Measuring the surface tension of yield stress fluids

Abstract

With the aim of studying the impact of capillary forces on the flow of yield stress fluids we investigate the properties of a film formed by withdrawing a blade from a bath of such a material. We show that before a progressive breakage of the film, the force amplitude reaches a maximum which is independent of the initial depth of penetration and the timing for blade lifting, but increases with the material yield stress and the blade thickness. This critical force is shown to reflect both capillary and viscous effects, even at vanishing blade velocity. We demonstrate that the ratio of this force to the blade perimeter provides the surface tension of the yield stress fluid in the limit of a low (≪1) capillary number (ratio of yield stress times the blade thickness to surface tension). Moreover we show that all our data for the force to perimeter ratio fall along a master curve which may be used to deduce the surface tension from measurements obtained at a capillary number up to 1, even if viscous effects are significant. Finally Carbopol gels appear to have almost the same value of surface tension whatever their yield stress, but this value is almost 10% smaller than that of pure water.