Contact angle relaxation on low energy surfaces

The spontaneous relaxation of the water—vapour interface, in contact with the “low energy” solids poly(ethylene terephthalate) (PET) and poly(methyl methacrylate) (PMMA), has been investigated after forced advancing 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 might be anticipated, due to extraneous phenomena such as bulk evaporation/condensation/absorption. It is clear that even for a pure low viscosity liquid such as water, an imbalance in interfacial energies in the neighbourhood of the three-phase contact line (TPCL) exists for extended periods. Measurements have been made at both high and low water vapour pressures. In the advancing case, the TPCL is pinned and equilibration occurs via an essentially evaporative mechanism. In the receding case, the TPCL is mobile and the approach to equilibrium, which also occurs by an evaporative mechanism, is much slower. Indeed our results lead us to question whether equilibrium in the receding case is experimentally accessible. Contact angle hysteresis was found to be significantly smaller in magnitude than that typically measured within seconds or a few minutes of the cessation of forced TPCL movement.