Instability of the three-phase contact region and its effect on contact angle relaxation

Contact angle relaxation was measured for captive air bubbles placed on sol id surfaces of varying degrees of heterogeneity, roughness, and stability, in water. The experimental results indicate that both advancing and needing contact angles undergo slow relaxation in these water-air-solid systems, d ue to instabilities of the three-phase contact line region. It is shown tha t the advancing contact angle decreases and the receding contact angle incr eases for many systems over a period of a. few hours. Also, examples of rev erse progressions are reported. Additionally, in extreme cases, the contact angle oscillates down and up, over and over again, preventing the system f rom stabilization/equilibration. Four different mechanisms are proposed to explain the contact angle relaxation. These include (i) pinning of the thre e-phase contact line and its slow evolution; (ii) the formation of microdro plets on the solid surface and their coalescence with the base of the gas b ubble, which causes dynamic behavior of the three-phase contact line; (iii) deformation of the solid surface and its effect on the apparent contact an gle; and (iv) chemical instability of the solid.