Yield stress and elasticity influence on surface tension measurements

TL;DR

This study investigates how yield stress and elasticity affect surface tension measurements in carbopol gels, revealing that flow history and elastic properties influence effective surface tension readings.

Contribution

It introduces a method to measure effective surface tension considering flow history and models the influence of yield stress and elasticity on surface tension measurements.

Findings

Effective surface tension depends on flow history (stretching vs. compression).

Yield stress causes differences in measured surface tension based on deformation direction.

Elastic modulus relative to yield stress determines if the measured surface tension reflects the true thermodynamic value.

Abstract

We have performed surface tension measurements on carbopol gels of different concentrations and yield stresses. Our setup, based on the force exerted by a capillary bridge on two parallel plates, allows to measure an effective surface tension of the complex fluid and to investigate the influence of flow history. More precisely the effective surface tension measured after stretching the bridge is always higher than after compressing it. The difference between the two values is due to the existence of a yield stress in the fluid. The experimental observations are successfully reproduced with a simple elasto-plastic model. The shape of successive stretching-compression cycles can be described by taking into account the yield stress and the elasticity of the gel. We show that the surface tension $\gamma_{LV}$ of yield stress fluids is the mean of the effective surface tension values only if the elastic modulus is high compared to the yield stress. This work highlights that thermodynamical quantities measurements are challenged by the fluid out-of-equilibrium state implied by jamming, even at small scales where the shape of the bridge is driven by surface energy. Therefore setups allowing deformation in opposite directions are relevant for measurements on yield stress fluids.