Witharana et al. Nanoscale Research Letters 2011, 6:231 NANO EXPRESS Open Access Stability of nanofluids in quiescent and shear flow fields Sanjeeva Witharana1*, Haisheng Chen2*, Yulong Ding1 Abstract An experimental study was conducted to investigate the structural stability of ethylene glycol-based titanium dioxide nanoparticle suspensions (nanofluids) prepared by two-step method. The effects of particle concentration, fluid temperature, shear rate and shear duration were examined. Particle size and thermal conductivity measurements in quiescent state indicated the existence of aggregates and that they were stable in temperatures up to 60°C. Shear stability tests suggested that the structure of nanoparticle aggregates was stable in a shear interval. | Witharana et al. Nanoscale Research Letters 2011 6 231 http content 6 1 231 o Nanoscale Research Letters a SpringerOpen Journal NANO EXPRESS Open Access Stability of nanofluids in quiescent and shear flow fields Sanjeeva Witharana1 Haisheng Chen2 Yulong Ding1 Abstract An experimental study was conducted to investigate the structural stability of ethylene glycol-based titanium dioxide nanoparticle suspensions nanofluids prepared by two-step method. The effects of particle concentration fluid temperature shear rate and shear duration were examined. Particle size and thermal conductivity measurements in quiescent state indicated the existence of aggregates and that they were stable in temperatures up to 60 C. Shear stability tests suggested that the structure of nanoparticle aggregates was stable in a shear interval of 500-3000 s-1 measured over a temperature range of 20-60 C. These findings show directions to resolve controversies surrounding the underlying mechanisms of thermal conduction and convective heat transfer of nanofluids. Introduction Nanofluids are suspensions of nano-sized particles in liquids where particle sizes are preferably below 100 nm. At modest particle concentrations the thermal conductivity forced convective heat transfer and critical heat flux of nanofluids were reported to be superior to respective base liquids 1-8 . In the backdrop of conventional heat transfer technologies approaching their upper limits nanofluids are seen as a potential contender for small- and large-scale thermal applications 9-12 . A number of attempts had been made in the past and postulates were put forward to explain the underlying mechanisms. Although yet inconclusive the nanoparticle aggregation in liquids is believed to be one of the principal mechanisms behind the enhanced thermal conductivity and convective heat transfer 13-16 . In either case the importance of particle aggregation and their stability were underlined. On the other hand .
