CONTACT-ANGLE OF WATER ON POLYMER SURFACES

Sessile droplet (advancing and receding) contact angles of water on surfaces of gelatin gel (15% solid), agar gel (2% solid), and polytetrafluoroethylene (PTFE) film were investigated at 20-degrees-C. The advancing contact angle of water increases with the postcoagulation time of the gels and reaches a stationary value (the equilibrium contact angle) in approximately 1 h. The value of the advancing contact angle of water is around 75 and 25-degrees for the gelatin gel and the agar gel, respectively, and 100-degrees for PTFE. The advancing contact angle is dependent on the relative humidity of air, and the lower the relative humidity, the higher is the contact angle. The gelatin gel shows a pronounced hysteresis of contact angle (discrepancy between advancing and receding contact angles), while the agar gel shows very little difference between the advancing and receding contact angles, partly due to the low advancing angle. Advancing and receding contact angles are nearly identical for the PTFE surface, and also the surface area under the droplet in the advancing and receding processes are identical. With gelatin hydrogels, the contact area does not decrease according to the decrease of drop volume during the receding process (merely flattens down the profile of the sessile droplet), whereas the contact area with the agar gel decreases with a slight hysteresis effect. The hysteresis effect observed with a polymeric film by the sessile droplet contact angle measurement is largely attributable to the creation of an attractive force between the water and polymer molecules, which results from the change of the surface configuration to establish a new equilibrium between liquid water and the surface state of the contacting polymer solid.