Synthesis and Characterization of Nanofluids …
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5.2 Factors Affecting Size Distribution of Particles
or Aggregation in Nanofluids
The distribution of particles throughout the nanofluid can be definitely a function
of the nanomaterial synthesis method as well as nanofluid synthesis method. But
there are several other factors that completely alter the distribution of the sizes of the
particles when dispersed in the nanofluid. Some of them are discussed here.
5.2.1 Time of Sonication During Synthesis of Nanofluid
As we have already seen in previous sections of this chapter, there are basically two
types of nanofluid synthesis methods. Amongst them, the two-step method involves
drying of nanoparticles and then re-dispersing them before application using either
mechanical means or ultrasonication, the latter being used at most of the times. But
this method also gives scope for the nanoparticles to form clusters and aggregates
and form a polydisperse nanofluid due to inefficient dispersion due to insufficient
ultrasonication (Mahbubul et al. 2015). Ma and Banerjee (2017) have specified that
there are four steps involved in the formation of particles, namely thermal decomposition, nucleation, diffusion growth and particle coagulation. Under the assumption
that there is no coagulation of the nanoparticles, they have observed that the nanofluid
shows almost monodisperse characteristics of size distribution and that the particle
size increases as the reaction is proceeding. Also, the coagulation of the particles at
the end of reaction happening due to Brownian motion-induced collisions produces
a polydisperse nanofluid.
Suganthi and Rajan (2012) studied the effect of ultrasonication time on the particle
size distribution of ZnO-based nanofluids prepared using water as a basefluid. They
have clearly mentioned that the hydrodynamic size of the nanoparticles reduces as the
ultrasonication time for dispersion increases. An optimum particle size distribution
that gives a minimum hydrodynamic size of the nanoparticles has been found out
as 3 h for 0.5 vol.% ZnO-based nanofluid. A further increase in ultrasonication
time leads to agglomeration of the nanoparticles resulting in a higher particle size.
Silambarasan et al. (2012) prepared TiO 2 particles using the stirred bead milling
approach and then dispersed it in water to produce nanofluid using ultrasonication
method. They found a wide size distribution of the particles produced by the stirred
bead milling method ranging from 40 to 900 nm. They found that sonication of 6 and
7 h almost de-agglomerates the larger-sized particles and narrows down the particle
size distribution of the TiO 2 nanoparticles between the sizes of 35 and 300 nm.
Sonication time of 7 h increases the percentage of smaller-sized particles even more.
37
5.2 Factors Affecting Size Distribution of Particles
or Aggregation in Nanofluids
The distribution of particles throughout the nanofluid can be definitely a function
of the nanomaterial synthesis method as well as nanofluid synthesis method. But
there are several other factors that completely alter the distribution of the sizes of the
particles when dispersed in the nanofluid. Some of them are discussed here.
5.2.1 Time of Sonication During Synthesis of Nanofluid
As we have already seen in previous sections of this chapter, there are basically two
types of nanofluid synthesis methods. Amongst them, the two-step method involves
drying of nanoparticles and then re-dispersing them before application using either
mechanical means or ultrasonication, the latter being used at most of the times. But
this method also gives scope for the nanoparticles to form clusters and aggregates
and form a polydisperse nanofluid due to inefficient dispersion due to insufficient
ultrasonication (Mahbubul et al. 2015). Ma and Banerjee (2017) have specified that
there are four steps involved in the formation of particles, namely thermal decomposition, nucleation, diffusion growth and particle coagulation. Under the assumption
that there is no coagulation of the nanoparticles, they have observed that the nanofluid
shows almost monodisperse characteristics of size distribution and that the particle
size increases as the reaction is proceeding. Also, the coagulation of the particles at
the end of reaction happening due to Brownian motion-induced collisions produces
a polydisperse nanofluid.
Suganthi and Rajan (2012) studied the effect of ultrasonication time on the particle
size distribution of ZnO-based nanofluids prepared using water as a basefluid. They
have clearly mentioned that the hydrodynamic size of the nanoparticles reduces as the
ultrasonication time for dispersion increases. An optimum particle size distribution
that gives a minimum hydrodynamic size of the nanoparticles has been found out
as 3 h for 0.5 vol.% ZnO-based nanofluid. A further increase in ultrasonication
time leads to agglomeration of the nanoparticles resulting in a higher particle size.
Silambarasan et al. (2012) prepared TiO 2 particles using the stirred bead milling
approach and then dispersed it in water to produce nanofluid using ultrasonication
method. They found a wide size distribution of the particles produced by the stirred
bead milling method ranging from 40 to 900 nm. They found that sonication of 6 and
7 h almost de-agglomerates the larger-sized particles and narrows down the particle
size distribution of the TiO 2 nanoparticles between the sizes of 35 and 300 nm.
Sonication time of 7 h increases the percentage of smaller-sized particles even more.