Xiaowei Zeng

Programmable hydrogenation of graphene for novel nanocages

Folded graphene has exhibited novel electrical and mechanical properties unmatched by pristine graphene, which implies that morphology of graphene adds the dimensionality of design space to tailor its properties. However, how to overcome the energy barrier of the folding process to fold the graphene with the specific morphology remains unexplored. Here we propose a programmable chemical functionalization by doping a pristine graphene sheet in a certain pattern with hydrogen atoms to precisely control its folding morphology. Molecular dynamics simulation has been performed to create a cross-shaped cubic graphene nanocage encapsulating a biomolecule by warping the top graphene layer downward and the bottom graphene layer upward to mimic the drug delivery vehicle. Such a paradigm, programmable enabled graphene nanocage, opens up a new avenue to control the 3D architecture of folded graphene and therefore provides a feasible way to exploit and fabricate the graphene-based unconventional nanomaterials and nanodevices for drug delivery.

The role of cohesive zone properties on intergranular to transgranular fracture transition in polycrystalline solids

A cohesive zone model is employed to simulate the fracture evolution and crack propagation in polycrystalline solids. Numerical simulations of fracture growth with various cohesive zone properties are presented and the simulation results capture the fracture transition from intergranular to transgranular mode. Three different random Voronoi grain cell tessellations are presented to study the grain size effects. The simulation results show that the intergranular to transgranular fracture transition in the polycrystalline solid is sensitive to key cohesive law parameters such as fracture energy and cohesive strength along grain boundaries and in grain cells. This study also provides evidence that tensile strength of polycrystalline solid increases as grain cell size decreases.

Contribution of extrafibrillar matrix to the mechanical behavior of bone using a novel cohesive finite element model

The mechanical behavior of bone is determined at all hierarchical levels, including lamellae (the basic building block of bone) that are comprised of mineralized collagen fibrils and extrafibrillar matrix. The mechanical behavior of mineralized collagen fibrils has been investigated intensively using both experimental and computational approaches. Yet, the contribution of the extrafibrillar matrix to bone mechanical properties is poorly documented. In this study, we intended to address this issue using a novel cohesive finite element (FE) model, in conjunction with the experimental observations reported in the literature. In the FE model, the extrafibrillar matrix was considered as a nanocomposite of hydroxyapatite (HA) crystals bounded through a thin organic interface modeled as a cohesive interfacial zone. The parameters required by the cohesive FE model were defined based on the experimental data reported in the literature. This hybrid nanocomposite model was tested in two loading modes (i.e. tension and compression) and under two hydration conditions (i.e. wet and dry). The simulation results indicated that (1) the failure modes of the extrafibrillar matrix predicted using the cohesive FE model were closely coincided with those experimentally observed in tension and compression tests; (2) the pre-yield deformation (i.e. internal strain) of HA crystals with respect to the applied strain was consistent with that obtained from the synchrotron X-ray scattering measurements irrespective of the loading modes and hydration status; and (3) the mechanical behavior of the extrafibrillar matrix was dictated by the properties of the organic interface between the HA crystals. Taken together, we postulate that the extrafibrillar matrix plays a major role in the pre-yield deformation and the failure mode of bone, thus, giving rise to important insights in the ultrastructural origins of bone fragility.

Biomechanical Cell Model by Liquid-Crystal Elastomers

AbstractIn this work, a soft matter cell model is proposed to simulate the cellular cytoskeleton network and motions of intermediate filaments during cell contact and adhesion, in an attempt to explain mechanical information exchange between cells and their extracellular environment. In particular, the cell is modeled as liquid-crystal elastomers. A microscale adhesive model has been introduced to describe the interaction between receptors and ligands. A Lagrange-type mesh-free Galerkin formulation and related computational algorithms for the proposed cell and adhesive contact model have been developed and implemented. A comparison study with experimental data has been conducted to validate the parameters of the cell model. By using the soft matter cell model, the soft adhesive contact process between cells and extracellular substrates with different stiffness has been simulated. The simulation shows that cell motion includes gliding as well as rolling forward along the substrate during the spreading process.

Numerical simulations of dynamic fracture growth based on a cohesive zone model with microcracks

AbstractA cohesive finite element model is employed to study the dynamic crack growth mechanisms in different materials. Dynamic crack propagation is analyzed numerically for a 2D square specimen with prescribed initial microcracks subjected to tensile loading conditions. In the cohesive zone model, the initial microcracks or defects are set up as traction-free interfacial surfaces in the specimen plane. The phenomena of microcrack initiation, nucleation, growth, coalescence, and propagation are captured from the simulation. The numerical simulation results have shown that the initially prescribed mircocrack or defect direction will result in a different macrocrack propagation path and crack branching path.

An improved interfacial bonding model for material interface modeling

An improved interfacial bonding model was proposed from potential function point of view to investigate interfacial interactions in polycrystalline materials. It characterizes both attractive and repulsive interfacial interactions and can be applied to model different material interfaces. The path dependence of work-of-separation study indicates that the transformation of separation work is smooth in normal and tangential direction and the proposed model guarantees the consistency of the cohesive constitutive model. The improved interfacial bonding model was verified through a simple compression test in a standard hexagonal structure. The error between analytical solutions and numerical results from the proposed model is reasonable in linear elastic region. Ultimately, we investigated the mechanical behavior of extrafibrillar matrix in bone and the simulation results agreed well with experimental observations of bone fracture.

Thermo-electromechanical response of a ferroelectric perovskite from molecular dynamics simulations

Based on a shell model potential obtained from first principles calculations, we performed molecular dynamics simulations to investigate the electromechanical response of a ferroelectric perovskite under finite temperature and electric field. We characterize the switching paths by which a homogeneous polarization reorientation process would take place in the prototypical ferroelectric PbTiO(3). We observe the hysteresis loop and butterfly electric-strain curve and obtain finite temperature piezoelectric coefficients in good agreement with experiments

Soft Matter Modeling of Biological Cells

In this work, we review some of our recent work on developments of soft matter models for cells to study the focal adhesion of endothelial cells as well as stem cells, in an attempt to explain mechanical information exchange between the cells and their extracellular environment. Particularly, we model the macroscale endothelial cell as a hyperelastic medium, and the stem cell as a liquid crystal elastomer. A nanoscale adhesive model is introduced to describe the interaction between receptors and ligands. We have developed and implemented a Lagrange type meshfree Galerkin formulation and related computational algorithms for the proposed cell and adhesive contact model. A comparison study with experimental data has been conducted to validate the parameters of the cell model. By using the soft matter cell model, we have simulated the soft adhesive contact process between cells and extracellular substrate. The soft matter cell model presented in this work is a primitive one, but it may have provided a useful approach for more realistic and more accurate modeling of cells, especially stem cells.

Multiscale Biomechanical Modeling of Stem Cell-Extracellular Matrix Interactions

A multiscale cell model has been developed to study the mechanotransduction of pluripotent stem cells in an attempt to explain the mechanical information exchange between the cells and their extracellular environment leading to a biologic response. In the model, the macroscale cell is modeled as liquid crystal with a hyperelastic nucleus. A nanoscale adhesive model was introduced to describe the interaction between receptors and ligands. We have developed and implemented a Lagrange type meshfree Galerkin formulation and related computational algorithms for the described cell and adhesive contact model. A comparison study with experimental data was conducted to validate the parameters of the cellular computational model. By using a soft matter physics modeling approach, we have simulated the adhesive contact process between cells and different extracellular matrix substrates. The simulation shows that the cell can sense substrate elasticity by responding via cell spreading, altered cell contact configurations, and altered molecular conformations.

Tough and strong bioinspired nanocomposites with interfacial cross-links

Strength and toughness are two mechanical properties that are generally mutually exclusive but highly sought-after in the design of advanced composite materials. There has only been limited progress in achieving both high strength and toughness in composite materials. However, the fundamental underlying mechanics remain largely unexplored, especially at the nanoscale. Inspired by the lamellar structure of nacre, here a layered graphene and polyethylene nanocomposite with tunable interfacial cross-links is studied via coarse-grained molecular dynamics simulations in order to achieve both high strength and toughness. Our simulations indicate that, as the cross-link density increases from 0 to about 25%, strength and toughness of the nanocomposite experience a surprising 91% and 76% increase respectively. This strengthening mechanism can be well explained by the extent of increased nonbonded contacts between polymer chains (van der Waals interaction) during the stretch and exceptional stretchability of each polymer chain (dihedral interaction) due to interfacial cross-links by comparing nanocomposites with and without cross-links. As the strength of cross-links increases, both mechanical strength and toughness of graphene-based polymer nanocomposite increase as expected. This may be attributed to the intra-chain bond and angle interactions among polymer chains, which may be negligible for nanocomposites with weak cross-links but play a key role in enhancing both strength and toughness for nanocomposites with strong cross-links. Overall, our findings unveil the fundamental mechanism at the nanoscale for tough-and-strong polymer composites via interfacial cross-linking as well as offer a novel way to design bioinspired nanocomposites with targeted properties via tunable interfacial cross-linking.