Influence of dispersion liquid and silica concentration on rheological properties of shear thickening fluids (STFs)

The rheological behavior of shear thickening fluid suspensions synthesized using three dispersion liquids, namely polyethylene glycol, glycerin, and diethylene glycol, having different numbers of hydroxyl groups at the ends of the chain and distinct chain lengths, was researched. The primary objective of this study is to investigate the effects of the length of the chain molecular, the number of –OH groups at the ends of the chain, and the density of dispersion liquids on the rheological behavior. Evaluations were made by taking into account the thickening ratio which expresses the maximum change in the viscosity of the fluid relative to the initial viscosity and the thickening period which states the difference between the shear rate at which the maximum viscosity is obtained and the critical shear rate. As a result of the evaluation made by considering these parameters, the rheological performance of shear thickening fluid suspensions synthesized with liquids having longer molecular chain lengths, higher –OH number, and higher density came to the fore. Samples synthesized with glycerin, which have more hydroxyl groups at the molecular chain ends, provided a more stable distribution by making stronger hydrogen bonds with silica. This situation significantly reduced the thinning behavior in the first region of the rheology curves and provided a stable and continuous thickening behavior after the critical shear rate. In addition, with the increase in the silica ratio, the thickening situation changed from continuous to discontinuous. Increment of silica also decreased the critical shear rate while increasing the initial and maximum viscosity. Increasing the silica content from 22 to 26% resulted in the thickening ratio increasing by 686% from 6.6 to 45 in the samples synthesized with polyethylene glycol while decreasing the thickening period from 559 to 41.2. Similar situations are observed in the samples synthesized with glycerin and diethylene glycol. All of the samples obtained exhibited a reversible behavior rheologically. When the applied shear rate was removed, the sample returned to its former fluid state. Moreover, suspensions synthesized by mixing dispersion liquids showed superior performance compared to single-liquid samples. It is thought that the dispersion liquids interact to form a branched network by making more bonds both with each other and with the silica particles, and it provides an increase in the resistance of the fluid against deformation under high shear stress.