Ahmad Jabbarzadeh

Wall slip in the molecular dynamics simulation of thin films of hexadecane

A molecular dynamics simulation of a thin liquid film as it is sheared between two planar walls is reported. The model liquid is composed of linear chain molecules of hexadecane (C16H34) with intramolecular architecture such as bond stretching, angle bending and dihedral potentials included in the model. Designing a model that can mimic the planar shear flow enables us to study important questions on the effects of the wall properties on the slip between the liquid film and the wall. Different properties of the wall such as wall density, wall stiffness and wall–fluid interaction strength have been studied to determine the slip between the wall and fluid. The slip has been investigated for strong and weak adsorbing surfaces at various shear rates. The results emphasize the importance of adsorption on the degree of slip. The dependence of slip on the film thickness is also demonstrated.

Odd-even effects on the structure, stability, and phase transition of alkanethiol self-assembled monolayers.

Molecular dynamics simulations were conducted to predict the structural properties and phase transition temperatures of n-alkanethiols CH(3)(CH(2))(n-1)SH (Cn, 4 ≤ n ≤ 22) self-assembled monolayers (SAMs) on Au (111) surfaces. We studied the effects of chain length on the structural properties, including tilt and orientation angles, and on phase transition temperature. We found clear dependence of the structural properties, on both the number of carbon atoms, n; and on n being odd or even. Alkanethiols with n ≤ 7 show liquid-like behavior and large rotational mobility, whereas those with n ≥ 12 are well-ordered and stable. For 12 ≤ n ≤ 15, odd-even effects are observed, where for n = odd, larger tilt angles, oriented in the direction of their next next nearest neighbor (NNNN), and for n = even, lower tilt angles, mostly tilted toward next nearest neighbor (NNN), were observed. For 15 ≤ n ≤ 19, we find tilt angle and orientation to be independent of n. For all alkanethiols, a gradual decrease of the tilt angle occurred by increasing the temperature from 300 to 420 K. Order-disorder phase transitions occurred at a certain temperature. This was signified by abrupt instabilities in the tilt orientation angle. This transition temperature showed an enhancement of ∼67-100 °C over the melting point of the corresponding n-alkane bulk system. This enhancement depended on n, and was larger for n = odd. Overall, we found that odd alkanethiols show better structural and thermal stability, and smaller gauche defects.

The effect of branching on slip and rheological properties of lubricants in molecular dynamics simulation of Couette shear flow

Abstract Molecularly thin liquid films of alkane in extreme conditions in a thin film lubrication regime have been investigated. To get an insight into the effects of molecular architecture in the behaviour of these thin lubricant films we have studied six different molecules, mainly isomers of C 30 . In this work the effect of branching on rheological properties and behaviour of lubricant film is examined. Our study shows viscosity and normal stress effects depend on the degree of branching. Dynamics of the molecules and their orientation are also affected by the degree of branching. A weaker layering near the wall is observed for branched molecules. Slip between the wall and lubricant film also was larger for branched molecules. Branched molecules had less tendency to change their orientation under the flow. The results obtained here could be helpful in designing new lubricants at the molecular level.

Rheological properties of thin liquid films by molecular dynamics simulations

In this paper we present the molecular dynamics simulations of thin fluids films sheared in Couette flow geometry between two structured plane walls. An NVT ensemble of atoms was chosen and simulation conducted in isothermal conditions. To keep the temperature at the required level a Gaussian thermostat was employed. This method was shown to be superior to the simple velocity rescaling method, especially at high shear rates. The Gaussian thermostat method gave results for viscosity in good agreement with the results of other researchers who used the reservoir method. The results for density and velocity profiles were obtained for a wide range of simulation parameters. The effects of shear rate and wall-fluid interaction strength were investigated in detail over a wide range of parameters. The material functions and normal stress differences were also obtained and the effects of shear rate and wall strength parameter on these properties were studied. The effect of film thickness on the viscosity was investigated and was compared with what we found for bulk fluid using the SLLOD algorithm. the existence of a non-Newtonian region with shear-thinning effect is found and examined for various films. The results suggest an increase in viscosity for thinner films in the Newtonian regime, though this is valid only for a limited range of wall-fluid interaction strength. A decrease in viscosity was also observed when the attraction force of the wall was increased.

Nanorheology of molecularly thin films of n-hexadecane in Couette shear flow by molecular dynamics simulation

Abstract In this work the rheological and structural properties of n-hexadecane have been studied by molecular dynamics simulation. The model consists of two structured atomic walls between which the fluid is sheared by moving the walls in opposite directions. The fluid consists of chains of n-hexadecane molecules. Each molecule has 16 interaction sites where each site on the molecule represents a CH2 or CH3 group. The Lennard–Jones potential governs the intermolecular interactions. Stretching, angular and torsional potentials are used for the intramolecular interactions to preserve the integrity of the molecules. An isothermal simulation of the Couette shear flow is conducted to reveal the rheological properties of n-hexadecane at high Weissenberg numbers in films as thin as 1 nm. The results obtained show an increase in the average viscosity of hexadecane as the film thickness is decreased to scales comparable to the molecular diameter of the chain segments. These results agree with recent experimental findings for very thin films, revealing shear thinning and normal stress difference effects which are an indication of non-Newtonian behaviour. Structural properties such as the density profiles, bond angle and dihedral angle distribution functions and average end-to-end distance of the molecules are obtained for films of different thickness and at different shear rates. The effects of the wall–fluid interaction strength on the fluid properties are also investigated in different adsorption limits. It seems that adsorption is a determining factor in the properties of these ultrathin films. The results indicate different shear responses depending on the adsorption limit of the surface.

Effect of load on structural and frictional properties of alkanethiol self-assembled monolayers on gold: some odd-even effects

We have conducted molecular dynamics simulations to study the frictional properties of alkanethiols CH(3)(CH(2))(n-1)SH (Cn, 12 ≤ n ≤ 15) self-assembled monolayers (SAMs) on Au(111) surfaces, under various loading and shearing conditions. For the examined alkanethiols, we found some evidence of the friction coefficient being dependent on the number of carbon atoms in the molecule being odd or even. Alkanethiols with n = odd show consistently higher friction coefficients than those with n = even. Such odd-even effect seems to be independent of the sliding velocity. However, the effect is significant only at lower loads (<700 MPa). The structural origin of this odd-even effect has been discussed. The effect of loading on the structure is also studied. For dodecanethiol (n = 12) we find the film responds to increased loading initially by increasing the tilt and then by deformation of individual molecules. SAM-Au contacts under shear show periodic storage and release of energy and a clear stick-slip pattern in the shear stress, film thickness, and the tilt and tilt orientation angles.

Low friction lubrication between amorphous walls: Unraveling the contributions of surface roughness and in-plane disorder

Using molecular dynamics simulations, we show that dodecane films confined between amorphous surfaces at 300 K retain liquid-like behavior down to film thicknesses of at least 1.8 nm and possibly smaller. This is in stark contrast to the behavior of films confined between crystalline surfaces which show an abrupt transition to a very high viscosity state at a film thickness of 4 nm. We show that it is the small increase in surface roughness in going from crystalline to amorphous walls, rather than the in-plane disorder, that is responsible for disrupting the crystalline bridges found in the crystal-confined films. The main consequences of the in-plane disorder are the removal of the orientational pinning of the local domain alignment and the reduction of the critical thickness at which the transition to film rigidity appears.

Effect of compression on self-assembled monolayers: a molecular dynamics study

Molecular dynamics simulation has been used to study the effect of pressure on self-assembled monolayers (SAM) of n-alkanethiols [CH3(CH2)n?1, n?=?12?15] on Au(1?1?1) substrates. Mechanical properties of the monolayer compressed by another bare Au substrate (SAM?Au contact) or by another SAM (SAM?SAM contact) have been calculated. The effects of pressure and molecular length on structural and physical properties such as tilt angle, radius of gyration and gauche defects have been studied. We find odd?even effects in response to pressure for SAM?Au contacts where even alkanethiols show larger resistance against compression. For SAM?Au contacts, odd alkanethiols compared with even ones show considerably larger gauche defects at the functional end of the chains. However, for SAM?SAM systems the odd?even effect is weak and molecular structural deformations are lower than the SAM?Au contacts. For all SAMs examined here we found a fully elastic response in compression and decompression stages under pressures between 0.5 and 4?GPa. The calculated Youngs moduli (E) are reported for C12, C13, C14 and C15 for both SAM?Au and SAM?SAM contacts. The results show that the modulus is mostly independent of the chain length and of SAM being a SAM?Au or SAM?SAM contact.

A parallel algorithm for molecular dynamics simulation of branched molecules

To get an insight into the effects of molecular architecture in the behaviour of thin lubricant films we have devised an algorithm for simulation of branched molecules. We have used this algorithm successfully to simulate branched isomers of C30. However the algorithm is flexible enough to be used for the simulation of more complex branched molecules. The resulting algorithm can be used in molecular dynamics simulation of branched molecules and could be helpful in designing new materials at the molecular level.  2002 Elsevier Science B.V. All rights reserved.

Parallel simulation of shear flow of polymers between structured walls by molecular dynamics simulation on PVM

A parallel algorithm has been developed for the simulation of Couette shear flow between structured walls. The algorithm is designed to simulate the shear flow of atomic and molecular fluids. The parallel link-cells model is used for parallelization with some modifications for accomodating the nonperiodic boundaries in the wall direction. Some techniques are also introduced for handling the non-homogeneous nature of the flow in the proximity of the physical walls in order to achieve a balanced workload between processors. PVM (Parallel Virtual Machine) is employed as the message passing paradigm for communication between the processors. The algorithm has been tested for a number of benchmarks with different sizes for simulating the shear flow of n-hexadecane. The maximum number of processors used was 28 DEC Alpha 500/256 workstations which were connected by a 100 Mbits/s Ethernet. A maximum speedup of 11 was obtained with 28 processors. The efficiency ranged from 92% to 40% depending on the number of processors and the system size.