Effect of Femtosecond Laser Irradiation on the Thermal Conductivity and Stability of Alumina Nanofluids

Abstract

Nanofluid is a new class of heat transfer fluid containing particles in the size range from 5 nm to about the 100 nm, which play an essential role in the development of energy-efficient heat transfer equipment. However, nanoparticles have a tendency to agglomerate and settle down when they are dispersed in the base fluids, because of their high surface area and surface activity. The sedimentation and agglomeration of particle can lower the thermal conductivity k of nanofluids. In this work, we analyzed for the first time the effect of femtosecond laser irradiation on the thermal conductivity and stability of alumina nanoparticle suspensions. The three-omega method with a microfabricated ac thermal sensor and a commercial device to measure zeta-potential and dynamic light scattering (DLS) were employed for measuring the thermal conductivity and stability of treated alumina nanofluid, respectively. The experimental results show that stability increased significant with the number of laser pulses at relatively a low laser fluence. Besides, there is a strong correlation between k and stability of alumina nanofluid as a function of pulses. Also, the stability of the laser-treated nanofluid was maintained for a long time (>28 days). On the other hand, size reduction and shape change in the suspended nanoparticles were observed at high laser fluence irradiation. The thermal conductivity was enhanced when the size was reduced with changes in the shape. Analysis of the physical mechanisms responsible for the stability enhancement is currently under way. The developed laser-treatment technique is expected to be employed in a variety of heat transfer and chemical engineering applications.