Electrorheology and particle dynamics of single-wall-carbon-nanotube suspensions under shear and electric fields.

摘要

Electrorheological (ER) fluids are smart materials consisting of polarizable particles in an insulating liquid. Under an electric field, the dispersed particles develop an induced dipole moment and interact with each other to form chains or fibrous structures. This anisotropic microstructure enables ER fluids to have reversible changes on their macroscopic rheological properties, such as apparent viscosity and yield stress. As such, electrorheological fluids have potential application in the control of devices such as dampers, clutches, and robotics.;Single-wall-carbon-nanotubes (SWNTs), because of their nanoscale size, large aspect ratio and high polarizability, are of interest as a possible dispersed phase of novel, highly efficient ER fluids. In this work, we experimentally demonstrated for the first time the ER response of dilute SWNT suspensions, with a more-than-doubling of the apparent viscosity at moderate shear rates for a SWNT volume fraction of just Φ = 1.5×10-5. By systematically varying the shear rate and electric field, we found that the electrorheological response can be interpreted in terms of an electrostatic-polarization model, where the governing parameter was a modified Mason number giving the ratio of viscous to dipole-dipole forces. Analysis of the electrostatic forces suggested that the magnitude of the electrorheological response in the dilute SWNT suspension, which was much higher than conventional electrorheological fluids of comparable volume fractions, was due to the high aspect ratio of the nanotubes.;Further studies of the particle dynamics and electrorheology of SWNT suspensions were made to better understand the possible connection between the macroscopic rheology and microscopic particle dynamics. Using an optical polarization-modulation method and a modified concentric-cylinder viscometer, the first experimental measurements were made of ensemble-averaged SWNT orientation angles under combined shear flow and electric fields. The particle-orientation response was found to occur on time scales one to two orders of magnitude faster than the macroscopic electrorheological response, indicating that the particle orientation does not directly affect the apparent viscosity at these low concentrations. Consistent with the theory developed by Mason and coworkers for ellipsoidal particles, the equilibrium particle-orientation angles for various shear rates and electric fields collapsed when plotted against a parameter giving the ratio of electrostatic-to-shear-flow torques. However, the measured equilibrium orientation angles for the SWNTs showed poor quantitative agreement with the classical model. Analysis of the electrostatic interaction torques between large-aspect-ratio SWNTs showed that the interactions are significant in spite of the diluteness of the suspension, and likely account for the discrepancy between the measurement and predicted particle orientation angles.