Vahid Mortazavi

Study of contact angle hysteresis using the Cellular Potts Model

We use the Cellular Potts Model (CPM) to study the contact angle (CA) hysteresis in multiphase (solid-liquid-vapour) systems. We simulate a droplet over the tilted patterned surface, and a bubble placed under the surface immersed in liquid. The difference between bubbles and droplets was discussed through their CA hysteresis. Dependency of CA hysteresis on the surface structure and other parameters was also investigated. This analysis allows decoupling of the 1D (pinning of the triple line) and 2D (adhesion hysteresis in the contact area) effects and provides new insight into the nature of CA hysteresis.

Wear-Induced Microtopography Evolution and Wetting Properties of Self-Cleaning, Lubricating and Healing Surfaces

Adhesion, friction, and stiction constitute a significant problem for small-sized applications, such as microelectromechanical systems (MEMS) and magnetic storage devices. It has been suggested to use surface texturing to decrease the adhesion force between components in these devices using the so-called Lotus effect (surface roughness induced superhydrophobicity and self-cleaning). However, for applications with high friction, such as head-tape interfaces, wear can deteriorate the microstructure, so the Lotus effect will not last for a long time. To overcome this, we suggest using equilibrium surface roughness, which provides Lotus effect properties. We suggest a thermodynamic model for microtopography evolution and equilibrium roughness due to wear and combine this model with the Wenzel and Cassie theories of wetting. The equilibrium roughness is governed by the minimization of deterioration using the minimum entropy production principle. On the other hand, self-healing/cleaning/lubrication is governed by minimum energy and entropy principles, so the combination of the two approaches is discussed. Examples of evolution of wetting properties due to wear of several cases of rough profiles are discussed and certain recommendations on how to decrease friction are formulated.

Friction-induced vibrations and self-organization : mechanics and non-equilibrium thermodynamics of sliding contact

Introduction: Friction as a Fundamental Force of Nature in the History of Mechanics Historical Background Certain Philosophical Concepts of Mechanics Hilberts Sixth Problem Nolls Axiomatic Mechanics P. Zhilins Approach The Study of Friction Summary References Vibrations and Stability at the Bulk and at the Interface Linear Vibrations in Systems with Single, Multiple, and Infinite Number of Degrees of Freedom Stability Analysis Non-Linear Vibration and Stability Analysis Bifurcations, Catastrophes, and Chaos Multi-Scale Systems Self-Organization: Different Types Example: Benard Cells Self-Organization in Tribology Asymptotic Transition from a 3D (Bulk) to a 2D (Interface) System Summary References Principles of Non-Equilibrium Thermodynamics and Friction Thermodynamic Potentials and Equations Irreversible Processes and Non-Equilibrium Extremal Principles and Stability of Frictional Motion Stability Criterion for Frictional Sliding Summary References Bibliography Fundamentals of Friction Empirical Laws of Friction Mechanisms of Friction Non-Linear Character of Friction Thermodynamics of Friction Summary References Wear and Lubrication Mechanisms of Wear Empirical Laws of Wear Thermodynamics of Wear: Entropy Generation and Degradation Lubricated Contact Summary References Bibliography Friction-Induced Instabilities and Vibrations Mechanics of Elastic Contact and Stability of Frictional Sliding Velocity Dependency of Coefficient of Friction and Stability Criterion Thermoelastic Instabilities Adams-Martins Instabilities Radiation of Elastic Waves by Friction Interaction of Elastic Waves with Friction Friction Reduction and Self-Organized Patterns due to Friction-Induced Vibrations Summary References Bibliography Friction-Induced Vibrations and Their Applications Brakes and Vehicles Music and Sound Generation Nature Summary References From Frictional Instabilities to Friction-Induced Self-Organization Self-Organization, Instabilities, and Friction Stability of Frictional Sliding with Coefficient of Friction Dependent on Temperature Running-In as a Self-Organized Process Frictional Turing Systems Modeling of the Formation of Tribofilms Stick-Slip Motion and Self-Organization Summary References Self-Lubrication Principles of Self-Healing and Self-Lubricating Materials Various Approaches to Self-Lubrication Summary References Outlook Index

Stability of Frictional Sliding With the Coefficient of Friction Depended on the Temperature

Friction-induced instabilities can be caused by different separate mechanisms such as elastodynamic or thermoelastic. This paper suggests another type of instability due to the temperature dependency of the coefficient of friction. The perturbations imposed on the surface temperature field during the frictional sliding can grow or decay. A stability criterion is formulated and a case study of a brake disk is performed with a simple model without including effects of transforming layer and chemical/physical properties change with temperature. The disk is rigid and the coefficient of friction depends on temperature. We show that the mechanism of instability can contribute to poor reproducibility of aircraft disk brake tests reported in the literature. We propose a method to increase the reproducibility by dividing the disk into several sectors with decreased thermal conductivity between the sectors. [DOI: 10.1115/1.4006577]

Green and Biomimetic Tribology

Green Tribology is a new area of tribology studying ecological approaches, methods, and applications in tribology. The major premise of green tribology is minimization of friction and wear, and reduction or elimination of lubrication. It includes eco-friendly materials and coating, green lubricants (biodegradable lubricants) and biomimetic surfaces used in tribological application. Using sustainable chemistry and engineering principles, biomimetic approaches and surface texturing are other concerns of green tribology. The area also takes degradation of surfaces, coatings, and components into consideration during design stages. The tribology of renewable sources of energy can also be considered as a relatively new challenge. We discuss areas and principles of green tribology and current developments in biomimetic materials and surfaces.

Studies of Shannon Entropy Evolution due to Self-Organization During the Running-In

When frictional sliding is initiated, the coefficient of friction is often high during the initial transient running-in process. After that, the coefficient of friction reaches its stationary value. Running-in is interpreted as friction-induced self-organization stage in that two sliding surfaces adjust to each other due to surface roughness evolution. Shannon entropy was proposed as a surface roughness parameter, and its decrease can be used as a simple test for self-organization. Sliding experiments were conducted on the hard steel plate using a soft Al-Mg alloy pin under both dry and lubricated conditions. Based on the results of the surface profile evolution, obtained by an optical profilometer, during running-in, we discuss change of Shannon entropy for various surface textures. Various textures which are characterized in terms of roughness parameters were produced on the steel plates. We compare how self-organization occurs for different textures during running-in stage.Copyright

Self-Organization at the Frictional Interface

Despite the fact that self-organization during friction has received relatively little attention of the tribologists so far, it has a potential for the creation of self-healing and self-lubricating materials, which are of importance for the green or environment-friendly tribology. The principles of the thermodynamics of irreversible processes and of the nonlinear theory of dynamical systems are used to investigate the formation of spatial and temporal structures during friction. The transition to the self-organized state with low friction and wear occurs through the destabilization of the steady-state (stationary) sliding. The criterion for the destabilization is discussed and examples like formation of a protective film and slip waves are discussed. Some cases like running-in stage, elastic structures, and Turing pattern formation as evidences of self-organization are studied. A special self-healing mechanism may be embedded into material by coupling corresponding required forces. The analysis provides a structure–property relationship which can be applied for the design optimization of composite self-lubricating and self-healing materials for various ecologically friendly applications and green tribology.

Pattern Formation During Frictional Mechanical Contact

Friction and wear are usually viewed as irreversible processes, which lead to energy dissipation (friction) and material deterioration (wear). On the other hand, it is known that under certain circumstances frictional sliding can provide mechanisms for the spatial and temporal pattern-formation (self-organization). We discuss the contact mechanics aspects of the pattern-formation and suggest quantitative metrics and criteria for this effect including the stability criteria for the transition from the steady-state sliding to pattern-formation.Copyright