6
D. P. Barai et al.
2 Synthesis Techniques of Nanofluids
Preparation of nanofluid is the technique of evenly and uniformly dispersing the
nanoparticles in the basefluid. This may seem to be simple and not so important, but
this process largely affects several properties of the nanofluid. Also, the lab-scale
preparation of nanofluids may possibly determine the complexity of preparing and
using nanofluids in large-scale applications. There are two methods of nanofluid
preparation: the two-step and one-step methods.
2.1 Two-Step Method
Two-step method is the most common and has been extensively used for preparing
nanofluids. As the name indicates, there are two steps that are carried out in this
method during preparation of nanofluids. In the first step, dry powder is produced
as nanoparticles, nanofibres or nanotubes. Then in the second processing step, nanomaterial is directly distributed in the basefluid with the use of dispersing devices
like magnetic stirrer (Duangthongsuk and Wongwises 2009; Kavitha et al. 2012; Liu
et al. 2005), homogenizer (Liu et al. 2005; Wen and Ding 2005) or by using ultrasound devices like probe or bath (Duangthongsuk and Wongwises 2009; Hwang
et al. 2007; Manimaran et al. 2014). Mostly, ultrasound device or a higher shear
homogenizer is commonly used to stir nanopowders with the basefluids. As this
method permits a gap between the synthesis and dispersion steps, many researchers
have used readily-available nano-sized particles for the preparation of nanofluids in
various basefluids (Xuan and Li 2000). It is a common observation that the two-step
process is highly used for preparing metal oxide nanoparticles-based nanofluids than
the metallic nanoparticles-based nanofluids (Eastman et al. 2001). In the two-step
method, drying the nanoparticles, their transportation and storage are further process
steps that cannot be neglected. The main advantage of this method is that it is easily
scalable. However, the major disadvantage is the formation of agglomerates because
of the attraction between the particles due to their high surface energy (Mohammed
et al. 2011). Two-step method is favourable to almost every kind of fluids (Wang and
Mujumdar 2007). The illustration of two-step method is given in Fig. 1.
2.2 One-Step Method
In one-step method, the process takes place in single step in accordance to its name.
This process is composed of both, the production of the nanoparticles and the synthesis of nanofluids. Numerous single-step processes have been developed for the
D. P. Barai et al.
2 Synthesis Techniques of Nanofluids
Preparation of nanofluid is the technique of evenly and uniformly dispersing the
nanoparticles in the basefluid. This may seem to be simple and not so important, but
this process largely affects several properties of the nanofluid. Also, the lab-scale
preparation of nanofluids may possibly determine the complexity of preparing and
using nanofluids in large-scale applications. There are two methods of nanofluid
preparation: the two-step and one-step methods.
2.1 Two-Step Method
Two-step method is the most common and has been extensively used for preparing
nanofluids. As the name indicates, there are two steps that are carried out in this
method during preparation of nanofluids. In the first step, dry powder is produced
as nanoparticles, nanofibres or nanotubes. Then in the second processing step, nanomaterial is directly distributed in the basefluid with the use of dispersing devices
like magnetic stirrer (Duangthongsuk and Wongwises 2009; Kavitha et al. 2012; Liu
et al. 2005), homogenizer (Liu et al. 2005; Wen and Ding 2005) or by using ultrasound devices like probe or bath (Duangthongsuk and Wongwises 2009; Hwang
et al. 2007; Manimaran et al. 2014). Mostly, ultrasound device or a higher shear
homogenizer is commonly used to stir nanopowders with the basefluids. As this
method permits a gap between the synthesis and dispersion steps, many researchers
have used readily-available nano-sized particles for the preparation of nanofluids in
various basefluids (Xuan and Li 2000). It is a common observation that the two-step
process is highly used for preparing metal oxide nanoparticles-based nanofluids than
the metallic nanoparticles-based nanofluids (Eastman et al. 2001). In the two-step
method, drying the nanoparticles, their transportation and storage are further process
steps that cannot be neglected. The main advantage of this method is that it is easily
scalable. However, the major disadvantage is the formation of agglomerates because
of the attraction between the particles due to their high surface energy (Mohammed
et al. 2011). Two-step method is favourable to almost every kind of fluids (Wang and
Mujumdar 2007). The illustration of two-step method is given in Fig. 1.
2.2 One-Step Method
In one-step method, the process takes place in single step in accordance to its name.
This process is composed of both, the production of the nanoparticles and the synthesis of nanofluids. Numerous single-step processes have been developed for the