Credit: © 2007 AIP

Controlling the viscosity of liquid suspensions is important in diverse areas, ranging from preventing heart attacks to the transportation of crude oil through underground pipelines. Liquid suspensions containing nanometre-sized particles are known as nanofluids, and these substances often exhibit unique properties, such as an enhanced heat transfer capability without problems related to clogging and sedimentation.

The dynamic viscosity of nanofluids is, however, not very well understood, especially the effect of the size of nanoparticles in the suspension. Now, Frederic Ayela and colleagues1 from Institut Néel, Grenoble and Université Claude Bernard Lyon 1 in France, have measured the viscosity of silicon dioxide nanofluids under strong shear rates. Using capillary micro-viscometers with integrated local pressure probes, the team was able to directly measure the nanofluid hydrodynamics at the micrometre scale.

An anomalous enhancement of viscosity was observed with increasing volume concentration, owing to size effects of aggregates formed from the nanoparticles. These results do not, however, lead to any direct relationships between the viscosity and thermal conductivity enhancements. The strong shearing forces in the microchannels are thought to affect the size of the aspect ratio of the agglomerates, and may be used for controlling nanofluid viscosity inside microchannels.