Surface spinodals and extended wetting in fluids and polymer solutions

Abstract

A novel concept of ‘‘surface’’ spinodal decomposition is introduced in the context of wetting and prewetting first order transitions for simple fluids in contact with a bounding wall. Qualitative and semiquantitative discussions are made within the framework of a phenomenological Landau theoretical model for semi-infinite systems with a (short-ranged) wall potential. A concept of ‘‘extended wetting’’ is also proposed, whereby even for T<TW (the wetting temperature) a macroscopically thick ‘‘wetting’’ layer can form near the wall upon rapidly quenching the solution into the coexistence region. These ideas are carried over to polymer solutions and blends near their consolute points where the characteristic time scales should be more manageable.

Fundamentals of Phase Separation in Polymer Blend Thin Films

“…They found that, at two-phase coexistence, the wall is always wetted with the preferred phase, but that there is strong enhancement of the preferred component at the wall even away from two phase coexistence. In disagreement with previous work by Nakanishi and Pincus [31], Schmidt and Binder argued that it should be possible to observe second-order wetting in a binary polymer mixture (in which mean-field descriptions should work well, and the spinodal should be well defined rather than smeared out by fluctuations, as it would be in simple fluid systems). The disagreement arises from a difference in the magnitude of the polymer wall interactions assumed by the authors, with the assumptions of Schmidt and Binder probably being much more physical, since the latter work assumes that the interactions between polymer segments and surfaces is of comparable magnitude to that between simple molecules and surfaces, as explained in section 2.3.3.…”

“…Perhaps they were first to write the surface energy in a phenomenological form like in equation (2.3). Using the same description of a simple bulk fluid in contact with a surface, Nakanishi and Pincus later studied 'surface spinodal decomposition' (where the surface gives directionality to spinodal decomposition) for fluids near a wall [31], describing the model as "the simplest model for wetting where an incremental chemical potential δ∆µ = h 1 k B T is introduced favouring one component, say B, of a binary fluid mixture near the wall" with an enhancement term describing an "enhancement near the wall in the effective molecular couplings".…”

“…Nakanishi and Pincus studied wetting for simple fluids in a semi-infinite geometry in which the bounding wall preferred the liquid (as opposed to the vapour) phase [31]. The work was carried over to polymer systems too.…”

“…They showed that the equilibrium states of the system, along with first order and second order phase transitions could be easily visualised using phase portraits. Nakanishi and Pincus utilised the method of Pandit and Wortis to study wetting transitions for simple fluid systems in contact with a wall/surface [31]. Their analysis included a discussion of possible metastably wet states, and the idea of extended wetting and 'surface spinodals'.…”

“…Nakanishi studied the effects of finite film thickness for films of a binary fluid mixture confined by symmetric walls, using mean field theory for a lattice gas [31]. It was found that all the sharp transitions found in semi-∞ systems are smeared out in finite systems, and that true wetting no longer occurs (note that it is still usual to use the term 'wetting' in finite systems to denote when having only one phase in contact with a surface is favourable).…”