Viscosity and dynamic surface tension measurement: A guideline for appropriate measurement

Dynamic surface tension measurements play a critical role in interfacial activities for liquids with varying viscosities. Understanding the rate at which the interface attains the equilibrium, for surface tension measurements, after the formation of a new interface is of significant interest. Although surface tension is independent of dynamic viscosity, the time required for a new surface to form (equilibrium/relaxation time) is influenced by viscosity. The measured surface tension value is accurate only once these viscous effects have subsided. Therefore, the relaxation time represents the minimum surface age value achievable during the measurement process. We experimentally established the minimum surface age needed to measure the dynamic and static surface tension of a fluid with a specific viscosity using three widely used methods: the Pendant drop method, the Wilhelmy plate method, and the Bubble pressure method. We propose a guideline with a phase plot that helps to choose the most suitable method and the youngest achievable surface age for an accurate measurement, independent of viscous effects. This guideline enables users in diverse applications such as 3D printed clothing, spray paint, coating, etc., to accurately measure dynamic and static surface tension without being influenced by viscosity effects.