  Molecular dynamics simulations are used to study the spreading of binary
polymer nanodroplets in a cylindrical geometry. The polymers, described by the
bead-spring model, spread on a flat surface with a surface-coupled Langevin
thermostat to mimic the effects of a corrugated surface. Each droplet consists
of chains of length 10 or 100 monomers with ~350,000 monomers total. The
qualitative features of the spreading dynamics are presented for differences in
chain length, surface interaction strength, and composition. When the
components of the droplet differ only in surface interaction strength, the more
strongly wetting component forms a monolayer film on the surface even when both
materials are above or below the wetting transition. In the case where the only
difference is the polymer chain length, the monolayer film beneath the droplet
is composed of an equal amount of short chain and long chain monomers even when
one component (the shorter chain length) is above the wetting transition and
the other is not. The fraction of short and long chains in the precursor foot
depends on whether or not both the short and long chains are in the wetting
regime. Diluting the concentration of the strongly wetting component in a
mixture with a weakly wetting component decreases the rate of diffusion of the
wetting material to the surface and limits the spreading rate of the precursor
foot, but the bulk spreading rate is not affected until the more strongly
wetting component is removed completely.
