Uma Shantini Ramasamy

Effect of Molecular-Scale Features on the Polymer Coil Size of Model Viscosity Index Improvers

Temperature-induced changes in coil size have been proposed as a mechanism underlying the functionality of viscosity index improving polymers. Here, molecular dynamics simulations are used to characterize the effect of temperature on the coil size of model additive polymers. The simulations reproduce experimental observations, where only some polymers increase in size with increasing temperature. The results also reveal that the presence of oxygen atoms in the polymer structure is a key factor in determining whether the polymer expands or contracts. This new simulation approach provides a general methodology for investigating temperature-induced coil size changes in polymeric lubricant additives.

Predicting Pressure–Viscosity Behavior from Ambient Viscosity and Compressibility: Challenges and Opportunities

AbstractThe potential for using an empirical expression to predict the piezoviscous response of a fluid from its pressure–volume behavior is explored. This approach is particularly promising since the variation of volume with pressure can be obtained relatively easily using atomistic simulations that are based on the molecular structure of the fluid. The accuracy of predictions made using the proposed method is evaluated, and its limitations are discussed in terms of sources of error and potential means of minimizing that error going forward.

Review of Viscosity Modifier Lubricant Additives

This article reviews viscosity modifiers, additives that increase the viscosity of lubricating oils. Viscosity modifiers are high molecular weight polymers whose functionality is derived from their thickening efficiency, viscosity–temperature relationship, and shear stability. There are now many different additive chemistries and architectures available, all of which have advantages and disadvantages, and affect solution viscosity through different mechanisms. Understanding these mechanisms and how they impart additive function is critical to the development of new viscosity modifiers that enable lubricants to function more efficiently over a wide range of temperatures.

Stability and Structure of Nanometer-Thin Perfluoropolyether Films Using Molecular Simulations

Methods to determine the stability of thin films are needed to ensure that lubricating layers maintain their integrity and so can reliably protect and lubricate moving interfaces. Here, we compare two models of perfluoropolyether and various methods of calculating disjoining pressure as a means of characterizing stability using molecular dynamics simulation. As has been shown previously, we find that energy methods, based on the variation of film--substrate interaction energy with film thickness, provide a good estimate of disjoining pressure. Further, we find that more rapid computational methods based on the density in the film, though they may yield a poorer estimate of the magnitude of the disjoining pressure, yield correct predictions for thin film stability.