CONTACT-ANGLE OF WATER ON POLYMER SURFACES

Sessile droplet (advancing and receding) contact angles of water on su rfaces of gelatin gel (15% solid), agar gel (2% solid), and polytetraf luoroethylene (PTFE) film were investigated at 20-degrees-C. The advan cing contact angle of water increases with the postcoagulation time of the gels and reaches a stationary value (the equilibrium contact angl e) in approximately 1 h. The value of the advancing contact angle of w ater is around 75 and 25-degrees for the gelatin gel and the agar gel, respectively, and 100-degrees for PTFE. The advancing contact angle i s dependent on the relative humidity of air, and the lower the relativ e humidity, the higher is the contact angle. The gelatin gel shows a p ronounced hysteresis of contact angle (discrepancy between advancing a nd receding contact angles), while the agar gel shows very little diff erence between the advancing and receding contact angles, partly due t o the low advancing angle. Advancing and receding contact angles are n early identical for the PTFE surface, and also the surface area under the droplet in the advancing and receding processes are identical. Wit h gelatin hydrogels, the contact area does not decrease according to t he decrease of drop volume during the receding process (merely flatten s down the profile of the sessile droplet), whereas the contact area w ith the agar gel decreases with a slight hysteresis effect. The hyster esis effect observed with a polymeric film by the sessile droplet cont act angle measurement is largely attributable to the creation of an at tractive force between the water and polymer molecules, which results from the change of the surface configuration to establish a new equili brium between liquid water and the surface state of the contacting pol ymer solid.