Sonication-Driven Colloidal Structure and Rheological Behavior of MWCNT-Based Nanofluids

Nanofluids, comprising nanoparticles dispersed in conventional fluids, have shown remarkable performance enhancements in various applications such as heat transfer, sensors, and tribology. Among the diverse nanoparticles available, multi-walled carbon nanotubes (MWCNTs) have gained significant attention due to their exceptional mechanical and electrical properties, as well as high thermal conductivity. To fully leverage these advantages, it is essential to comprehend the fundamental mechanisms through which MWCNTs influence nanofluid characteristics. Viscosity emerges as a key property of nanofluids, providing crucial insights into their colloidal structure and fluid dynamics. This research focuses on examining the rheological behavior of multi-walled carbon nanotube (MWCNT) nanofluids, with particular emphasis on the effects of sonication, a widely employed preparation technique.
Our study evaluated morphological characteristics of MWCNTs and rheological behavior of MWCNT-based nanofluids depending on the sonication intensity. Morphological analysis revealed that MWCNTs exhibit bundling and entanglement properties, attributable to their high surface-to-volume ratio and van der Waals interactions. These morphological features significantly impact the colloidal structure of MWCNT nanofluids, which undergoes transformations under varying sonication conditions from the initial stages of preparation. Furthermore, MWCNT-based nanofluids showed sonication-dependent rheological behavior, which the viscosity increases with the sonication intensity and decreases at higher intensity of sonication. To elucidate the relationship between rheological behavior and colloidal structure, we quantified sonication intensity using the amplitude, total energy, and time of sonication. Our findings indicate that the size-phase of MWCNTs in colloidal structures transitions from debundling to nano-cutting as sonication energy density increases. This transition plays a crucial role in determining the physical properties of nanofluid. In the debundled state, randomized disentanglement causes MWCNT aggregates to occupy a larger volume within the fluid. This leads to increased viscosity due to higher flow resistance. Conversely, nano-cutting at elevated energy densities results in the formation of shorter, exfoliated nanotubes. In this state, MWCNTs behave like rigid rods, and their self-alignment under shear flow contributes to a decrease in nanofluid viscosity. Our research has established a correlation between viscosity and sonication energy, enabling the estimation of MWCNT nanofluid colloidal structure from the early stages of preparation. This study significantly contributes to the utilization of MWCNTs in nanofluids and facilitates the application of specific colloidal structures in fields where fluid dynamics play a critical role.
In this study, we confirmed that changes in the colloidal structure of MWCNT-based nanofluids due to varying sonication intensities are reflected in the rheological behavior of the nanofluids. Furthermore, we explained the mechanism of relationship between sonication and rheological behavior by using the colloidal structure. The implications of this research extend beyond theoretical understanding, offering practical insights for nanofluid preparation and application. By manipulating sonication parameters, it becomes possible to tailor the rheological properties of MWCNT nanofluids for specific applications, such as thermal management, lubrication, and advanced sensing technologies. Furthermore, this study paves the way for future investigations into the optimization of nanofluid properties, the development of novel nanoparticle-fluid combinations, and the exploration of innovative applications leveraging the unique characteristics of carbon nanotube-based nanofluids.