Experimental study and modeling of the energy density and time-dependent rheological behavior of carbon nanotube nanofluids with sonication

Nanofluids are colloidal suspensions of nanoparticles in liquids. Nanoparticles have superior properties, which means that they can improve the performance characteristics of nanofluids. To press their advantages, the characteristics of nanofluids should be investigated to understand their underlying mechanisms. In this sense, fluidity is an important concept for examining the colloidal structure and interactions among nanoparticles based on their morphologies and intrinsic properties. Viscosity determines the fluidity of nanofluids, thereby indicating their degree of resistance to deformation and molecular dynamicity. Herein, we experimentally studied the rheological behavior of sonicated nanofluids with multi-walled carbon nanotubes (MWCNTs), which had long, tube-like, and flexible structures. The change in viscosity at an increasing energy density (E rho) due to sonication at the specific total energy (E-T) indicates that the nanotubes experienced a size phase change from "debundling" to "nano-cutting". These phases agree well with the results of the hydrodynamic radii from dynamic light scattering (DLS) measurements. In the debundling phase, the unordered colloidal structure due to randomized entanglement increased the resistance to shear flow, thereby leading to a higher viscosity. With higher E-rho , the nanotubes broke owing to nano-cutting. In this phase, the short and exfoliated nanotubes functioned as rigid rods in the nanofluids and tended to align easily under shear flow. Finally, we discovered a well-fitted correlation between the viscosity of the nanofluids and sonication conditions (i.e., E-rho and E-T). We found that the characteristic change in the colloidal structure of the MWCNT nanofluids was reflected in their rheological behavior during their preparation via sonication. Furthermore, the relationship between sonication conditions and viscosity helped us to predict the colloidal structure of the MWCNT nanofluids in the early stages of preparation. We expect that this work will contribute to promote the availability of MWCNTs in various application fields in terms of rheology and intermolecular structure. (C) 2022 Elsevier Ltd. All rights reserved.