Surface Configuration Change in Dry Air

The extent of surface configuration change is dependent on the differential surface tension given by Eq. (25.8), and the value of y varies depending on the equilibration conditions of the surface before the new polymer/air interface was created. Because the value of ys cannot be smaller than y, the extent of surface configuration change 

the driving force for the reverse process is much less than for the wetting process. Consequently a simple drying of the wetted sample does not cause a significant hydrophobic recovery of the surface. From the viewpoint of interaction forces, the repulsive force between fluorine atoms and water molecules is not replaced by the attractive force in the similar magnitude, but the repulsive force is merely removed. The reverse process, therefore, may be more appropriately considered as a redistribution process of certain chemical moieties, which is driven by the local differential chemical potential of each species. 

In the case of a polymer surface treated to make it hydrophilic, the value of js is significantly large, and its reduction by virtue of surface configuration change is generally observed. A plasma-modified hydrophilic surface often loses its gained hydrophilicity in a week to 2 months depending on the nature of substrate polymer and of the treatment. 

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