Sound propagation in light-modulated carbon nanosponge suspensions

Single-walled carbon nanotube bundles dispersed in a highly polar fluid are found to agglomerate into a porous structure when exposed to low levels of laser radiation. The phototunable nanoscale porous structures provide an unusual way to control the acoustic properties of the suspension. Despite the high sound speed of the nanotubes, the measured speed of longitudinal-acoustic waves in the suspension decreases sharply with increasing bundle concentration. Two possible explanations for this reduction in sound speed are considered. One is simply that the sound speed decreases because of fluid heat induced by laser light absorption by the carbon nanotubes. The second is that this decrease results from the smaller sound velocity of fluid confined in a porous medium. Using a simplified description of convective heat transport, we estimate that the increase in temperature is too small to account for the observed decrease in sound velocity. To test the second possible explanation, we calculate the sound velocity in a porous medium, using a self-consistent effective-medium approximation. The results of this calculation agree qualitatively with experiment. In this case, the observed sound wave would be the analog of the slow compressional mode of porous solids at a structural length scale of order of 100 nm.