Mohammed S. Mayeed

Conformation of perfluoropolyethers in the ultrathin liquid films on solid flat surfaces - effect of polar interaction [hard disks]

Applying the reptation algorithm to a simplified perfluoropolyether (PFPE) Z and Zdol (qualitative) bead-spring off-lattice polymer model an NVT Monte Carlo simulation has been performed to simulate the bulk and the ultrathin film polymer conformation. Bead-bead Lennard-Jones (LJ) potential is used in the bulk and with this substrate-bead LJ potential is added in the ultrathin film condition. In addition, dipole-dipole polar interaction potential is adopted at the two ends of Zdol. Simulation is performed with similar molecular weights of Z and Zdol under the assumption of room temperature and pressure condition. Maximum volume fraction of approximately 80-82%, typical of a melt at room condition, is considered for both the bulk and the ultrathin film condition. The calculated average radius of gyration (Rg) is nearly equal to the experimental value both being in bulk situation. The average components of Rg in the x, y and z directions Rgx, Rgy and Rgz are almost equal to each other in the bulk denoting the average shape of a polymer as a sphere. In the ultrathin films of several nano-meters of film thickness a substrate surface at the bottom and a top restriction at film heights of different nano-meters are introduced keeping the maximum volume fraction similar to that of the bulk. The results will be important for the design of the ultrathin liquid films of PFPE.

Prediction of protein conformation in water and on surfaces by Monte Carlo simulations using united-atom method

The united-atom method has been used to model an avian pancreatic polypeptide (APP) in water and the adsorption process of an albumin subdomain (AS) onto graphite surface to observe the capability of this lumped modelling approach to generate structures observed in protein data bank (PDB) and from atomistic modelling. The subdomain structure of a protein is simplified by the united-atom approximation where the side chains and peptide groups are represented by lumped spheres. The total potential energy of the adsorption process involves the interaction between these lumped spheres by means of virtual bond chain interaction and the interaction of the spheres with the graphite surface by means of Lennard-Jones potential. The protein/polypeptide structure has been perturbed by Monte Carlo with energy minimisation to obtain the global minimum. Results on the APP in water showed a near-to-experimental PDB conformation revealing the two α-helix structures of this small protein molecule with the root mean square deviation among carbon backbone atoms of 5.9 Å. Protein adsorption on biosurfaces has been made by modelling AS, which has 60 amino acids. The surface is graphite, which is characterised by its hydrophobicity. Graphite was chosen because of its widely used applications in certain implants that interact with blood. Our simulation results showed final conformation close to that obtained by atomistic modelling. It also proved that the whole pattern of intramolecular hydrogen bonds was distorted. The model also demonstrated the random conformation of the original α-helix secondary structures of AS consistent with experimental and atomistic results. While atomistic simulation works well for simulating individual small proteins, the united-atom model is more efficient when simulating macromolecular and multiple protein adsorption where time and limiting computer capacity are key factors.

Finite Element Thermal/Mechanical Analysis of Transmission Laser Microjoining of Titanium and Polyimide

Detailed analysis of a residual stress profile due to laser microjoining of two dissimilar biocompatible materials, polyimide (PI) and titanium (Ti), is vital for the long-term application of bio-implants. In this work, a comprehensive three-dimensional (3D) transient model for sequentially coupled thermal/mechanical analysis of transmission laser (laser beam with wavelength of 1100 nm and diameter of 0.2 mm) microjoining of two dissimilar materials has been developed by using the finite element code ABAQUS, along with a moving Gaussian laser heat source. First the model has been used to optimize the laser parameters like laser traveling speed and power to obtain good bonding (burnout temperature of PI>maximum temperature of PI achieved during heating> melting temperature of PI) and a good combination has been found to be 100 mm/min and 3.14 W for a joint-length of 6.5 mm as supported by the experiment. The developed computational model has been observed to generate a bonding zone that is similar in width (0.33 mm) to the bond width of the Ti/PI joint observed experimentally by an optical microscope. The maximum temperatures measured at three locations by thermocouples have also been found to be similar to those observed computationally. After these verifications, the residual stress profile of the laser microjoint (100 mm/min and 3.14 W) has been calculated using the developed model with the system cooling down to room temperature. The residual stress profiles on the PI surface have shown low value near the centerline of the laser travel, increased to higher values at about 165 μm from the centerline symmetrically at both sides, and to the contrary, have shown higher values near the centerline on the Ti surface. Maximum residual stresses on both the Ti and PI surfaces are obtained at the end of laser travel, and are in the orders of the yield stresses of the respective materials. It has been explained that the patterned accumulation of residual stresses is due to the thermal expansion and contraction mismatches between the dissimilar materials at the opposite sides of the bond along with the melting and softening of PI during the joining process.

Experimental study of the replenishment of ultrathin liquid perfluoropolyether films on carbon surfaces

An experimental study is performed on the spreading and replenishment of ultrathin liquid films of perfluoropolyether Zdol on the same carbon coated disk. Carbon coated disks are partially dip coated, producing a spreading edge of the lubricant film, using Zdol with molecular weights of 2000 and 4000 g/mol. Away from the spreading edge of the film on the same carbon coated disk, the film is scratched by a semispherical slider. An ellipsometer is then used to measure the spreading and replenishment profiles consecutively with time. Diffusion coefficients are calculated from the spreading profiles and then used to simulate the replenishment profiles from the scratched profiles by solving the diffusion equation. The results are compared with the experimentally measured replenishment profiles with time from the same initially scratched profiles. The scratch depths are 1–3 nm for the sample with an initial film thickness of 5–7 nm. The comparison shows that the replenishment of ultrathin liquid films on solid ...

Density variation in the ultrathin liquid perfluoropolyether films on solid surfaces

Variation of density in the ultrathin liquid perfluoropolyether films on solid surfaces has been studied by Monte Carlo simulations of Lennard-Jones systems. The liquid film is 8.75 nm thin film consisting of polymer molecules assuming the structure of perfluoropolyether Z having molecular weights of 3840, 2500 and 1700 g/mol and interacting among themselves by Lennard-Jones potential. The substrate is assumed to be continuous without atomic structure and exerting Lennard-Jones potential on liquid molecules in the ultrathin film. The system temperature is considered to be 25 °C and the liquid molecules also have the gravitational potential. It is found that the bead density decreases towards the surface in a thinner sublayer in the ultrathin liquid film above the surface and the thickness of this sublayer just above the surface may increase with the increase of molecular weight of the of polymers in the film. Repulsive potential of the surface further decreases the bead density near the surface. The results are compared with the experimental results of the pefluoropolyether lurbricants by X-ray reflectivity (Shouji, et al., 1998).

Finite Element Thermal Analysis for Microscale Laser Joining of Nanoscale Coatings of Titanium on Glass/Polyimide System

Finite element thermal analysis and comparison with experiments of microscale laser joining of biocompatible materials, polyimide (PI) and nanoscale coating of titanium (Ti) on glass (Gl), is vital for the long-term application of bio-implants and important for the applications of nanoscale solid coatings. In this study, a comprehensive three dimensional (3D) transient simulation for thermal analysis of transmission laser micro-joining of dissimilar materials has been performed by using the finite element (FE) code ABAQUS, along with a moving Gaussian laser heat source. The laser beam (wavelength of 1100 nm and diameter of 0.2 mm), moving at an optimized velocity (100 mm/min), passes through the transparent PI, gets absorbed by the absorbing Ti, and eventually melts the PI to form the bond. The laser bonded joint area is 6.5 mm long on three different Ti coating thicknesses of 400, 200 and 50 nms on Gl surface. Non-uniform mixed meshes have been used and optimized to formulate the 3D FE model and ensure very refined meshing around the bond area. During the microscale laser heating finite element modeling shows widths of PI surface experiencing temperatures above the glass transition temperature are similar to the widths of bonds observed in experiments for coating thicknesses of 400 and 200 nms of Ti on Gl. However, for the case of 50 nm coating bond width using finite element analysis cannot produce and is lower than the bond width observed experimentally.© 2009 ASME

Roughness and undulations of the free surfaces of ultrathin liquid films of perfluoropolyethers on solid surfaces-effect of polar interaction

A rouse-like algorithm applied to a simplified bead spring off-lattice polymer model on NVT Monte Carlo (MC) simulation for perfluoropolyether Z and Zdol (qualitative) has been performed. The surface roughness with no evaporation and undulations of the free surfaces during evaporation of the ultrathin film polymers has been presented. A bead-bead Lennard-Jones (LJ) potential is used in the bulk, and a substrate-bead LJ potential is added in the ultrathin film condition. In addition, a dipole-dipole polar interaction potential is adopted at the two chain ends of Zdol. Simulation is performed with approximately 4000 g/mol molecular weight under the assumption of room temperature and pressure condition. It is observed that the calculated average radius of gyration is almost equal to the experimental value both for the bulk situation. Surface roughness of ultrathin liquid films of 2-nm film thickness has been calculated for Z and Zdol. A top solid surface has been activated over the ultrathin film on the bottom surface and the film has been allowed to evaporate toward the top surface. Undulations of the free surfaces have been observed along with the desorption of the films with increasing MC cycle and compared between Z and Zdol.

Characterization of balsa wood mechanical properties required for continuum damage mechanics analysis

Balsa wood has attained commercial importance because of its exceptional lightweight and its insulating properties. The use of balsa as core material in sandwich construction for aircraft and for other lightweight structures has greatly increased the use of this unique material. In the present study, the mechanical properties of balsa wood required for the continuum damage mechanics analysis are calculated using experimental, analytical, and numerical results. Extensive experiments are carried out that evaluate the tension, compression, and shear properties. Equivalent modulus has been calculated from the bi-modulus behavior of balsa wood obtained from the experiments. The properties that could not be obtained from the experiments were calculated using analytical solutions. The mechanical properties obtained were compared with the results available in literature and the numerical solutions using LS-DYNA. Finally, the mechanical properties are calibrated using GENOA—a commercial software based on the wood cell constituent properties and experimental results.

Surface perturbations on the perfluoropolyether molecules in the melt and the gas-like conditions

A Rouse-like algorithm applied to a simplified bead-spring off-lattice polymer model an NVT [N (number of atoms), V (volume), and T (temperature)] Monte Carlo (MC) simulation for perfluoropolyether (PFPE) Z and Zdol (qualitative) has been performed. Simulation is performed with approximately 4000 g/mol molecular weight under the assumption of room temperature and pressure conditions. Simulations have been carried out for both the ultrathin liquid films (melts) of PFPE on solid surfaces and gas-like states of PFPE molecules. Top solid surfaces at several separation distances (3, 4, 6, and 8 nm) have been activated over the ultrathin films (2-nm film thickness) only on the bottom surfaces. The critical separation distances for the direct perturbations (to destabilize the films) of the top surfaces on the ultrathin liquid films only on the bottom surfaces have been calculated. The statistical critical distances of perturbations of the surfaces on the gas-like states of PFPE molecules have also been calculated. Critical separation distances between the parallel solid surfaces to inhibit the destabilization of the films considering vapor phases over the ultrathin films on solid surfaces have also been proposed. Here, effects of polar interaction have been elucidated. Meniscus bridges between the two parallel solid surfaces have also been calculated for several separation distances.

Study of Convective Heat Transfer for Turbulent Flow of Nanofluids Through Corrugated Channels

Numerical study of turbulent heat transfer of nanofluid through a corrugated channel is presented. The finite volume method is used to solve the transport equation for the momentum, energy and turbulence quantities adopting a single phase approach. The corrugated channels are sine-shaped, V-shaped and rectangular shaped with amplitude (a/H) and wave length (lambda/H) of 0.15 and 1 respectively. Three different nano particles such as aluminum oxide (Al2O3), Copper (Cu) and titanium dioxide (TiO2) with different volume fraction (1%, 3% and 5%) using water as the base fluid are analyzed for a range of Reynolds number from 2000 to 14,000 with constant heat flux at the corrugated walls. Realizable k-epsilon turbulence model with enhanced wall treatment is considered. For all three geometries, average Nusselt number for the corrugated section is obtained. The result reveals that increasing the Reynolds number and volume fraction of nanoparticle in the base fluid has an effect of increasing the heat transfer rate. The sine shaped channel gives a better heat transfer enhancement compared to other two geometries up to Re = 8200 for 3% volume fraction of Al2O3-water nanofluid. At Reynolds number higher than this, rectangular shaped corrugated channel gives better heat transfer rate. Among the nano-particles, Al2O3 gives a higher heat transfer rate in all three corrugated channels. The enhancement of heat transfer is about 10.5% to 50.15% compared to water for the flow of Al2O3-water nanofluid through sine wave channel depending on the Reynolds number and volume fraction.