Abstract
The surface critical behaviour of fluids was studied by extensive computer simulations of the liquid–vapour coexistence curves of water and the LJ fluid in pores. The local order parameter , that is the difference between the densities of the coexisting phases at distance z from the surface, follows the surface critical behaviour of the ordinary transition of Ising magnets, Δρ∼τβ1
, where τ = 1 − T/T
c is the temperature deviation from the bulk critical temperature T
c and β1 ≈ 0.8. The crossover from bulk to surface critical behaviour occurs when the distance to the wall is about two bulk correlation lengths ξ−. The temperature dependence of the local coexistence curve diameter
shows mainly regular behaviour. To provide equivalence of the surface critical behaviour of fluids and Ising magnets, we propose to generalize the definition of the order parameter of the phase transition of Ising magnets in the presence of a surface field. The difference
of the local magnetizations in the coexisting phases, and not the local magnetization m
I,II(z) itself, measures the dissimilarity of the coexisting phases and should be considered as the correct order parameter. The order parameter
contributes to the total magnetization up to T
c only. Accordingly, the average magnetization
is induced by the surface field and does not vanish above T
c.
Keywords:
Acknowledgements
We are grateful to K. Binder, M. Fisher, R. Evans, H.W. Diehl and J. Indekeu for stimulating and helpful discussions. Financial support from DFG (SPP 1155) and BMBF (grant 01SF0303) is gratefully acknowledged.