The influence of fumed silica content, dispersion energy, and humidity on the stability of shear thickening fluids

Shear thickening fluids (STFs) are smart materials that change from liquid to solid reversibly when undergoing critical stresses. These materials are good alternatives to improve applications where energy dissipation is important, for example, in the fabrication of liquid body armor and shock absorbing protective gear. However, as much as it is known about the effect of several variables on their properties, such as particle concentration and medium viscosity, the stability of these colloidal dispersions over time and over shearing is not yet well understood. The development and design of new applications depend on predicting for how long the material will keep its properties. In this project, we studied the influence of fumed silica content, ultrasonication energy used during dispersion of the silica particles, and humidity during storage to analyze the changes in properties of STFs. The influence of shearing magnitude on their properties was also studied. STFs with higher amounts of silica and produced using less dispersion energy showed the highest viscosity peak on initial tests, but they were also the least stable over time, due to flocculation of the particles. In stable samples, water absorption led to a large loss of maximum viscosity. The presence of humidity on samples diminished the overall viscosity, but did not prevent the sample from becoming a gel if the parameters used resulted in an unstable STF. Shearing the STF reduced its maximum viscosity, being more evident in samples with higher viscosity. Graphical abstract