Affirmation of the effect of pH on shake-gel and shear thickening of a mixed suspension of polyethylene oxide and silica nanoparticles

The mixture of silica nanoparticles and polyethylene oxide (PEO) shows unique rheological behaviors, such as reversible gelation and shear thickening, when moderate shear is applied. These behaviors are attributed to the transient bridging network, which is formed via the simultaneous adsorption of a PEO chain onto multiple silica nanoparticles. The adsorption affinity depends on pH and thus, we expect that these behaviors change with pH. Nevertheless, the effect of pH on shake-gel behavior and shear thickening has not yet been systematically examined. In order to improve our understanding, we attempt to study the influence of pH and ionic strength on the relaxation time of the gel and the viscosity of the mixture. Our experimental results demonstrate that shake-gel and shear thickening can be observed in the pH range of 8.0-9.9. Moreover, the relaxation time required for the gel recovery to sol increases and the critical shear rate at which the viscosity begins to rise decreases as the pH decreases. Furthermore, we determined that irreversible shake-gels can be obtained in a narrow range of PEO concentrations at the pH of approximately 8.0. In conclusion, we determined that the relaxation time of gel is longer at low pH, and the critical shear rate decreases with the decrease in interparticle repulsion of silica.

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