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Mechanical properties of porous materials depend on their micro-architectural characteristics. Freeze casting is an effective method to fabricate micro-architectured porous scaffolds. Three key characteristics generated during freeze casting are wall thickness, number of domains at the cross section, and transverse bridges connecting adjacent walls. To specifically study the effect of these structural characteristics on the mechanics and anisotropic compressive properties of scaffolds, we utilize additive manufacturing, i.e., 3D printing, to fabricate strictly designed cubic scaffolds with varying one characteristic at a time. We then compare strength, toughness, resilience, stiffness, and strain to failure in three orthogonal directions of the scaffolds, including longitudinal and transverse directions. To compare these multidimensional mechanics in a single diagram, we use a previously developed radar chart method to evaluate different scaffolds and unravel the effect of the structural characteristics. We find that the multidimensional mechanics can be effectively tuned by the micro-architectural characteristics. Notably, the buckling resistance of the scaffolds depends on all three structural characteristics. Our results show that an increased number of domains leads to enhanced toughness in all three directions. Increasing wall thickness leads to enhanced mechanical properties but comes at the price of losing small-sized pores, which is not favored for certain applications. In addition, adding transverse bridges increases not only the transverse strength of the scaffolds but also the longitudinal strength as they also enhance the buckling resistance. Our study provides important insights into the structure-property relationships of 3D-printed micro-architectured porous scaffolds. [ABSTRACT FROM AUTHOR]

Mechanical properties of porous materials depend on their micro-architectural characteristics. Freeze casting is an effective method to fabricate micro-architectured porous scaffolds. Three key characteristics generated during freeze casting are wall thickness, number of domains at the cross section, and transverse bridges connecting adjacent walls. To specifically study the effect of these structural characteristics on the mechanics and anisotropic compressive properties of scaffolds, we utilize additive manufacturing, i.e., 3D printing, to fabricate strictly designed cubic scaffolds with varying one characteristic at a time. We then compare strength, toughness, resilience, stiffness, and strain to failure in three orthogonal directions of the scaffolds, including longitudinal and transverse directions. To compare these multidimensional mechanics in a single diagram, we use a previously developed radar chart method to evaluate different scaffolds and unravel the effect of the structural characteristics. We find that the multidimensional mechanics can be effectively tuned by the micro-architectural characteristics. Notably, the buckling resistance of the scaffolds depends on all three structural characteristics. Our results show that an increased number of domains leads to enhanced toughness in all three directions. Increasing wall thickness leads to enhanced mechanical properties but comes at the price of losing small-sized pores, which is not favored for certain applications. In addition, adding transverse bridges increases not only the transverse strength of the scaffolds but also the longitudinal strength as they also enhance the buckling resistance. Our study provides important insights into the structure-property relationships of 3D-printed micro-architectured porous scaffolds. [ABSTRACT FROM AUTHOR]

It is crucial to investigate the dynamic mechanical behavior of materials at the nanoscale to create nanostructured protective systems that have superior ballistic impact resistance. Inspired from recent experimental advances that enable ballistic materials testing at small scales, here we report a comparative analysis of the dynamic behavior of nanoscale thin films made from multilayer graphene (MLG), polymer, gold, and aluminum under high-speed projectile impact. We employ atomistic and coarse-grained (CG) molecular dynamics (MD) simulations to measure the ballistic limit velocity (V50) and penetration energy (Ep) of these nanoscale films and investigate their distinctive failure mechanisms over a wide range of impact velocities (Vi). For the local penetration failure mechanism observed in polymer and metal films, we find that the intrinsic mechanical properties influence Ep at low Vi, while material density tends to govern Ep at high Vi. MLG films uniquely show a large impact propagation zone (IPZ), which transfers the highly localized impact energy into elastic deformation energy in a much larger area through cone wave propagation. We present theoretical analyses that corroborate that the size of IPZ should depend not only on material properties but also on a geometrical factor, specifically, the ratio between the projectile radius and film thickness. This study clearly illustrates how material properties and geometrical factors relate to the ballistic penetration energy, thereby allowing a quantitative comparison of the nanoscale ballistic response of different materials. [ABSTRACT FROM AUTHOR]

Graphene oxide (GO) shows promise as a nanocomposite building block due to its exceptional mechanical properties. While atomistic simulations have become central to investigating its mechanical properties, the method remains prohibitively expensive for large deformations and mesoscale failure mechanisms. To overcome this, we establish a coarse-grained (CG) model that captures key mechanical and interfacial properties, and the non-homogeneous effect of oxidation in GO sheets. The CG model consists of three types of CG beads, representing groups of pristine sp² carbon atoms, and hydroxyl and epoxide functionalized regions. The CG force field is parameterized based on density functional-based tight binding simulations on three extreme cases. It accurately quantifies deterioration of tensile modulus and strength at the expense of improving interlayer adhesion with increasing oxidation of varying chemical compositions. We demonstrate the applicability of the model to study mesoscale phenomena by reproducing different force vs. indentation curves in silico, corroborating recent experimental observations on how chemistry near contact point influences properties. Finally, we apply the model to measure the fracture toughness of pristine graphene and GO. The critical stress intensity factor (Kc) of graphene is found to be the highest, and epoxide-rich GO also possesses higher Kc compared to hydroxyl-rich GO. [ABSTRACT FROM AUTHOR]

Predicting the macroscopic fracture energy of highly cross-linked glassy polymers from atomistic simulations is challenging due to the size of the process zone being large in these systems. Here, we present a scale-bridging approach that links atomistic molecular dynamics simulations to macroscopic fracture properties on the basis of a continuum fracture mechanics model for two different epoxy materials. Our approach reveals that the fracture energy of epoxy resins strongly depends on the functionality of epoxy resin and the component ratio between the curing agent (amine) and epoxide. The most intriguing part of our study is that we demonstrate that the fracture energy exhibits a maximum value within the range of conversion degrees considered (from 65% to 95%), which can be attributed to the combined effects of structural rigidity and postyield deformability. Our study provides physical insight into the molecular mechanisms that govern the fracture characteristics of epoxy resins and demonstrates the success of utilizing atomistic molecular simulations toward predicting macroscopic material properties. [ABSTRACT FROM AUTHOR]

Natural protective materials offer unparalleled solutions for impact-resistant material designs that are simultaneously lightweight, strong, and tough. Particularly, the Bouligand structure found in the dactyl club of mantis shrimp and the staggered structure in nacre achieve excellent mechanical strength, toughness, and impact resistance. Previous studies have shown that hybrid designs by combining different bioinspired microstructures can lead to enhanced mechanical strength and energy dissipation. Nevertheless, it remains unknown whether combining Bouligand and staggered structures in nanofibrillar cellulose (NFC) films, forming a discontinuous fibrous Bouligand (DFB) architecture, can achieve enhanced impact resistance against projectile penetration. Additionally, the failure mechanisms under such dynamic loading conditions have been minimally understood. In our study, we systematically investigate the dynamic failure mechanisms and quantify the impact resistance of NFC thin films with DFB architecture by leveraging previously developed coarse-grained models and ballistic impact molecular dynamics simulations. We find that when nanofibrils achieve a critical length and form DFB architecture, the impact resistance of NFC films outperforms the counterpart films with continuous fibrils by comparing their specific ballistic limit velocities and penetration energies. We also find that the underlying mechanisms contributing to this improvement include enhanced fibril sliding, intralayer and interlayer crack bridging, and crack twisting in the thickness direction enabled by the DFB architecture. Our results show that by combining Bouligand and staggered structures in NFC films, their potential for protective applications can be further improved. Our findings can provide practical guidelines for the design of protective films made of nanofibrils. [ABSTRACT FROM AUTHOR]

Lipases secreted by psychrotrophic bacteria are known to be heat resistant and can remain active even after the thermal processing of milk products. Such enzymes are able to destabilize the quality of milk products by causing a rancid flavor. Rapid detection of a small amount of heat-resistant lipase-producing psychrotrophic bacteria is crucial for reducing their adverse effects on milk quality. In this study, we established and optimized a novel loop-mediated isothermal amplification (LAMP) assay for the detection of Pseudomonas fluorescens in raw cow milk, as the most frequently reported heat-resistant lipase-producing bacterial species. Pseudomonas fluorescens-specific DNA primers for LAMP were designed based on the lipase gene sequence. Reaction conditions of the LAMP assay were tested and optimized. The detection limit of the optimized LAMP assay was found to be lower than that of a conventional PCR-based method. In pure culture, the detection limit of the LAMP assay was found to be 4.8 × 101 cfu/reaction of the template DNA, whereas the detection limit of the PCR method was 4.8 × 102 cfu/reaction. Evaluation of the performance of the method in P. fluorescens-contaminated pasteurized cow milk revealed a detection limit of 7.4 × 101 cfu/reaction, which was 102 lower than that of the PCR-based method. If further developed, the LAMP assay could offer a favorable on-farm alternative to existing technologies for the detection of psychotrophic bacterial contamination of milk, enabling improved quality control of milk and milk products. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations on nanoindentation of circular monolayer molybdenum disulfide (MoS2) film are carried out to elucidate the deformation and failure mechanisms. Typical force–deflection curves are obtained, and in-plane stiffness of MoS2 is extracted according to a continuum mechanics model. The measured in-plane stiffness of monolayer MoS2 is about 182 ± 14 N m−1, corresponding to an effective Young’s modulus of 280 ± 21 GPa. More interestingly, at a critical indentation depth, the loading force decreases sharply and then increases again. The loading–unloading–reloading processes at different initial unloading deflections are also conducted to explain the phenomenon. It is found that prior to the critical depth, the monolayer MoS2 film can return to the original state after completely unloading, while there is hysteresis when unloading after the critical depth and residual deformation exists after indenter fully retracted, indicating plasticity. This residual deformation is found to be caused by the changed lattice structure of the MoS2, i.e. a phase transformation. The critical pressure to induce the phase transformation is then calculated to be 36 ± 2 GPa, consistent with other studies. Finally, the influences of temperature, the diameter and indentation rate of MoS2 monolayer on the mechanical properties are also investigated. [ABSTRACT FROM AUTHOR]