Surface spinodals and extended wetting in fluids and polymer solutions

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.