AbstractIn weakly collisional sheaths, potential wells and barriers could appear. These can cause the presence of repulsed and trapped ions in the region surrounding a negatively biased Langmuir probe, invalidating the orbital motion limited (OML) theory. In this work, a Particle‐In‐Cell/Test‐Particle Monte Carlo (PIC/TPMC) model of Ar plasma at medium gas pressure has been developed in order to show the effect of ion‐neutral collisions. In fact, the results of the present simulation for different discharge pressures demonstrate the complex structure of electric potential around the probe and the failure of analytical theories. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)