The spreading of polymer droplets is studied using molecular dynamics simulations. To study the dynamics of both the precursor foot and the bulk droplet, large hemispherical drops of 200 000 monomers are simulated using a bead-spring model for polymers of chain length 10, 20, and 40 monomers per chain. We compare spreading on flat and atomistic surfaces, chain length effects, and different applications of the Langevin and dissipative particle dynamics thermostats. We find diffusive behavior for the precursor foot and good agreement with the molecular kinetic model of droplet spreading using both flat and atomistic surfaces. Despite the large system size and long simulation time relative to previous simulations, we find that even larger systems are required to observe hydrodynamic behavior in the hemispherical spreading droplet.

The adhesion and friction between pairs of ordered and disordered self-assembled monolayers on SiO2 are studied using molecular dynamics. The disorder is introduced by randomly removing chains from a well ordered crystalline substrate and by attaching chains to an amorphous substrate. The adhesion force between monolayers at a given separation increases monotonically with chain length at full coverage and with coverage for fixed chain length. Friction simulations are performed at shear velocities between 0.02-2 m/s at constant applied pressures between 200 and 600 MPa. Stick-slip motion is observed at full coverage but disappears with disorder. With random defects, the friction becomes insensitive to chain length, defect density, and substrate.

The crossover to Rouse-like behavior for the self-diffusion constant D, the viscosity η, and the equilibrium structural statistics of n-alkanes (6 ≤ n ≤ 66) is studied numerically. For small n the chains are non-Gaussian and the meanwhere a depends on the interaction model. At constant density, the Rouse model is used to extract the monomeric friction coefficient ζ and the viscosity η independently from the diffusion constant D and the longest relaxation time τ R . ζ D extracted from D is nearly independent of chain length while ζ τ obtained from τ R is much larger than ζ D for small n. The viscosity measured in a non-equilibrium molecular dynamics simulation is closely approximated by the value of η determined from τ R while η inferred from D is smaller for small n. For n > ∼ 60, the two estimates for both ζ and η agree as predicted from the Rouse model. D calculated from three interaction models is studied for increasing n and compared to experimental data.

Silicon micromachines in microelectromechanical systems (MEMS) are coated with self-assembled monolayers (SAMs) in order to reduce the wear and stiction that are commonplace during operation. Recently, perfluorinated SAMs have been the focus of attention because they have better processing properties than hydrocarbon SAMs. In this study, we perform molecular dynamics simulations that model adhesive contact and friction for perfluorinated alkylsilane (Si(OH) 3 (CF 2 ) 10 CF 3 ) self-assembled monolayers (SAMs), which are commonly used in MEMS devices. Amorphous silica is used as the substrate for the SAMs in the simulations. The frictional behavior is investigated as a function of applied pressure (50 MPa-1 GPa) for a shear velocity of 2 m/s and compared to recent simulation results of hydrocarbon alkylsilane SAMs. The microscopic friction coefficient for the perfluorinated SAMs is the same as was measured for the hydrocarbon SAMs, but the shear stress is slightly larger than in the case of the hydrocarbon SAMs on amorphous silica.

As motivation builds to consider mechanics of nanometer scale objects, it is increasingly advantageous to implement models with finer resolution than standard continuum approaches. For such exercises to prove fruitful, these models must be able to quantify continuum thermomechanical quantities; furthermore, it may be necessary to do so on a sub-system level in order to assess gradients or distributions in a given property. Herein we review the calculation of stress, heat flux, and temperature in atomic scale numerical simulations such as the molecular dynamics method.