Surface critical behavior of fluids: Lennard-Jones fluid near weakly attractive substrate

TL;DR

This study uses computer simulations to analyze the surface critical behavior of Lennard-Jones fluids near weakly attractive substrates, revealing universal surface critical phenomena similar to the Ising model.

Contribution

It demonstrates the universal surface critical behavior of fluids near weakly attractive surfaces and relates it to the Ising model's surface transitions.

Findings

Order parameter follows a tau^(beta_1) scaling law with beta_1 ~ 0.8

Crossover from bulk-like to surface-like behavior occurs at a correlation length about half the distance to the surface

Surface critical behavior is consistent with the ordinary surface transition in the Ising model

Abstract

The phase behavior of fluids near weakly attractive substrates is studied by computer simulations of the coexistence curve of a Lennard-Jones (LJ) fluid confined in a slitlike pore. The temperature dependence of the density profiles of the LJ fluid is found to be very similar to the behavior of water near hydrophobic surfaces (Brovchenko et al. J.Phys.: Cond.Matt. v.16, 2004). A universal critical behavior of the local order parameter, defined as the difference between the local densities of the coexisting liquid and vapor phases at some distance z from the pore walls, Deltarho(z) = (rho_l(z) - rho_v(z))/2, is observed in a wide temperature range and found to be consistent with the surface critical behavior of the Ising model. Near the surface the dependence of the order parameter on the reduced temperature tau = (T_c - T)/T_c obeys a scaling law ~ tau^(beta_1) with a critical exponent beta_1 of about 0.8, corresponding to the ordinary surface transition. A crossover from bulk-like to surface-like critical behavior with increasing temperature occurs, when the correlation length is about half the distance to the surface. Relations between the ordinary and normal transitions in Ising systems and the surface critical behavior of fluids are discussed.