This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada. The fellowships provided by the Swiss National Foundation and the Swiss Freie Akademische Gesellschaft for S.S. and by the Programme Qurbrcois de Bourses d'Excellence for C.F.M.B. are gratefully acknowledged. The authors are grateful to Prof. D. Guay at INRS-l~nergie et Matrriaux for his assistance with AFM measurements and to Profs. M. Wertheimer and J. Pelletier;
and to Mr. R. Antaki for stimulating discussions;