Spreading of a drop on a contaminated substrate

Most wetting or spreading problems are treated with surface and interfacial free energies taken as constants for the system in question. We consider h ere the case where a two-dimensional drop (ribbon) is deposited on a solid substrate bearing a very fine layer of a contaminant which modifies the eff ective drop/solid interfacial free energy. As the contaminant diffuses into the drop, the interfacial free energy, and thus the capillary balance, evo lve. This has strange effects on wetting behaviour, including causing a wet ting 'overshoot'. Possible applications are briefly discussed. (C) 2000 Aca demie des sciences/Editions scientifiques et medicales Elsevier SAS.