CONTACT-ANGLE RELAXATION ON LOW-ENERGY SURFACES

The spontaneous relaxation of the water-vapour interface, in contact w ith the ''low energy'' solids poly(ethylene terephthalate) (PET) and p oly(methyl methacrylate) (PMMA), has been investigated after forced ad vancing and receding movement. A tensiometric apparatus has been used to probe the three-phase contact zone during relaxation for periods of up to 24 h. The measured force-time trace differs from that which mig ht be anticipated, due to extraneous phenomena such as bulk evaporatio n/condensation/absorption. It is clear that even for a pure low viscos ity liquid such as water, an imbalance in interfacial energies in the neighbourbood of the three-phase contact line (TPCL) exists for extend ed periods. Measurements have been made at both high and low water vap our pressures. In the advancing case, the TPCL is pinned and equilibra tion occurs via an essentially evaporative mechanism. In the receding case, the TPCL is mobile and the approach to equilibrium, which also o ccurs by an evaporative mechanism, is much slower. Indeed our results lead us to question whether equilibrium in the receding case is experi mentally accessible. Contact angle hysteresis was found to be signific antly smaller in magnitude than that typically measured within seconds or a few minutes of the cessation of forced TPCL movement.