Sound propagation in light-modulated carbon nanosponge suspensions

Abstract

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.

Stimulated Brillouin scattering in dispersed graphene

“…( 8), we can see that its growth is unlikely determined by the Brillouin frequency shift (0 -B), whose change should be rather negative upon irradiation. 29 Thermodynamic studies show a remarkable and monotonous decrease of viscosity (both shear and bulk) of liquids 36 and steam 35 in a large range of temperature and pressure growth. It means that if the Eq.…”

“…Having varied refractive index, density, acoustic velocity, Brillouin line width and thermo-optic coupling coefficient, a comparative examination of the efficiency of each parameter on the Brillouin gain factor reduction has been made. This approach implies the priority of small variations of the substance characteristics in the observed changes of the net effect, and its reliability is based on numerous observations of the relatively small changes in refractive index, 4,28 acoustic velocity, 29,30 thermal expansion coefficient 31 and frequency shifts, 0 -B, 29 occurring at high power laser action on nano-carbon species in suspensions, as well as on the moderate character of known temperature and pressure dependences of density, speed of sound, viscosity, refractive index, and thermal expansion coefficient of pure NMP [32][33][34] and water 32,35 . Some of the parameters depend on others, such as refractive index and acoustic velocity on density.…”