Study of Pre-Shearing Protocol and Rheological Parameters of Shear Thickening Fluids Containing Nano Particles

Abstract

Suspensions having viscoelastic property are ubiquitous and find applications in variety of fields. One such suspension is that of the shear thickening fluid, which is non- Newtonian in nature and shows an increase in viscosity when subjected to shear. Thus, these fluids have the ability to change from liquid to solid phase when they undergo shear and then return back to their original state when the shear is released. This reversible nature is highly useful in defence applications. Previous studies show that the shear thickening fluids have the potential of impact and stab resistance and can be used as a coating material on Kevlar fabric to be used as a soft body armour. With modifications in the fluids, these armours can be made more durable and light weight, which will help in increasing the flexibility of the soldier. Like most of the viscoelastic materials, shear thickening fluids are also made up of polymers, and have a tendency to store its previous shear history, thereby making it a memory fluid. In the present work, a protocol has been developed in order to remove this shear history from the fluids and thus enhance its properties. The shear thickening fluid for which such protocol has been developed is a bi-dispersed mixture of fumed silica (11 nm) and simple silica (400 nm). The past history was removed using the pre-shearing technique. Such technique has been used for the first time for any such bi-dispersed suspension composed of different sized particles. The rheological protocol developed for such a shear thickening fluid is quite different from that reported in the past. Using this method it can be observed how much initial shear and rest period is required by the sample so as to become stable and provide reproducible rheological results. The outcomes have been found to indicate a significant increase in critical viscosity of the material. Another important feature which affects the thickening behaviour of these fluids is the particle shape. Elongated particles provide high viscosity to the fluid whereas spherical particles show thickening behaviour at higher shear rates. Thus, composite shear thickening fluid was prepared using halloysite nanotubes (rods) and fumed silica (spheres) and its rheological properties were studied. A better non-flocculated structure was obtained at 1% halloysite and 20% fumed silica composition compared to that of only spherical particle STF. The oscillatory tests showed that this composition was also suitable as a high impact resistant material as no gel formation takes place rather the fluid behaves like a dispersed sol. Furthermore, composites having higher percentages of halloysite nanotubes were also prepared and a comparison of the rheological properties of the three halloysite nanotubesfumed silica composites was done.