modifications we expect to be necessary are study of a three-dimensional model, allowing fluctuations in directions perpendicular to the array axis, and inclusion of hydrodynamic interactions between the particles. We defer these tasks to future work. 

## Acknowledgments 

We thank O. N. Mesquita, U. Agero and J. G. Moreira for helpful comments. This work was supported by CNPq and Fapemig, Brazil. 

[1] B. Schmittmann and R. K. P. Zia, in Phase Transitions and Critical Phenomena , edited by C. Domb and J. L. Lebowitz (Academic Press, London, 1995), Vol. 17; Phys. Rep. 301 , 45 (1998). 

[2] J. Marro and R. Dickman, Nonequilibrium Phase Transition in Lattice Models (Cam- bridge University Press, Cambridge, 1999). 

[3] Katz, J. L. Lebowitz, and H. Spohn, Phys. Rev. B 28 , 1655 (1983). 

[4] K.-t. Leung, B. Schmittmann, and R. K. P. Zia, Phys. Rev. Lett. 62 , 1772 (1989). 

[5] J. T. MacDonald, J. H. Gibbs, and A. C. Pipkin, Biopolymers 6 , 1 (1968). 

[6] E. D.Andjel, M. Bramson, and T. M. Ligget, Prob. Theory Rel. Fields 78 , 231 (1988). 

[7] T. E. Harris, J. Appl. Probab. 2 , 323 (1965). 

[8] F. Sptizer, Adv. Math. 5 , 2 (1970). 

[9] J. T. MacDonald and J. H. Gibbs, Biopolymers 7 , 707 (1969). 

[10] K. Heckmann, in Passive Permeability of Cell Membranes , edited by F. Kreuzer and J. F. G . Slegers (Plenum, New York, 1972), Vol. 3, p. 127. 

[11] A. Schadschneider, Phys. A 285 , 101 (2001). 

