Atomic Scale Sliding and Rolling of Carbon Nanotubes

A carbon nanotube is an ideal object for understanding the atomic scale aspects of interface interaction and friction. Using molecular statics and dynamics methods different types of motion of nanotubes on a graphite surface are investigated. We found that each nanotube has unique equilibrium orientations with sharp potential energy minima. This leads to atomic scale locking of the nanotube. The effective contact area and the total interaction energy scale with the square root of the radius. Sliding and rolling of nanotubes have different characters. The potential energy barriers for sliding nanotubes are higher than that for perfect rolling. When the nanotube is pushed, we observe a combination of atomic scale spinning and sliding motion. The result is rolling with the friction force comparable to sliding.Comment: 4 pages (two column) 6 figures - one ep

More Than Meets the Eye in Bacterial Cellulose: Biosynthesis, Bioprocessing, and Applications in Advanced Fiber Composites

Bacterial cellulose (BC) nanofibers are one of the stiffest organic materials produced by nature. It consists of pure cellulose without the impurities that are commonly found in plant-based cellulose. This review discusses the metabolic pathways of cellulose-producing bacteria and the genetic pathways of Acetobacter xylinum. The fermentative production of BC and the bioprocess parameters for the cultivation of bacteria are also discussed. The influence of the composition of the culture medium, pH, temperature, and oxygen content on the morphology and yield of BC are reviewed. In addition, the progress made to date on the genetic modification of bacteria to increase the yield of BC and the large-scale production of BC using various bioreactors, namely static and agitated cultures, stirred tank, airlift, aerosol, rotary, and membrane reactors, is reviewed. The challenges in commercial scale production of BC are thoroughly discussed and the efficiency of various bioreactors is compared. In terms of the application of BC, particular emphasis is placed on the utilization of BC in advanced fiber composites to manufacture the next generation truly green, sustainable and renewable hierarchical composites

Recombinant biosynthesis of bacterial cellulose in genetically modified Escherichia coli

Bacterial cellulose (BC) exhibits unique properties such as high purity compared to plant-based cellulose; however, commercial production of BC has remained a challenge, primarily due to the strain properties of cellulose-producing bacteria. Herein, we developed a functional and stable BC production system in genetically modified (GM) Escherichia coli by recombinant expression of both the BC synthase operon (bcsABCD) and the upstream operon (cmcax, ccp Ax). BC production was achieved in GM HMS174 (DE3) and in GM C41 (DE3) by optimization of the culture temperature (22 °C, 30 °C, and 37 °C) and IPTG concentration. BC biosynthesis was detected much earlier in GM C41 (DE3) cultures (3 h after IPTG induction) than those of Gluconacetobacter hansenii. GM HMS174 (DE3) produced dense fibres having a length of approximately 1000–3000 μm and a diameter of 10–20 μm, which were remarkably larger than the fibres of BC typically produced by G. hansenii

Quantum heat transfer through an atomic wire

We studied the phononic heat transfer through an atomic dielectric wire with both infinite and finite lengths by using a model Hamiltonian approach. At low temperature under ballistic transport, the thermal conductance contributed by each phonon branch of a uniform and harmonic chain cannot exceed the well-known value which depends linearly on temperature but is material independent. We predict that this ballistic thermal conductance will exhibit stepwise behavior as a function of temperature. By performing numerical calculations on a more realistic system, where a small atomic chain is placed between two reservoirs, we also found resonance modes, which should also lead to the stepwise behavior in the thermal conductance.Comment: 14 pages, 2 separate figure

Modeling, Simulation, and Experiments of Coating Growth on Nanofibers

This work is a comparison of modeling and simulation results with experiments for an integrated experimental/modeling investigation of a procedure to coat nanofibers and core-clad nanostructures with thin film materials using plasma enhanced physical vapor deposition. In the experimental effort, electrospun polymer nanofibers are coated with metallic materials under different operating conditions to observe changes in the coating morphology. The modeling effort focuses on linking simple models at the reactor level, nanofiber level and atomic level to form a comprehensive model. The comprehensive model leads to the definition of an evolution equation for the coating free surface around an isolated nanofiber. This evolution equation was previously derived and solved under conditions of a nearly circular coating, with a concentration field that was only radially dependent and that was independent of the location of the coating free surface. These assumptions permitted the development of analytical expressions for the concentration field. The present work does not impose the above-mentioned conditions and considers numerical simulations of the concentration field that couple with level set simulations of the evolution equation for the coating free surface. Further, the cases of coating an isolated fiber as well as a multiple fiber mat are considered. Simulation results are compared with experimental results as the reactor pressure and power, as well as the nanofiber mat porosity, are varied. (C) 2008 American Institute of Physics

Structure of aluminum atomic chains

First-principles density functional calculations reveal that aluminum can form planar chains in zigzag and ladder structures. The most stable one has equilateral triangular geometry with four nearest neighbors; the other stable zigzag structure has wide bond angle and allows for two nearest neighbors. An intermediary structure has the ladder geometry and is formed by two strands. All these planar geometries are, however, more favored energetically than the linear chain. We found that by going from bulk to a chain the character of bonding changes and acquires directionality. The conductance of zigzag and linear chains is 4e^2/h under ideal ballistic conditions.Comment: modified detailed version, one new structure added, 4 figures, modified figure1, 1 tabl

Multiscale Modeling, Simulations, and Experiments of Coating Growth on Nanofibers. Part Ii. Deposition

This work is Part II of an integrated experimental/modeling investigation of a procedure to coat nanofibers and core-clad nanostructures with thin-film materials using plasma-enhanced physical vapor deposition. In the experimental effort, electrospun polymer nanofibers are coated with aluminum materials under different operating conditions to observe changes in the coating morphology. This procedure begins with the sputtering of the coating material from a target. Part I [J. Appl. Phys. 98, 044303 (2005)] focused on the sputtering aspect and transport of the sputtered material through the reactor. That reactor level model determines the concentration field of the coating material. This field serves as input into the present species transport and deposition model for the region surrounding an individual nanofiber. The interrelationships among processing factors for the transport and deposition are investigated here from a detailed modeling approach that includes the salient physical and chemical phenomena. Solution strategies that couple continuum and atomistic models are used. At the continuum scale, transport dynamics near the nanofiber are described. At the atomic level, molecular dynamics (MD) simulations are used to study the deposition and sputtering mechanisms at the coating surface. Ion kinetic energies and fluxes are passed from the continuum sheath model to the MD simulations. These simulations calculate sputtering and sticking probabilities that in turn are used to calculate parameters for the continuum transport model. The continuum transport model leads to the definition of an evolution equation for the coating-free surface. This equation is solved using boundary perturbation and level set methods to determine the coating morphology as a function of operating conditions. (c) 2005 American Institute of Physics

Multiscale Modeling, Simulations, and Experiments of Coating Growth on Nanofibers. Part I. Sputtering

This paper is Part I of an integrated experimental/modeling investigation of a procedure to coat nanofibers and core-clad nanostructures with thin-film materials using plasma-enhanced physical vapor deposition. In the experimental effort, electrospun polymer nanofibers are coated with aluminum under varying operating conditions to observe changes in the coating morphology. This procedure begins with the sputtering of the coating material from a target. This paper focuses on the sputtering process and transport of the sputtered material through the reactor. The interrelationships among the processing factors for the sputtering and transport are investigated from a detailed modeling approach that describes the salient physical and chemical phenomena. Solution strategies that couple continuum and atomistic models are used. At the continuum scale, the sheath region and the reactor dynamics near the target surface are described. At the atomic level, molecular-dynamics (MD) simulations are used to study the sputtering and deposition mechanisms. Ion kinetic energies and fluxes are passed from the continuum sheath model to the MD simulations. These simulations calculate sputtering and sticking probabilities that in turn are used to calculate parameters for the continuum reactor model. The reactor model determines the concentration field of the coating material. (c) 2005 American Institute of Physics