycle gradually decrease and the structural relaxation proceeds through collective, irreversible displacements of atoms. With increasing cycle number, the amplitudes of the volume and potential energy oscillations are significantly reduced, and the potential energy minima saturate to a constant value that depends on the thermal cycling amplitude and the initial cooling rate. In the steady state, the glasses thermally expand and contract but most of the atoms return to their cages after each cycle, similar to limit cycles found in periodically driven amorphous materials. The results of tensile tests demonstrate that the elastic modulus and the yielding peak, evaluated after the thermal treatment, acquire maximum values at a particular thermal cycling amplitude, which coincides with the minimum of the potential energy.

Searching for materials with improved or perhaps completely novel properties involves an iterative process intended to successively narrow the gap between some initial starting point and the desired design target. This can be viewed as an optimization problem in a high-dimensional search space, often with many dozens of material parameters that need to be tuned. To tackle this, the evolutionary process in biology has been a source of inspiration in developing effective search algorithms. However, reaping the full benefits of bioinspired searches for materials design requires some thought. Here, we go beyond traditional black box algorithms and take a broader view of computational evolution strategies. We discuss recent strategies that exploit knowledge about the material configuration statistics and we highlight the advantages when time-varying environments are considered. Throughout, we emphasize that the search strategies themselves can be viewed as a nonequilibrium dynamical process in design space.