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 $D_c\left(\theta \right)$ has unusual behavior as a function of the coverage $\theta$. It first increases, but after a maximum goes to zero as $\theta \to 1$. 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 $D_c\left(\theta \right)$ which agrees well with the simulations.

• Received 30 January 1996

DOI:https://doi.org/10.1103/PhysRevLett.76.4003