LAUSR

Abstract The breakpoints of the profiles of the advancing (θa) and receding (θr) contact angles can be used to determine the critical micelle concentration of surfactant solution. Although the breakpoints of θa and θr matched with that of surface tension for some surfactant solutions (i.e. SDS), the physical meaning behind these breakpoints is yet to be ascertained. This study attempts to interpret these breakpoints by examining the evaporation process of the sessile drops of SDS, Triton X-100 and C12E6 solutions on parafilm. The surface area perturbance, during the period θa and θr were detected, resulted in a noticeable deviation of the L-G and L-S interfacial tensions (γlg and γls) from their respective equilibrium states. As θa and θr were detected during the L-G and L-S area perturbations, these deviations of γlg and γls played a crucial role in the relaxation profiles of θa and θr. Furthermore, the relaxations of θa and θr were affected by the extent up to which the γlg and γsl re-equilibrated with the drop concentration. This extent of re-equilibration, in turn, was dependent on the dynamic surface tension of a perturbed interface. Subsequently, it was found out that the breakpoints of θa and θr indicated the lowest bulk concentrations at which the γlg and γsl re-equilibrated when either θa or θr was detected using the evaporation method.