System closed fluid, work done

Solvent-structure (mediated] forces enter when h is so small that overlap occurs between the liquid layers adjacent to the two boundaries. Close to these surfaces the ordering of the fluid molecules differs from that in the bulk, as discussed in sec. 11.2.2. If h is reduced, the overlap of two such layers requires work to be done by or on the system, that is, it leads to an additional contribution to G(h) and to /7(h). 

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