Phase Separation in Porous Media
The equilibrium behavior of homogeneous solids and fluids near the critical point is a subject that is quite well in hand, but the dynamics of phase separation is a subject still alive with complexity and interest. Here we discuss an embellishment of the above two problems that has caught the interest of the phase transition community in recent years, namely phase transitions in the presence of externally imposed random fields1. In a magnetic system, this field is indeed an externally imposed magnetic field, its direction varying randomly from spin site. In binary mixtures, with which we are concerned here, the “field” is actually a chemical potential difference µ(r) that plays the role of the field. Instead of favoring one spin direction over another, it favors, at the point r, the presence of a molecule of species A, say, over that of B in an A-B mixture. In the present instance, the source of this random chemical potential µ(r) is porous glass or a gel, saturated with the liquid mixture. If the glass were, say, in the form of a flat sheet or a test tube instead of a porous block, its preference for one of the two components would be described by the theory of wetting2. One might reasonably expect that such systems should be described by random field theory, rather than wetting theory, when the correlation length ξ of the mixture is large compared to the pore size; in this case the attractive force for one of the components over the other is random on all relevant spatial scales. The experiments to be described do not fully bear out this expectation, in that some of the effects seen in large pore-systems also seem to fit well into the random field picture.
KeywordsPorosity Quartz Agarose Polyacrylamide Turbidity
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