On the rheological properties of Silica – Poly(ethylene-oxide) dispersions: "Shake gels" III The effect of salt concentration

Abstract

The investigation focused on the rheological impact of sodium chloride and calcium chloride on PEO silica water mixtures, specifically studying their gelation/shear thickening behaviour referred to as "shake gels". Monitoring viscosity over time at a constant shear rate revealed a significant increase in viscosity by several orders of magnitude at a specific time point. Interestingly, higher concentrations of sodium chloride and calcium chloride correlated with shorter times required for gel formation.Following gel formation, dropping the shear rate facilitated the observation of the gel's relaxation back into a liquid state. Notably, higher concentrations of sodium chloride or calcium chloride resulted in slower relaxation processes. Furthermore, a notable difference emerged: at a consistent shear rate, calcium chloride exhibited faster gelation compared to sodium chloride, while during relaxation, calcium chloride relaxed at a slower pace than sodium chloride. This behaviour can be attributed to the effect of salt on polymer molecules. The presence of salt results in reduction in Debye length and polymer gets contracted. Consequently, shearing becomes more easier even though at lower shear rate as salt concentration increases. In contrast, during the relaxation process, polymer restructuring is hindered due to electrostatic repulsion and Debye length decreases, resulting in slower relaxation as salt concentration increases. The variations in gelation and relaxation behaviour between different salts can primarily be attributed to the differing ionic strengths exhibited by these salts.