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Non-Tech : US Global Nanospace (USGA) -- Ignore unavailable to you. Want to Upgrade?

To: scion who wrote (49)	2/21/2004 11:07:48 PM
From: scion	Respond to of 132

Mixing up nanotubes causes complications 19 February 2004 As applications of polymer-nanotube composites take off, mixing up suspensions of nanotubes will be an increasingly important processing operation. But researchers at the National Institute of Standards and Technology (NIST), the University of Kentucky and Michigan Technological University, all in the US, have found that certain mixing conditions may actually cause the nanotubes to separate. “We were using light scattering to look at how simple shearing flows align semi-dilute carbon nanotubes suspended in a polymer,” Erik Hobbie of NIST told nanotechweb.org, “and we noticed that at low shear rates the tubes were aggregating, while at high shear rates they dispersed.” Hobbie and colleagues used an optical flow cell to measure the force needed to mix different concentrations of multiwalled carbon nanotubes. They found that under some weak shear conditions the initially homogenous suspensions grew macroscopic striped domains. These patterns showed extended correlation along the direction of vorticity (or rotation) and periodicity along the direction of flow, as well as negative first normal stress differences. Scientists have already seen such behaviour in nematic liquid-crystalline polymers, but it is rare and not well understood. “Practically speaking our work is important because it shows that nanotubes can actually de-mix (instead of mix) under simple shearing flows, which are often used to homogenize multi-phase complex fluids,” added Hobbie. “Although this tendency emerges at low-to-modest rates of flow, it is relevant because almost all processing flows are pressure driven, which leads to broad regions of weak-to-no shear in the middle of a channel or pipe. Nanotubes in these regions clump together instead of mixing, in effect ruining the process.” Now the researchers are investigating using simple flows to orient and eventually fractionate carbon nanotubes. “We have had some modest success with this and will be showing it at the March Meeting of the American Physical Society in Montreal,” said Hobbie. The scientists are also studying flow-induced nanotube clustering in concentrated nanotube suspensions, and are “finding some fascinating results that are related to both the concept of ‘jamming’ and ‘elastic turbulence’”. The researchers reported their results in Physical Review Letters. About the author Liz Kalaugher is editor of nanotechweb.org. nanotechweb.org