Adding small particles into a fluid in cooling and heating processes is one of the methods to increase the rate of heat transfer by convection between the fluid and the surface. In the past decade, a new class of fluids called nanofluids, in which particles of size 1-100 nm with high thermal conductivity are suspended in a conventional heat transfer base fluid, have been developed. It has been shown that nanofluids containing a small amount of metallic or nonmetallic particles, such as Al 2 O 3 , CuO, Cu, SiO 2 , TiO 2 , have increased thermal conductivity compared with the thermal conductivity of the base fluid. In this work, effective thermal conductivity models of nanofluids are reviewed and comparisons between experimental findings and theoretical predictions are made. The results show that there exist significant discrepancies among the experimental data available and between the experimental findings and the theoretical model predictions.

We present a method to fill 2−5-nm-diameter channels of closed multiwalled carbon nanotubes (MWNT) with an aqueous fluid and perform in situ high-resolution observations of fluid dynamic behavior in this confined system. Transmission electron microscope (TEM) observations confirm the successful filling of two types of MWNTs and reveal disordered gas/liquid interfaces contrasting the smooth curved menisci visualized previously in MWNT with diameter above 10 nm. Electron energy loss spectroscopy (EELS) and energy dispersive spectrometry (EDS) analyses, along with TEM simulation, indicate the presence of water in MWNT. A wet−dry transition on the nanometer scale is also demonstrated by means of external heating. The results suggest that when ultrathin channels such as carbon nanotubes contain water, fluid mobility is greatly retarded compared to that on the macroscale. The present findings pose new challenges for modeling and device development work in this area.

A hydrothermal method of catalytic nanotube synthesis has been shown to produce high-aspect-ratio, multiwall, capped carbon nanotubes, which are hollow and contain a high-pressure encapsulated aqueous multicomponent fluid displaying clearly segregated liquid and gas by means of well-defined curved menisci. Thermal experiments are performed using electron irradiation as a means of heating the contents of individual nanotubes in the high vacuum of a transmission electron microscope (TEM). The experiments clearly demonstrate that TEM can be used to resolve fluid interface motion in nanochannels. Good wettability of the inner carbon walls by the water-based fluid is shown. Fully reversible interface dynamic phenomena are visualized, and an attempt is made to explain the origin of this fine-scale motion. Experimental evidence is presented of nanometer-scale liquid films rapidly moving fluid along the nanochannel walls with velocities 0.5 μm/s or higher.