Nanotubes as shock absorbers
doi:10.1038/nindia.2011.85 Published online 21 June 2011
Researchers have uncovered new phenomena associated with a particle's viscosity and flow by suspending multiwalled carbon nanotubes (MWNTs) in an organic solvent. The newly explored properties of MWNTs in suspension could have important applications in the design of smart materials such as soft body armours and shock absorbers.
An increase in fluid viscosity with rising flow rate is known as shear thickening. It is of great interest in materials science for designing smart materials. Fluids that exhibit shear thickening contain dense suspensions of either Brownian or non-Brownian spherical or rod-like particles. These particles come out of suspension in the form of flocs or flakes.
The mechanism of shear thickening in non-Brownian flocs is distinct from that of Brownian particles driven by hydrodynamic interactions. However, the shear thickening behaviour of non-Brownian fractal flocs, such as those formed by MWNTs suspended in an organic solvent, has yet to be explored. To investigate this area, the researchers suspended MWNTs at very low weight fractions (around 1–5%) in the organic solvent N methylpyrrolidone. They then used a rheometer to study how the suspension behaves in the presence of varying shear stresses.
The researchers varied the shear stress in the range of 0.01–20 Pa. In the initial state — known as the jammed state — the flocs were locked together. However, for shear stresses greater than 0.01 Pa, the MWNT flocs aggregated under flow to form denser structures that transform into rolling log-like flocs of length 200–500 μm.
At stresses of around 0.05 Pa, the MWNT flocs moved to a shear thinning state, in which the floc size decreased due to the shear stress. This state precedes the shear thickening state. "By controlling the flow processing conditions suitably in polymer nanotube composites, it is possible to enhance their mechanical strength and electrical conductivity significantly at low nanotube weight fractions," says lead researcher A. K. Sood.
The authors of this work are from: the Department of Physics, Indian Institute of Science, and Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, India.