Associate ProfessorAhmad Jabbarzadeh Khoei

Project Opportunities

Title: Nano-Rheology and Nano-Tribology: Atomistic Simulation of Boundary Lubrication

Summary of opportunity:

 This research seeks to find practical ways to reduce friction in boundary lubrication regime by a novel virtual nano-tribometer/ nano-rheometer.

Opportunity synopsis:

Successful manufacturing and application of miniaturized mechanical parts in the Micro/Nano Electro Mechanical Systems and other nano-devices with moving parts greatly depend on our ability to reduce friction, wear and energy dissipation. That requires understanding of the atomic origins of friction, high viscosity and rigidity of confined ultra-thin lubricant films and the interplay of surface and lubricant characteristics.Various types of lubricants with nonpolar and functionalised molecules confined by realistic smooth and rough surfaces are studied through direct atomistic simulations. The project is focused on the areas related to lubrication problems in boundary level and their rheological properties, with emphasis on self assembled monolayers (SAM).

--------------------------------------------------

Project Opportunities

Title: Tribological properties of biopolymers in human joints

Summary of opportunity:

In this project we will use advanced molecular simulation techniques to model the lubrication process in synovial joints to shed light on these puzzles.

Opportunity synopsis:

The lubrication process in mammalian joints is characterised by extremely low friction coefficient, resilient and protective damping and almost no wear. Despite many existing theories on macro-scale our understanding of molecular origin of such a marvelous lubrication performance is poor. The complex biopolymers at the surface of the cartilage are thought to play a major role in efficient lubrication.The goal is to understand the behaviour of these biopolymers, relate their biomechanical properties to their structure that varies with diseases and age and find practical solutions that can be applied to artificial joints used in hip/knee replacement operations.

--------------------------------------------------

Project Opportunities

Title: Polymer Crystallization

Summary of opportunity:

Using advanced molecular simulation techniques the project seeks to establish the relationship between molecular structure and complex interplay of thermodynamic and processing parameters on the rheology of the melt and crystallization.

Opportunity synopsis:

It is well known in the polymer industry that the crystallization of polymer melts is dependant on many thermodynamic and processing factors such as cooling rate, temperature, and shear rate. The structure and shape of the individual molecules also significantly affect the results. The structure of the crystallized material and as a result its mechanical properties greatly depend on these factors.

--------------------------------------------------

Project Opportunities

Title: High performance computational nanotechnology

Summary of opportunity:

This project seeks to develop these techniques for molecular simulations of liquid and solid states using parallel computational techniques. The developed algorithms will be used on supercomputers and cluster based platforms.

Opportunity synopsis:

One of the challenges faced in computational nanotechnology is the immense computational power needed to simulate systems at atomic and molecular level. Even a small sample of materials few nanometers thick contain many thousand atoms. So calculation of the properties and simulation of mater at such a scale require developing novel computational techniques to enhance the efficiency and allow for larger systems to be simulated for longer time spans.

----------Current Research Projects----------

• Nano-Rheology and Nano-Tribology: Atomistic Simulation of Boundary Lubrication
• Boundary conditions and wall slip at the fluid-solid interface
• Characterizing material properties by molecular level simulations
• Novel 3D nano-structures, the origin of high rigidity for ultra-thin liquid films
• Low friction states of films only a few nanometer thick
• Nano-rheology and complex flow through nano-channels
• Linking material properties and molecular architecture a route to design of customized purpose materials
• Using molecular simulations to study crystallization of polymers

Nano-Rheology and Nano-Tribology: Atomistic Simulation of Boundary Lubrication
Successful manufacturing and application of miniaturized mechanical parts in the Micro/Nano Electro Mechanical Systems and other nano-devices with moving parts greatly depend on our ability to reduce friction, wear and energy dissipation. That requires understanding of the atomic origins of friction, high viscosity and rigidity of confined ultra-thin lubricant films and the interplay of surface and lubricant characteristics. This research seeks to find practical ways to reduce friction in boundary lubrication regime by a novel virtual nano-tribometer/ nano-rheometer. Various types of lubricants with nonpolar and functionalised molecules confined by realistic smooth and rough surfaces are studied through direct atomistic simulations. The project is focused on the areas related to lubrication problems in boundary level and their rheological properties, with emphasis on self assembled monolayers (SAM).


Boundary Condition and Wall Slip at the Fluid-Solid Interface
Understanding the boundary condition of flow near solid surfaces is very important in many applications such as polymer processing, adhesion and lubrication problems. For computational rheology of viscoelastic flows using constitutive equations applying appropriate boundary conditions is essential to obtain meaningful results. Using molecular dynamics simulations we explore various parameters that affect the flow condition near the surface. Surface energy, roughness characteristics and topography and atomic order as well as molecular architecture, size and interaction energies are among the many parameters that are investigated.


Characterizing Material Properties by Molecular Level Simulations
Using molecular dynamics simulations bulk properties of many low molecular weight liquids can now be calculated quantitatively thanks to well tested molecular potential available. An example is the zero shear rate viscosity of various alkanes. The system of interest is usually put under shear over a range of accessible rates by molecular dynamics simulations 10⁷-10¹² s^-1). The Newtonian plateau then reveals the zero-shear viscosity. Below the results are shown for a linear and a branched alkane.


Novel 3D Nano-Structures, the Origin of High Rigidity for Ultra-Thin Liquid Films
Thin films of liquid, confined between atomically smooth surfaces, exhibit a transition characterised by an enormous increase in shear viscosity (by a factor of 10⁶ in some cases) as the film thickness is decreased to somewhere between 6 and 8 molecular layers. This rheological transition has been observed for lubricating liquids as diverse as linear alkanes and near spherical cyclohexane and siloxanes, a ubiquity that strongly suggests a general underlying cause. Despite extensive study over the last 15 years, fundamental questions remain unresolved concerning both the phenomenon itself (i.e. whether the transition is continuous or discontinuous, a consequence of surface contaminants or an intrinsic property of the surface) and the physical process responsible. We have shown that the transition to rigidity in a realistic simulation of a dodecane film between mica surfaces is the result of the formation of crystalline bridges across the film. This transition leads to a novel solid state in which the crystal bridges organise themselves into a mosaic structure, translationally disordered, but with a long range tetratic orientational order. The dynamic heterogeneity associated with the in-plane organisation of these crystal bridges accounts for the striking difference in how the shear viscosity and the diffusion constant vary with film thickness and temperature.


Low Friction States of Films Only A Few Nanometer Thick
The effective viscosity of confined lubricant films less than 6-7 molecular layers is usually enhanced by many orders of magnitude. For dodecane the high friction film has a strong in-plane order with "mosaic-like" structures that extend across the film and effectively form "crystalline bridges" resulting in high friction. Using molecular dynamics simulations, we have identified three routes to lower the friction. We show that the structure of confined films and their response to shearing are affected by atomic in-plane order and smoothness of the confining surfaces, the relative orientation of two crystalline surfaces and the direction of shear. We show a small increase in surface roughness in going from crystalline to amorphous surfaces can lead to a much lower friction. We demonstrate that misaligning (twisting) one surface with respect to the other by 45x results in a much lower effective viscosity. Application of shear for extended times induces alignment of lubricant molecules into a nematic-like order with ultra-low effective viscosity. The magnitude of reduction in the friction and the physical process through which it happens varies for each of these three routes. Depending on the method used, destruction of crystalline bridges, multilayer or fault plane slip provides a route for dramatic reduction in friction.


Nano-Rheology and Complex Flow Through Nano-Channels
Generally in MD simulation the averages are obtained for the entire space of the simulation box. However, in situations that involve non-isotropy and inhomogeneity, or when we are interested in the properties of the fluid on a certain point of space one needs to calculate the time average properties locally. For this purpose the volume of the simulation box is divided into a number of cells. To get good statistics each cell should be occupied by a few tens of particles. The properties that vary only in two dimensions are often calculated easily. This then involves dividing the simulation box in two dimensions into a number of sampling cells.All the local properties such as the stress tensor, velocity and density can be measured by this technique. Meshing in three dimensions has been attempted, but it requires much larger systems to acquire good statistics.

We have used to study complex flows through nano-channels. Two examples are the shown here Flow over a nano-cylinder in a nano rectangular box. The fluid is made of dodecane molecules. The calculated stream-lines and velocity field are shown.


Linking Material Properties and Molecular Architecture en route to Design of Customized Purpose Materials
The shape of molecules in liquids (polymers, lubricants etc) dictates their dynamical behaviour and mechanical properties. Understanding these structural effects is crucial in synthesising new materials with desired properties. In most cases especially for the high molecular weight molecules, it is still very difficult to determine the structure of by experiments. Experimental techniques such as NMR are useful in determining the number of branches for polymers. However in measuring the length of branches NMR cannot differentiate branches that have more than 6 to 10 atoms. Using highly active catalysts we can control short and long chain branching. This produces polymers with different rheological properties. A molecular simulation such as the Monte-Carlo method or molecular dynamics (MD) is an alternative approach to understand the structure-property relationships. The advantage of these methods is that highly detailed architectures can be simulated with controlled molecular weight and shape of the molecules. This is especially useful to study moderate length molecules usually a limited number of molecules are examined in the simulation because of the large number of calculations required. Also, for larger molecules the relaxation time is much longer and beyond the available simulation time. So the molecular weight of the simulated molecules is usually below the entanglement molecular weight. Although the molecules in these simulations are short, they can still capture much of the behaviour of real polymer melts. These model molecules often reproduce non-linear behaviour and normal stress differences that are observed for polymer melts. MD simulations have shown that liquids that often appear Newtonian in experiments at low shear rates, show non-Newtonian behaviour if they are sheared at large enough shear rates, such as occur in MD simulations. Shear thinning is now thought to be a universal phenomenon at high enough shear rates. For molecules with long relaxation times shear thinning occurs at low shear rates, but for Newtonian fluids with very low relaxation times it happens at much larger shear rates. MD simulation can examine the properties of the fluid at very much higher shear rates than those possible to achieve in the laboratory. There are situations such as hard-disk lubrication and flow in nano-channels where the fluid experiences extremely high shear rates. So these simulations can also be used to capture the effect of molecular shape on the rheological behaviour in such applications. An understanding of such structural effects no doubt can add to our knowledge and gives help in devising theoretical models for longer polymers.


Using Molecular Simulations to Study Crystallization of Polymers
We have studied the crystallization of alkanes using molecular dynamics simulation. Moderate system sizes have been simulated ranging from chains of C20 to C60. Model PE molecules are in the simulation box with periodic boundaries in all three directions. The temperature is dropped gradually according to the cooling rate and then the system is monitored.