Stokes formula and density perturbances for driven tracer diffusion in an adsorbed monolayer

We study the intrinsic friction of monolayers adsorbed on solid surfaces from a gas phase or vapor. Within the framework of the Langmuir model of delocalized adsorption, we calculate the resistance offered by the mobile adsorbate's particles to some impure tracer molecule, whose diffusive random motion is biased by a constant external force. We find that for sufficiently small driving forces the force exerted on the tracer shows viscous-like behavior. We derive then the analog of the Stokes formula for two-dimensional adsorbates, calculate the corresponding friction coefficient and determine the stationary particle distribution in the monolayer as seen from the driven impurity.