No abstract

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 ͑u͒ has unusual behavior as a function of the coverage u. It first increases, but after a maximum goes to zero as u ! 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 ͑u͒ which agrees well with the simulations.[ S0031-9007(96) The spreading dynamics of molecularly thin oil films on solid substrates has gained substantial interest recently [1][2][3][4][5]. This is not only due to its obvious significance for technical applications, but also to the richness of the structures (layering, fingering) in the observed film profiles that sensitively depend on the molecular structure and interactions. As a typical feature, the development of molecularly thin so-called precursor films is observed [1]. The precursor film profile may be smooth or exhibit pronounced molecular layering effects ("dynamical layering").At the foremost tip of the film, the flow dynamics develops into surface diffusion of single molecules, i.e., a 2D gas. In some experiments [2-4], it has been found that the transition from the dense submonolayer to the dilute film occurs much more rapidly than expected from simple molecular diffusion. The measured profiles are not Gaussian but assume a steeper shape that can be well fitted by a spherical cap [3] (see also Ref. [4]). An explanation for the late-time profiles has been suggested in terms of a transition of the fluid from a nonvolatile 3D to a volatile 2D phase where the attractive intermolecular interactions are weaker [2,4]. However, it is the aim of the present study to provide an alternative microscopic explanation for the steep density profiles observed in cases where desorption of the molecules can be neglected. By studying the diffusive properties of chainlike molecules, we demonstrate that such profiles can be generated by entropic repulsion alone.To this end, we have performed a systematic study of diffusion and submonolayer spreading of athermal, flexible chains. We concentrate on the coverage dependence of collective and tracer diffusion coefficients D c ͑u͒ and D t ͑u͒, respectively. In addition to being important for submonolayer spreading dynamics, the diffusion of such molecules is of fundamental theoretical interest. Despite considerable experimental [6] and theoretical [7] work on adatom diffusion on surfaces there are only a few studies of more complicated molecules [8].We use the fluctuating bond (FB) model with Monte Carlo (MC) simulations to extract both D c ͑u͒ and D t ͑u͒ from the relevant correlation functions in equilibrium. These results are complemented by analysis of simulated density profiles together with the nonlinear diffusion equation. We show that, while D t ͑u͒ is a decreasing function of u as expected, D ...

No abstract