6
Nanofluids
6.1
Definition
Nanofluids are stable suspensions of nanoparticles in a liquid. In order to avoid
coagulation of the particles, the particles must be coated with a colloid stabilizer, a
surfactant, acting as distance holder that is stable in the liquid. This distance holder
must also overcome the tendency to form van der Waals bond clusters or, even more
difficult, the mutual attraction of magnetic particles. The two most important
mechanisms of colloid stabilization are depicted schematically in Figure 6.1: steric
stabilization (Figure 6.1a) and stabilization by electrostatic charges (Figure 6.1b).
Typically, nanofluids contain up to 10 vol% of nanoparticles and usually more than
10 vol% of surfactant. Either oil or water is used as a carrier liquid and the
suspensions are designed in such a way that Brownian molecular movement
thwarts the sedimentation of the particles.
6.2
Nanofluids for Improved Heat Transfer
The high heat capacity of nanoparticles, coupled with the possibility of producing
stable suspensions, has many technical applications. Notably, such stable suspensions
may show unprecedented combinations of two or more of the properties or features
that are required in thermal systems, namely a high heat capacity (see Section 7.3), a
good thermal conductivity, and good compatibility with technical systems. However,
even when the heat capacity of nanoparticles is relatively high, because of the small
amount of particles added to the liquids, the actual effect is possibly marginal. One
typical application of nanofluids containing nanoparticles is as a coolant, since the
addition of only a few volume percent of nanoparticles to a liquid coolant can
significantly improve its thermal conductivity, yet have no negative influence on its
heat capacity. In fact, the high heat capacity of nanoparticles can actually improve the
heat capacity of a coolant. Figure 6.2 shows, graphically, the heat capacity ratio of a
suspension consisting of ethylene glycol (which is, pure or diluted with water, widely
used as a coolant, e.g., in cars, or may be added to aqueous cooling liquids) as the liquid
Nanomaterials: An Introduction to Synthesis, Properties and Applications, Second Edition. Dieter Vollath.
Ó 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.
j123
Nanofluids
6.1
Definition
Nanofluids are stable suspensions of nanoparticles in a liquid. In order to avoid
coagulation of the particles, the particles must be coated with a colloid stabilizer, a
surfactant, acting as distance holder that is stable in the liquid. This distance holder
must also overcome the tendency to form van der Waals bond clusters or, even more
difficult, the mutual attraction of magnetic particles. The two most important
mechanisms of colloid stabilization are depicted schematically in Figure 6.1: steric
stabilization (Figure 6.1a) and stabilization by electrostatic charges (Figure 6.1b).
Typically, nanofluids contain up to 10 vol% of nanoparticles and usually more than
10 vol% of surfactant. Either oil or water is used as a carrier liquid and the
suspensions are designed in such a way that Brownian molecular movement
thwarts the sedimentation of the particles.
6.2
Nanofluids for Improved Heat Transfer
The high heat capacity of nanoparticles, coupled with the possibility of producing
stable suspensions, has many technical applications. Notably, such stable suspensions
may show unprecedented combinations of two or more of the properties or features
that are required in thermal systems, namely a high heat capacity (see Section 7.3), a
good thermal conductivity, and good compatibility with technical systems. However,
even when the heat capacity of nanoparticles is relatively high, because of the small
amount of particles added to the liquids, the actual effect is possibly marginal. One
typical application of nanofluids containing nanoparticles is as a coolant, since the
addition of only a few volume percent of nanoparticles to a liquid coolant can
significantly improve its thermal conductivity, yet have no negative influence on its
heat capacity. In fact, the high heat capacity of nanoparticles can actually improve the
heat capacity of a coolant. Figure 6.2 shows, graphically, the heat capacity ratio of a
suspension consisting of ethylene glycol (which is, pure or diluted with water, widely
used as a coolant, e.g., in cars, or may be added to aqueous cooling liquids) as the liquid
Nanomaterials: An Introduction to Synthesis, Properties and Applications, Second Edition. Dieter Vollath.
Ó 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.
j123