Alper T. Celebi

A phenomenological continuum model for force-driven nano-channel liquid flows

A phenomenological continuum model is developed using systematic molecular dynamics (MD) simulations of force-driven liquid argon flows confined in gold nano-channels at a fixed thermodynamic state. Well known density layering near the walls leads to the definition of an effective channel height and a density deficit parameter. While the former defines the slip-plane, the latter parameter relates channel averaged density with the desired thermodynamic state value. Definitions of these new parameters require a single MD simulation performed for a specific liquid-solid pair at the desired thermodynamic state and used for calibration of model parameters. Combined with our observations of constant slip-length and kinematic viscosity, the model accurately predicts the velocity distribution and volumetric and mass flow rates for force-driven liquid flows in different height nano-channels. Model is verified for liquid argon flow at distinct thermodynamic states and using various argon-gold interaction strengths. Further verification is performed for water flow in silica and gold nano-channels, exhibiting slip lengths of 1.2 nm and 15.5 nm, respectively. Excellent agreements between the model and the MD simulations are reported for channel heights as small as 3 nm for various liquid-solid pairs.

Electric field controlled transport of water in graphene nano-channels

Motivated by electrowetting-based flow control in nano-systems, water transport in graphene nano-channels is investigated as a function of the applied electric field. Molecular dynamics simulations are performed for deionized water confined in graphene nano-channels subjected to opposing surface charges, creating an electric field across the channel. Water molecules respond to the electric field by reorientation of their dipoles. Oxygen and hydrogen atoms in water face the anode and cathode, respectively, and hydrogen atoms get closer to the cathode compared to the oxygen atoms near the anode. These effects create asymmetric density distributions that increase with the applied electric field. Force-driven water flows under electric fields exhibit asymmetric velocity profiles and unequal slip lengths. Apparent viscosity of water increases and the slip length decreases with increased electric field, reducing the flow rate. Increasing the electric field above a threshold value freezes water at room temperature.

VIBRATION AND ELASTIC BUCKLING ANALYSES OF SINGLE-WALLED CARBON NANOCONES

This paper reports the result on elastic buckling and vibration behaviors of singlewalled carbon nanocones (SWCNCs) having the potential usage in atomic force microscope and scanning tunneling microscope tips. The modeling work employs the molecular mechanics based finite element approach in which Euler-Bernoulli beam element formulations are used with consistent mass matrix. Free-free, free-clamped and clamped-clamped boundary conditions are considered in vibration analysis of SWCNCs; on the other hand, axial compression and bending loading conditions are taken into account in elastic buckling behavior of SWCNCs. The effects of cone height and disclination or apex angles on the buckling force and natural frequencies of SWCNCs are investigated. Vibration analysis results indicate that the natural frequency decreases with increasing cone height in all types of SWCNCs, whereas it increases as the disclination angle increases. Buckling analysis results indicate that as the disclination angle increases, the critical buckling load increases in axial compression loading and decreases in bending loading. In addition, it is observed that bending loading is more critical than axial compression loading for buckling behavior of SWCNCs if the disclination angle increases. Cengiz Baykasoglu, Alper T. Celebi Esra Icer and Ata Mugan

Thermal-stress analysis of ceramic laminate veneer restorations with different incisal preparations using micro-computed tomography-based 3D finite element models

Main objective of this study is to investigate the thermal behavior of ceramic laminate veneer restorations of the maxillary central incisor with different incisal preparations such as butt joint and palatinal chamfer using finite element method. In addition, it is also aimed to understand the effect of different thermal loads which simulates hot and cold liquid imbibing in the mouth. Three-dimensional solid models of the sound tooth and prepared veneer restorations were obtained using micro-computed tomography images. Each ceramic veneer restoration was made up of ceramic, luting resin cement and adhesive layer which were generated based on the scanned images using computer-aided design software. Our solid model also included the remaining dental tissues such as periodontal ligament and surrounding cortical and spongy bones. Time-dependent linear thermal analyses were carried out to compare temperature changes and stress distributions of the sound and restored tooth models. The liquid is firstly in contact with the crown area where the maximum stresses were obtained. For the restorations, stresses on palatinal surfaces were found larger than buccal surfaces. Through interior tissues, the effect of thermal load diminished and smaller stress distributions were obtained near pulp and root-dentin regions. We found that the palatinal chamfer restoration presents comparatively larger stresses than the butt joint preparation. In addition, cold thermal loading showed larger temperature changes and stress distributions than those of hot thermal loading independent from the restoration technique.

NONLINEAR FRACTURE ANALYSIS OF CARBON NANOTUBES WITH STONE-WALES DEFECTS

In this paper, a molecular mechanic based finite element model is employed to investigate the effects of Stone-Wales defects on mechanical properties of armchair and zigzag carbon nanotubes by considering large deformation and nonlinear geometric effects. Nonlinear characteristic of the covalent bonds are obtained by using the modified Morse potential and effects of the large deformation and geometric nonlinearities are considered by updating the atomistic coordinates of the original nanotube structure at each load step. The results show that the fractures of all types of carbon nanotubes are brittle, but armchair nanotubes are stiffer than zigzag nanotubes and these defects significantly affect the mechanical performance of nanotubes. Fracture initiation and crack propagation direction issues are also studied. It is shown that the direction of crack propagation in armchair nanotube is in the maximum shear directions having an angle of ±45° along its circumference. Comparisons are made with the failure stress and strain results reported in literature that show good agreement with our results.