Diffusive spreading of chainlike molecules on surfaces

We study the diffusion and submonolayer spreading of chainlike molecules on surfaces. Using the fluctuating bond model, we extract the collective and tracer diffusion coefficients D-c and D-t with a variety of methods. We show that D-c(theta) has unusual behavior as a function of the coverage theta. It first increases, but after a maximum goes to zero as theta --> 1. We show that the increase is due to entropic repulsion that leads;to steep density profiles for spreading droplets seen in experiments. We also develop an analytic model for D-c(theta) which agrees well with the simulations.