4. Conclusions
We have demonstrated that 3.5 Pa was the highest shear stress which yielded “creep flow” (very slow flow dominated by elastic response) during a creep-recovery-creep test carried out for a fluid gel containing 0.2 wt% gellan gum and 0.22 M Na+ as gel-promoting ion. This was consistent with the response of a solid-like material under creep flow. A slight increase in shear stress up to 3.7 Pa provoked a dramatic change in the rheological response of the fluid gel studied since clear fluid-like behaviour was observed. Therefore, we assigned a value of 3.6 Pa for an apparent, or better, practical yield stress. Above the practical yield stress value, a shear rate close to the steady state value was quickly reached, the elastic recovery was negligible and the viscosity versus time curve illustrated the occurrence of thixotropy with an incomplete recuperation of the initial viscosity after 40 s of recovery. The instability of the viscosity versus time curves after the first creep test at 3.7 Pa was ascribed to the proximity to the yield stress of these fluid gels. Between 4.0 and 6.0 Pa, the response became more stable, but similar to that found at 3.7 Pa. Additionally,the shear rate dependence of viscosity derived from independent creep-recovery-creep tests illustrated a very shear thinning behaviour, with a power law index of 0.3. The results obtained support the conclusion that the presented creep-recovery-creep test is a good method to determine both the practical yield stress and the inception of non-linear timedependent rheological behaviour of very shear thinning structured materials, such as low-acyl gellan gum fluid gels.
