Mesoscopic rodlike molecules can nowadays be synthesized in large amounts. This process ends usually in a polydispersity of the rod lengths. We investigate polydisperse suspensions of mesoscopic rods using Monte-Carlo techniques. The model we use consists of a hard core and has an additional attractive part which mimics van-der-Waals interactions as well as depletion forces corresponding to the Asakura-Oosawa model. In bulk systems we observe a separation of short and long rods where the latter form a high ordered cluster which coexists with an isotropic fluid of short rods. The structure of the cluster and the length distribution in the two phases turns out to be determined by the strength of
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When the suspension is in contact with a substrate we find at higher pressures a sedimentation of long rods at the substrate. This effect can be enhanced by a corrugated substrate. At low pressures, however, the corrugation structure repels the long rods while short rods accumulate preferentially at the substrate.