Martin-Gallego et al. Nanoscale Research Letters 2011, 6:610 http://www.nanoscalereslett.com/content/6/1/610 where Ke and Kf are the effective thermal conductiv- ities of the suspension and the base fluid, respectively; α = Kp is the K ratio, Kp is the particle conductivity, n Kf is the particle shape factor (n = 6 for cylinders, n = 3 for spheres in which case, Equation 1 reduces to the Maxwell model), and is the particle volume fraction calculated using the true density of the nanotubes [31]. The theoretical thermal conductivities are calculated with both values due to the bent conformation adopted by CNTs when dispersed in a matrix; thus, their shape factor would be between 3 and 6. Figure 4 compares the experimental results for the MWNT samples with the theoretical ones from the proposed model. We observe an anomalous enhancement of the experi- mental K. This behavior could be related to two effects. The first effect is the presence of an organized structure of the molecules in the liquid state at the solid/liquid interface that facilities the coupling between the solid particles and the fluid [32]. The second effect could be contributions from the Brownian motions of the parti- cles that modify the heat transfer in the fluid [33]. We finally measured the K of the cured samples for some of the nanocomposites with a classical hot-plate apparatus. TEM microphotographs show a finely and homogeneous dispersion of the carbon nanostructures, MWNTs, and FGS in the cured epoxy samples (Figure 5). The improvements obtained for the cured MWNT Page 5 of 7 nanocomposites are approximately the same as those in the liquid state and are in agreement with the data found in the literature [27,28,34]. The cured FGS sample revealed a similar enhancement of the K as the MWNT sample. This increase of K in cured epoxy resin due to the addition of FGS has already been reported [35,36], but not the uncured/cured transition. This transition suggests that the results can be attributed to the differ- ences in the media surrounding the nanoparticles when the resin is in the liquid or solid state. While the FGS are dispersed in a liquid media, they are not able to transfer the heat because the vibrational modes are not compatible. However, when the FGS are surrounded by the more rigid cured matrix, the differences between the frequencies of vibrational modes are smaller and enable phonon coupling. This result corroborates a current the- ory postulating that the dominant factor in nanocompo- site heat conduction is the low frequency modes and their coupling with high vibrational modes at the inter- face [37,38]. This transition does not exist in the MWNT samples because the phonon transport through the inner tubes should be relatively unperturbed by the surrounding matrix. Conclusions We employed an easy and direct method based on the hot wire technique to measure the thermal conductivity of epoxy nanofluids. We also studied the differences in Figure 4 Comparison of the measured data for MWNT nanofluids and the theoretical values.