Revisiting the surface tension of liquid marbles: Measurement of the effective surface tension of liquid marbles with the pendant marble method

The effective surface tension of liquid marbles coated with polyvinylidene fluoride, polytetrafluoroethylene, lycopodium, carbon black and hydrophobized SiO2 powder particles is discussed. It was established with the pendant-droplet method under inflation and deflation (evaporation) of liquid marbles. The effective surface tension depends strongly on the marble volume and demonstrates the pronounced hysteretic behavior. The phenomenological model predicting the linear dependence of the effective surface tension of marbles on their inverse surface area is proposed. The model is validated experimentally. Three “surface phases” are distinguished under inflation and deflation of marbles, some of which are featured by the predicted linear dependence of the effective surface tension on the inverse surface area of marbles. It turned out that the notion of the effective surface tension of a surface covered with solid particles is ambiguous, since this quantity depends on the pathway of its measurement and on the marble size.