Nano-scale interactions of particles and drops with heterogenous surfaces

Recent technological developments enable one to study the behavior and interactions of particles and drops with heterogeneous surfaces at microscopic resolution, and investigate their possible applications. In this thesis, we use the microscopic calculational technique of molecular dynamics simulation, augmented by other continuum methods as appropriate, to study some prototypical examples. For applications to particle separation, we consider on the transport of particles by flow through a narrow channel of which one side has a stripe pattern of alternating wettabilities. We first consider van der Waals forces alone. The particle-wall interaction can either trap particles on the attractive stripes or deflect the trajectories of mobile particles away from the mean flow direction. Using molecular dynamics we determine how the migration angle of finite-sized rigid particles differs from the imposed fluid flow. The effects of electrostatic interactions are considered by decorating the particles and walls with opposite charges, resulting in significantly more trapping and larger deflection angles. We then use Langevin equations to simulate larger particles in the van der Waals case, and compare the results to the MD simulations. From the analysis of the associated Fokker-Planck equation we further obtain bounds on the deflection angle. The second problem involving fluid-solid interactions is that of nano-sized drop impact on a surface, which are flat, curved or pillared, with either homogeneous interactions or cross-shaped patterns of wettability. From the simulations we observe drop bouncing, sticking, spreading or disintegrating, depending on impact velocity and surface properties. In contrast to macroscopic observation, MD shows that the presence or absence of vapor has no effect on the onset of splashing. We argue that this difference is a direct consequence of drop size. For low velocity impacts, we compare MD results with continuum lattice Boltzmann methods at the same Reynolds and Weber numbers. In most situations we observe similar drop behavior at both length scales, with the best quantitative agreement for low impact velocities on wettable surfaces. We attribute the discrepancy for relatively high impact velocities to compressibility effects, while the disagreement on non-wetting surfaces is associated with different treatments of the liquid-solid boundary conditions.