Static and dynamic contact angles of water on polymeric surfaces

Static contact angle and dynamic (advancing and receding) contact angles of water on polymeric surfaces were investigated using microscope cover glasses coated with various plasma polymers of trimethylsilane and oxygen. By variation of the mole fraction of the TMS/oxygen mixture, glass surfaces having varying degrees of wettability were prepared. The advancing angle of a sessile droplet, which is independent of the droplet volume, is considered as the static contact angle of water on a polymeric surface, 0s, which is a parameter characteristic to a polymeric surface. The dynamic contact angle of water refers to the contact angle of which three-phase contact line is in motion with respect to the surface. The dynamic advancing (immersing) contact angle, 0(D,a), and receding (emerging) contact angle, 0(D,r), were measured by the Wilhelmy balance. The difference between 0(D,a) and 0(D,r) is mainly due to the direction of dynamic force acting on the three-phase contact line. The discrepancy between the immersion and the emersion buoyancy lines and the corresponding values of contact angles can be used to indicate the hysteresis due to the dynamic factor (the dynamic hysteresis). The dynamic hysteresis is largely determined by the critical immersion depth in which the three-phase contact line remains at the same place on the surface while the shape of meniscus changes when the motion of the sample is reversed. The dynamic hysteresis may contain the contribution of the change of static contact angle due to the surface-configuration change caused by the wetting of the surface (the intrinsic hysteresis). The dynamic hysteresis varies according to the value of cos 0(s), with the maximum at the threshold value around 0.6 and linearly decreases above this value, as the emersion line approaches the limiting buoyancy line determined by the surface tension of the liquid. The intrinsic hysteresis follows the same trend with the maximum at around 0.8. The three contact angles are related by cos 0(s) = (cos 0(D,a) + cos 0(D,r))/2.