Behavior of Surface-Functionalized Multiwall Carbon Nanotube Nanofluids during Phase Change from Liquid Water to Solid Ice

摘要

Multiwall carbon nanotube (MWCNT) nanofluids have been shown to enhance the crystallization process of water to ice. While the beneficial effects of MWCNTs on phase change processes are well-documented, little work has been conducted to investigate the behavior of MWCNTs during and after exposure to freezing conditions: In this work, the crystallization morphology of water droplets containing surface-functionalized hydrophilic MWCNTs was evaluated at three driving force temperatures and two concentrations of nanofluid. At low supercoolings, the MWCNTs are completely expelled from the crystal matrix due to slow solidification rates. At high supercoolings, the MWCNTs are embedded in the solid droplet within air volumes and interdendritic regions as a result of rapid crystallization speeds. The results show that the dispersion of MWCNTs within the solid ice matrix itself was not achieved at these levels of supercooling. Under all conditions, freezing of the colloidal system results in destabilization the MWCNTs and loss of dispersion. These effects are important considerations for applications requiring successful freeze/thaw cycling of nanofluid systems as well as in the storage and transport of colloidal suspensions.