Characterization of orientation state of carbon nanotubes in shear flow

To characterize and understand carbon nanotube (CNT) orientation state in thermoset composite processing, we surface treated multi-walled carbon nanotubes (MWNT) and suspended them in vinyl ester resin. The surface treatment prevented clustering of the nanotubes and maintained homogeneously separate nanotubes in the suspension ensuring good dispersion quality. The suspension was subjected to shear flow in a micro channel, a nano channel and across a micro cylinder. transmission electron microscope (TEM) was employed to characterize the position of the nanotubes at various locations in the suspension. Second order orientation tensor description was used to describe the MWNT orientations in these flow fields in the 2-D plane. The degree of MWNT alignment was found to increase with increasing shear. However, we did not observe near perfect alignment, as one would expect in case of short fiber suspensions. Although MWNTs diameter is of the order of nanometers, we show by an order of magnitude analysis that Brownian motion is not significant. The MWNT orientations are maintained even after the shear force is discontinued. (c) 2005 Elsevier Ltd. All rights reserved.