ABSTRACT
Regardless of the term ‘‘advancing’’ or ‘‘receding,’’ static contact angle measurement is performed while the three-phase contact line water/air/surface is stationary. By simply adding more water to the existing droplet a series of advancing contact angles are measured. The increase of droplet volume forces the three-phase contact line to always advance outward in the direction of the dry surface. The contact angle that is independent of the droplet size is taken as the advancing contact angle, which represents the surface property of a surface. The receding contact angle is measured after the advancing contact angles were measured in several steps by withdrawing a certain volume (the same volume aliquot used in the advancing contact angle measurements) from the droplet in several steps. Advancing and receding contact angles for various polymers are shown in Figures 26.5. Advancing and receding contact angles for the same polymers coated with plasma polymer of trimethylsilane (TMS) and of (TMSþO2) are shown in Figure 26.6. The following 10 different conventional polymer samples were used: polytetrafluoroethylene (PTFE), ultrahigh molecular weight polyethylene (UHMWPE), polypropylene (PP), low-density polyethylene (LDPE), high-density polyethylene (HDPE), polycarbonate (PC), polyethyleneterephthalate (PET), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF), and nylon 6 (nylon). Each polymer plate was cut approximately 20 25mm from a larger sheet with a thickness of 1mm, except for PVDF which was 0.8mm thick. Only untreated polymers including those prior to plasma treatment were cleaned in an ultrasonic soap water bath, thoroughly rinsed with deionized (DI) water, dried completely in air, and finally placed in a desiccator with calcium sulfate (CaSO4).
