Non-Equilibrium Systems: Spreading, Deformation, Adhesion and Friction

Johns Hopkins University, Baltimore MD

Investigators

Abstract

TECHNICAL EXPLANATION This research addresses fundamental non-equilibrium processes that impact many technologies: spreading, plastic deformation, adhesive failure and friction. Traditional engineering approaches to these phenomena start from macroscopic continuum equations with phenomenological constitutive relations and boundary conditions whose validity is questionable, particularly as dimensions approach the molecular scale. Molecular simulations will be used to test existing continuum descriptions, provide connections between molecular interactions and macroscopic behavior, develop new mesoscale models, and provide input for macroscopic models that cannot be obtained from experiment. Simulations of spreading will study the singular stresses that are predicted to exist near the contact line where a fluid interface intersects a solid. Several competing mechanisms have been proposed for removing the singularity and it is difficult to distinguish between them using experiments. Molecular dynamics and hybrid atomistic/continuum methods will be used to test new theories based on diffusion and variations in interfacial tension. The results will enable construction of mesoscale models for larger length and time scales. These will be applied to dynamics in more complex geometries to study contact angle hysteresis and multi-phase flow in porous media. NON-TECHICAL EXPLANATION Studies of deformation, adhesion and friction will focus on amorphous polymer glasses, which are a major component of many adhesives and structural materials. The connectivity of the molecules leads to entanglements that dramatically alter mechanical properties, particularly at large strains. A new method will be used to visualize entanglements and correlate their evolution to stress/strain curves during craze formation and shear deformation. Atomistic and hybrid atomistic/continuum simulations will be used to study the fracture energy and mechanisms in bulk and thin films. Different polymers will be studied to determine what aspects of molecular interactions select the failure mode. Work on friction will explore the relation between constitutive relations for friction and bulk deformation, the contrast between nanoscale and macroscopic friction coefficients for some polymers, and the effect of molecular alignment near the surface on friction. Intellectual Merit: The proposed research addresses fundamental issues about how molecular level processes control macroscopic properties and how behavior changes as dimensions approach the atomic scale. These issues are relevant to many processes and materials systems. The research will also serve as a testing ground for new hybrid techniques that couple very different descriptions of matter. The resulting atomistic/continuum algorithms are expected to be widely applicable to many other problems and systems with a wide range of length and time scales. Progress in developing mesoscale and continuum models from molecular simulations is also likely to enable related work in other fields. Broader Impact: An improved understanding of spreading, deformation, adhesion and friction would have broad impact in coating of materials, processing of polymers, and the design and formulation of improved adhesives and structural materials. The project will contribute to the workforce by training students and a postdoc in a wide range of interdisciplinary modeling methods. A new course on multiscale modeling will be developed to expose students from engineering and science departments to these and related techniques. Finally, outreach efforts will bring research results to a wider audience through the annual Physics Fair at Johns Hopkins and collaborations with local high school teachers and the Maryland Science Center.