Rheology of Nanofluids in Microchannels

Microfluidics is often presented for applications where only microliters sample volumes are available. But the benefits of microchannels do not reduce to a low consumption of fluids. From a physical and mechanical point of view, microfluidics can offer high shear rates combined with low Reynolds number and low viscous heating. It becomes possible to explore high shear rheology on a lab-on-chip. We have micromachined microviscometers to study the rheological properties of nanofluids under very high shear rates conditions. Nanofluids are fluid suspensions of solid nanoparticles. Recent experiments have indicated an anomalous increase in thermal conductivity of these suspensions. But less attention has been payed to the rheological properties of nanofluids. The few results concerning the viscosity of nanofluids exhibit scattered values higher than those of fluid suspensions of microparticles, because of a higher rate of collisions due to Brownian motion and shearing motion which enhance aggregation. These experiments were performed with commercially available rheometers over a limited range of shear rates. Our viscometers on chip are silicon — Pyrex microchannels (H ≈ 10 – 20 μm) equipped with local pressure drop sensors. Nanofluids under test were ethanol-based SiO2 nanoparticles. For particle sizes from 20 nm to 190 nm, and solid volume fractions from 1.4% to 7%, a newtonian behaviour has been observed up to 5.104 s−1. High shear rheology is the only way to reach high Peclet number values with nanoparticles in a laminar flow. It was possible to cover a wide range of Peclet number and to have Pe > 1 with diameter in the tens of nanometers range. Our results have demonstrated that an apparent solid volume fraction φa > φ, due to aggregation, was responsible of the increment of viscosity. More important was the demonstration that the shape of the clusters could be modified and that the ratio φa/φ could be lowered by a very high shear rate. Very high shearing rates in microchannels appear to be a way for nanofluids to converge to a well-defined value of viscosities.