A Nano to Macro Study of Friction and Adhesion in Granular MaterialsPublic Deposited
The study of granular matter is both scientifically intriguing and industrially important, yet it presents several unique characteristics and challenges. The discrete nature of granular materials often precludes the use of the continuum descriptions that are so commonly used for fluids, so alternative methods must be developed. Particles interact via their surfaces, which allows surface forces such as friction and adhesion to affect essentially any aspect of their collective behavior. The origins of these forces stem from molecular-level surface properties, so a study of friction and adhesion in granular flows requires a multi-scale approach. Experimental results presented here show that even nanoscale differences in surface roughness greatly affect the angle of repose in a rotating drum. Surprisingly though, rough and smooth particles with considerable differences in their angles of repose do not segregate, putting into question a common explanation for segregation. Particle dynamics simulations can connect inter particle friction and adhesion to bulk granular behavior. Simulation results for binary systems of particles that differ only in their adhesive or frictional properties show a strong dependence upon the relative concentration of particle types and particle interaction strength. Molecular systems display a similar range of behavior, which has prompted the application of the structure of thermodynamic solution theory to analyze the granular system. Friction and adhesion manifest themselves in different ways in granular materials depending upon the size of the particles. For micro and nanoparticles, van der Waals adhesion can be significant and even dominate particle behavior. Adhesion was incorporated into particle dynamics simulations and applied to systems of small particles in shearing, drum rotation, and to develop a novel type of mechanical damping device. On the molecular level, simulations capture the origins of friction and adhesion. Monte Carlo simulations were used to calculate the adsorption isotherms of model lubricants (long-chain alkanes) along with their density and orientation profiles in slit shaped pores. The adsorption behavior of the lubricant affects the stability of thin films at high temperatures and stresses. Molecular simulations predicted temperature and load combinations which can induce lubrication failure.