  (1) Introduction: Nanoparticles have multiple applications, including drug
delivery systems, biosensing, and carbon capture. Non-Einstein-like viscosity
reduction has been reported in nanoparticle-polymer blends at low nanoparticle
concentrations. More recently, a similar non-Einsteinian viscosity reduction
effect has been observed in aqueous ultra-low concentration carbon-based
nanofluids. (2) Methods: We use a boron nitride nanotube functionalized with
hydrophilic groups in rheological experiments to investigate the viscosity
reduction in ultra-low concentration nanofluids (0.1-10 ppm). We measure the
dynamic viscosity in an air atmosphere and methane (0-5 MPag) at low
temperatures (0-10 C). (3) Results: A negligible effect on the temperature
dependence of viscosity was found. Ultra-low concentrations of BNNT reduced the
viscosity of the nanofluid by up to 29% at 10 ppm in the presence of methane.
The results presented here were compared to similar studies on O-GNF and
O-MWCNT nanofluids, which also reported significant viscosity reductions. (4)
Conclusions: This work identified a non-Einsteinian viscosity reduction in BNNT
nanofluids, which was exacerbated by methane dissolved in the nanofluid.
