  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.
