Gels of cysteine/Ag-based dilute colloid systems and their rheological properties

Abstract

The rheology of various systems consisting in L-cysteine, Ag, and aqueous solutions of different electrolytes has been studied. These systems can form gels at very low concentration of cysteine not exceeding 0.036 wt%. In periodic low amplitude oscillations, the gels behave as linear elastic media with frequency independent modulus and low mechanical losses in a wide frequency range. The modulus values lie in the range of 10–100 Pa. Meanwhile these systems demonstrate the dualism in the rheological behavior. In the stress-controlled shearing, “gels” behave not like elastic but like visco-plastic media. At low stresses, they flow with Newtonian viscosity of the order of 103–105 Pa s. However at some critical stress (“the yield stress”), the apparent viscosity falls by 6–7 decimal orders. Special rheological experiments demonstrated that this is not a bulk effect but most likely a consequence of the stream separation (“shear banding”). This effect is reversible and the thixotropic formation of the initial structure and properties takes place in the time-scale of the order of minutes. It was argued that the structural coagulation network is formed by the Ag nano-particles stabilized against sedimentation by L-cystine appeared as a result of the cysteine oxidation.