ABSTRACT
For a highly hydrophobic polymer surface, which has minimal w interaction with water, the surface configuration change in the g interaction may be the major factor for the overall surface state change in the case of liquid water immersion. For a hydrophilic polymer surface, the dominant factor in the overall surface state change is the change of the mobility of polymers in the surface state because the w interaction of water with polymer is high (if the change involves the transformation of a dry polymer to a water-swollen polymer). If no change in the swelling (w interaction) is involved, such as the case when the surface of a swollen hydrogel in water is exposed to air, the surface configuration change due to the g interaction is the main event in the overall change of the surface state. In this case, the surface configuration change occurs in a nearly reversible manner because the chain mobility in highly swollen hydrogel is very high, and the surface configuration change can proceed nearly instantaneously. Because of this situation, the sessile droplet contact angle of water on a gelatin gel is always high, but the same surface under water shows very low contact angle (by bubble method) immediately after the surface was immersed in water.
