(402i) Computational Study of Lubrication Forces Between Colloidal Particles and Planar Surfaces
Lubrication forces arise whenever two surfaces, separated by a fluid, move relative to one another in close proximity. This hydrodynamic interaction plays an important role in colloidal aggregation and deposition processes. Lubrication forces are typically analyzed within the framework of continuum hydrodynamics, which dictates that the contact of the two surfaces cannot be achieved due to the infinite force required to squeeze out the interstitial fluid. Yet, there is some uncertainty as to the limiting behavior of lubrication forces as the separation between surfaces approaches microscopic dimensions, where a truly continuum description of the interstitial fluid is precluded. For example, molecular dynamics (MD) simulations of Challa and van Swol (2006, Phys. Rev. E 73, 016306) indicate that lubrication forces remain finite as the surfaces come into contact. These simulations are limited, however, to colloid-solvent size ratios of no more than 15 to 1, whereas some widely used colloidal entities of mesoscopic size (~1μm) have colloid-solvent size ratios of ~1000 to 1. Whether lubrication forces deviate from the continuum predictions at these larger ratios remains unclear. A further concern within the lubrication force regime includes the nature of the interaction of fluid particles with surfaces, specifically with regard to the development of suitable boundary conditions. Typical hydrodynamics calculations rely on either slip or no-slip boundary conditions; it is indefinite as to what extent, in reality, either of these types of boundary condition is applicable for length scales small enough to preclude a continuum representation of the interstitial fluid. To investigate in detail the behavior of lubrication forces between larger colloidal particles and surfaces, we utilize a novel MD simulation that includes a portion of the colloidal surface along with the interstitial fluid coupled to a fluid reservoir. This simulation enables us to probe hydrodynamic effects on length and time scales more relevant to colloids nearing 1μm in diameter. Furthermore, to evaluate the pertinency of slip and no-slip boundary conditions we variously characterize the surface with which the partial colloidal particle interacts as a hard, structureless surface; a purely attractive surface; or a surface composed of Lennard-Jones particles. Simulation results are compared with typical hydrodynamics calculations under both slip and no-slip boundary conditions. Additionally, there is some question as to how well lubrication forces are replicated in coarse-grained simulations of colloidal systems where the interstitial fluid is only represented in an averaged sense. As these techniques are invaluable in determining the behavior of colloidal particles interacting with surfaces, it is imperative that the proper lubrication results be included. To this end, MD simulation results from lubrication studies are correlated with mesoscopic simulations of colloidal systems to ensure that the correct lubrication effects are replicated for colloidal particles in the lubrication regime.