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Using molecular dynamics simulations, we study the roles of geometry, boundary conditions, and nanotwinned structure on the mechanical response of Cu ⟨ 110 ⟩ cylindrical nanopillars under tensile loading. Specifically, we investigate the stress–strain responses, deformation mechanisms, and defect evolution in nanopillars of varying height-to-diameter ( H / D ) ratios, capped with rigid layers to resemble hard coatings, and with microstructures consisting of columnar grains with and without nanoscale coherent twin boundaries (CTBs). Our results indicate that the disk-shaped nanopillars (DPs) with low H / D ratios exhibit high yield strength, significant triaxial tension stress, and pronounced plastic deformation. This deformation is accompanied by high defect density near the Cu/rigid layer interfaces, driven by dislocation pile up from multiple activated slip systems. Conversely, the deformed rod-shaped nanopillars of large ratios exhibit localized plastic deformation characterized by necking phenomena. Our results reveal the dominant role of Shockley type dislocations in plastic deformation, particularly in DPs, which display higher dislocation densities due to reduced dislocation annihilation events at free surfaces and pile up at interfaces. Notably, deformed DPs exhibit a unique vacancy-induced nanovoid nucleation mechanism at interfaces, resulting from the coalescence of vacancies generated during shearing of jogs. Additionally, the results demonstrate that dislocations glide between CTBs forming wide stacking faults, or traverse CTBs in a zigzag pattern as perfect dislocations, confirming a recently hypothesized cross-slip-like transmission in a recent experimental observation. Our simulations indicate that nanometer-spaced CTBs in Cu ⟨ 110 ⟩ nanopillars act as weak barriers to dislocation motion during tensile loading, leading to only a marginal enhancement of mechanical properties. [ABSTRACT FROM AUTHOR]

Using molecular dynamics simulations, we investigate the interaction of α-tocopherol (α-toc) with dipalmitoylphosphatidylcholine (DPPC), dimyristoylphosphatidylcholine (DMPC), palmitoyloleoylphosphatidylcholine (POPC), and palmitoyloleoylphosphatidylethanolamine (POPE) lipid bilayers. The goal is to develop a better understanding of the positioning and orientation of α-toc inside the bilayers; properties of significant relevance to α-toc anti-oxidant activity. We investigated bilayer systems with 128 lipids in the presence of either single or 14 α-toc molecules. The single α-toc bilayer systems were investigated via biased MD simulations in which the potential of mean force (PMF) and diffusivity were obtained as functions of the distance between α-toc head group and bilayer center. The higher α-toc concentration systems were investigated with unbiased MD simulations. For all four bilayers at both concentrations, the simulations show that the most probable location of the α-toc hydroxyl group is just below the lipid carbonyl group. Overall, the simulation results are in good agreement with existing experimental data except for the DMPC bilayer system for which some experiments predict α-toc to be located closer to bilayer center. The flip-flop frequency calculated shows that the α-toc flip-flop rate is sensitive to bilayer lipid type. In particular, α-toc has a much lower flip-flop rate in a POPE bilayer compared to the three PC lipid bilayers due to the smaller area per lipid in the POPE bilayer. For DMPC and POPC, the α-toc flip-flop rates are significantly higher at higher α-toc concentration and this appears to be related to the local structural disruption caused by α-toc clusters spanning the bilayer. [ABSTRACT FROM AUTHOR]

Numerical approaches are presented to minimize the statistical errors inherently present due to finite sampling and the presence of thermal fluctuations in the molecular region of a hybrid computational fluid dynamics (CFD) - molecular dynamics (MD) flow solution. Near the fluid-solid interface the hybrid CFD-MD simulation approach provides a more accurate solution, especially in the presence of significant molecular-level phenomena, than the traditional continuum-based simulation techniques. It also involves less computational cost than the pure particle-based MD. Despite these advantages the hybrid CFD-MD methodology has been applied mostly in flow studies at high velocities, mainly because of the higher statistical errors associated with low velocities. As an alternative to the costly increase of the size of the MD region to decrease statistical errors, we investigate a few numerical approaches that reduce sampling noise of the solution at moderate-velocities. These methods are based on sampling of multiple simulation replicas and linear regression of multiple spatial/temporal samples. We discuss the advantages and disadvantages of each technique in the perspective of solution accuracy and computational cost. [ABSTRACT FROM AUTHOR]

This article introduces a novel nanomechanical architecture to form pure metallic nanotubes or nanocoils via spontaneous self-rolling of initial planar free-standing bilayer thin films. Our molecular dynamics simulation results show that if the film is only a few nanometers thick, the spontaneous reorientation of the (001) top layer leads to rolling-up of the initially planar, free-standing, (001)/(111) bilayer into a tubular or coiled structure. The driving force for this process is given by the existence of an initial mismatch strain between the 2-nm-thick layers of different textures. Our detailed analysis of the reorientation process indicates that the bilayer self-rolling is determined by both energetic and kinetic processes characterizing the spontaneous structural reorientation of the top (001) textured layer to the (111) orientation of the substrate layer. Specifically, the analysis of the simulation results indicates that reorientation of the (001) top layer proceeds via a mechanism characterized by nucleation from multiple sites, propagation, and growth of the new (111)-oriented domains embedded in the original (001)-oriented layer. While individually the newly formed (111) domains grow free of defects, a region containing a surface dislocation-like linear defect forms at the boundary where two such domains meet. The equilibrium of the newly formed bilayer structure containing the surface dislocations is attained by multiple localized bendings of the bilayer structure about the direction coinciding with the dislocations line (the $$ [\bar{1}10] $$ direction). The spacing (density) of the nucleation is a function of temperature and influence the radius of curvature of the resulting structure. [ABSTRACT FROM AUTHOR]