Power-law creep and residual stresses in a carbopol gel

We report on the interplay between creep and residual stresses in a carbopol microgel. When a constant shear stress sigma is applied below the yield stress sigma(y), the strain is shown to increase as a power law of time, gamma(t) = gamma(0) + (t/tau)(alpha), with an exponent alpha = 0.39 +/- 0.04 that is strongly reminiscent of Andrade creep in hard solids. For applied shear stresses lower than some typical value sigma(c) similar or equal to 0.2 sigma(y), the microgel experiences a more complex, anomalous creep behavior, characterized by an initial decrease of the strain, that we attribute to the existence of residual stresses of the order of sigma(c) that persist after a rest time under a zero shear rate following preshear. The influence of gel concentration on creep and residual stresses are investigated as well as possible aging effects. We discuss our results in light of previous works on colloidal glasses and other soft glassy systems.