The rheological properties of colloidal suspensions and polymer solutions govern key operations in food processing (mixing, pumping) and determine the texture and swallowability of final products. In recent years, shear-induced gels (also known as shake-gels), which exhibit a fluid-like sol state at rest but transform into a non-flowable gel state under shear, have attracted attention as hydrocolloids whose rheological properties change reversibly in response to external flow. The shake-gels consist of mixtures of high molecular weight polymers and adsorptive nanoparticles. This mechanism is considered to originate from deformation of polymer chains in the flow field and the network formation mediated by transient polymer–particle bonds. Such behavior may provide a new strategy for controlling the rheological properties of suspensions according to shear history. However, the detailed mechanism of shake-gel formation remains unclear, and the conditions that promote gelation have not yet been fully understood.
In this study, we investigated the variation in the onset shear rate of shear-induced gelation under flow. As a representative model system for shear-induced gels, aqueous mixed suspensions composed of silica nanoparticles and poly(ethylene oxide) (PEO) were employed. Particular attention was paid to the polymer dosage per unit particle surface area denoted as Cp. Using a cone–plate viscometer, steady shear was applied to the suspensions initially in the sol state, and the critical shear rate at which shear thickening and subsequent gelation occurred was systematically determined.
The results revealed that the critical shear rate strongly depended on
C
p
. Specifically, shear-induced gelation was most readily triggered at an
intermediate
C
p
, whereas it was suppressed at higher
C
p
. This result can be explained as follows: when
C
p
was too low, polymer chains were insufficient to bridge particles, making
network formation difficult. Conversely, at higher
C
p
, polymer saturation on particle surfaces inhibited interparticle bridging.
At much higher
C
p
, the critical shear rate decreased again, likely due to enhanced polymer
chain overlap and entanglement under flow.
These findings suggest that both the balance between polymer adsorption and interparticle bridging and the flow-induced conformational response of polymer chains are key factors governing the onset of shear-induced gelation.