Considered are particles which are condensed homogeneously on a plane surface from the vapour phase. They are removed in circular or straight sinks. The edges of the sinks are partially masked by an array of small obstacles. The problem of surface diffusion is solved by a method of conformal

mapping following BETZ and PETERSOHN ¹. The obstacles cause an increase of particle density in a step-like fashion. The amplitude of the step depends on the degree of masking. As long as the sinks are not completely masked the density step is comparatively small and superimposed on the density function of unmasked sinks. The particle flux remains unchanged up to complete masking of the sinks.