DYNAMIC CONTACT ANGLES AND CONTACT-ANGLE HYSTERESIS OF PLASMA POLYMERS

Dynamic advancing and receding contact angles and the contact angle hy steresis for different plasma polymers deposited on microscope cover g lasses were investigated by the Wilhelmy plate method. The hysteresis loops observed by this dynamic method show two major factors: (1) the meniscus change at the transition region; (2) the wettability change o f solid surfaces. The first factor is due to the fact that three-phase contact line does not move in the transition region while the water l evel is forced to rise up or lower down. The second factor is due to t he change of surface state of the solid. Moreover, this factor can be related to the ''step change'' at the buoyancy slope of a hysteresis l oop when the wetting line is moved from a ''prewetted'' area to a ''dr y'' area of the same sample and also to the gap between the first and second immersion loops. Both factors are caused by the interaction for ces between solid surface and water molecules at the solid/liquid inte rface. Plasma polymer of the tetrafluoroethylene (TFE) showed the larg est hysteresis loop primarily due to the significant change of surface state after water immersion. Plasma polymer of hexamethyldisiloxane ( HMDSO) showed the smallest hysteresis loop due to the minimal change i n the surface state after water immersion. Water contact angles of pla sma polymers were clearly dependent on plasma conditions manifested by the discharge energy density.