Biomedical Engineering Reference
In-Depth Information
demonstrated that fluids with suspended nanoparticles, forming a stable
colloid and maintaining a quasi-single phase state, exhibit high heat
transport properties at a very low amount of dispersion (
1 vol%). The
term nanofluid was coined by Choi (1995) to describe this new class of
fluids. Nanofluids have now emerged as a promising field in nanotechnology
research. Among the properties of nanofluids that have been studied, the
largest volume of work has focused on characterization of the thermal
properties of nanofluids, particularly the high thermal conductivity they
display. There has also been research on modeling the anomalous increase in
thermal conductivity.
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11.2 Synthesis of nanofluids
Nanofluids can be synthesized by adding nanometer-sized particles to a
liquid. However, nanofluids are not simply solid-liquid mixtures. Some
special requirements are essential, namely an even and stable suspension,
adequate durability, negligible agglomeration of particles, and no chemical
change of the dispersed particles or fluid. The synthesis procedure of a
nanofluid is a key factor on which the thermal properties depend, and the
behavior of a nanofluid is highly dependent on the behavior of the base
fluids and the dispersed phases, particle concentration, size and morphol-
ogy, as well as the presence of dispersants or surfactants. The synthesis of
stable nanofluids with minimum agglomeration and controlled properties
like thermal conductivity and viscosity for heat transfer applications is the
main objective of the community dealing with nanofluid technology. The
techniques generally utilized for the synthesis of nanofluids are discussed in
the following sections.
11.2.1 One-step process
A one-step process is a bottom-up approach that combines the synthesis of
nanoparticles with the preparation of nanofluids, as the nanoparticles
synthesized (by physical/chemical vapor deposition or chemical methods)
are collected in the same fluid/medium. A number of techniques have been
reported for the synthesis of nanofluids by a one-step process, including
vacuum evaporation onto a running oil substrate (VEROS) (Yatsuya et al.,
1978), the direct condensation technique (Eastman et al., 2001), microwave
irradiation (Zhu et al., 2004), citrate reduction (Patel et al., 2003; Zhu et al.,
2004; Zhang et al., 2006a; Liu et al., 2006; Fuentes et al., 2008; Mishra et al.,
2009; Wang and Wei, 2009), submerged arc nanoparticle synthesis system
(SANSS) (Lo et al., 2005a, 2005b, 2007), multi-pulse laser ablation (Phuoc
et al., 2007), sputtering on running liquid (Tamjid and Guenther, 2010), to
name a few. A detailed summary of the different techniques by which
