Shear-induced reversible gelation of nanoparticle suspensions flocculated by poly(ethylene oxide)

The steady-shear viscosity and dynamic viscoelasticity were measured for suspensions of silica nanoparticles dispersed in aqueous solutions of poly(ethylene oxide) (PEO). When the particle and polymer concentrations are increased above some critical values, the suspensions show striking shear-thickening profiles and change from fluids with low viscosity to gel-like pastes in high shear fields. At the same time, the shear-thickening suspensions are converted from transparent to turbid systems. After the shear thickening, the suspensions reversibly regain the high fluidity and transparency in a quiescent state. Through the measurements of dynamic viscoelasticity and light scattering, it is confirmed that the timescales in mechanical relaxation are comparable to those of optical clearing in the recovering processes. The correspondence in timescales indicates that the structural changes due to flocculation determine the rheologial and optical properties. Therefore, the shear-thickening behavior of nanoparticle suspensions flocculated by polymer bridging can be attributed to the transient network formation by the shear-induced flocculation.