On the effect of relaxation time in interfacial tension measurement

The time scale to attain equilibrium for interfacial tension measurement by means of the well known Wilhelmy plate method has been investigated. Despite that interfacial tension is independent of liquid viscosity, the measurement technique shows considerable dependency on viscosity in terms of relaxation time. For most practical liquids, density variation is insignificant, however, the viscosity often varies in the order of magnitude and indeed dictates the waiting or relaxation time for the measurement system to acquire steady equilibrium state. In this study, the quantified argument is made to define the relaxation time based on the known liquid properties while measuring unknown interfacial tension. This is of particular importance when assessing the dynamic interfacial tension of highly viscous liquids containing surface active molecules, as it allows to predict the time after which a reliable measurement of the interfacial tension is possible.