111
Nanofluids
6
6.1
Background
Nanofluids are stable suspensions of nanoparticles in a liquid, generally, water or
oil. Suspensions are, in most cases unstable. The particles have the tendency to
agglomerate and to sediment. This tendency is avoidable by adding colloid stabilizers (surfactant) to the liquid. There are many possibilities to stabilize a colloid.
The most important are steric and electrostatic stabilization. Figure 6.1 displays
these two possibilities in a simplified way.
In Figure 6.1a, steric stabilization is sketched. Simply stated, steric stabilization
is obtained by adding long organic molecules perpendicular at the surface of the
particles. Now, these molecules act as distance holders, they avoid direct contact
in-between the particles and, therefore, agglomeration. In Figure 6.1a the distanceholder molecules are perpendicular to the surface. When they are not well ordered,
one obtains a transition to the gel network stabilization. When the molecules carry
electric dipole moments, they can form an electrostatic double layer. As the surfaces of the particles now carry electrostatic charges of equal sign, they repel each
other. (It is important to mention that the net charge of each particle remains nil.)
In both cases, the particles are kept at a certain distance, this thwarts the formation
of van der Waals bond clusters. Therefore, the Brownian molecular movements of
the particles, kept at a distance, prevents sedimentation of the particles.
As a general rule, one can say that a nanofluid contains up to 10 vol% particles
and approximately the same amount of surfactants.
6.2
Nanofluids for Improved Heat Transfer
In general, the heat capacity of nanoparticles is larger than that of liquids; additionally, the same is valid for the thermal conductivity. Therefore, it seems straightforward to add nanoparticles to coolants to improve their performance. In fact,
these expected phenomena were found. The remaining open questions are the
compatibility of the particles in the fluids with the technical environment to be
Nanoparticles – Nanocomposites – Nanomaterials: An Introduction for Beginners, First Edition. Dieter Vollath.
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.
Nanofluids
6
6.1
Background
Nanofluids are stable suspensions of nanoparticles in a liquid, generally, water or
oil. Suspensions are, in most cases unstable. The particles have the tendency to
agglomerate and to sediment. This tendency is avoidable by adding colloid stabilizers (surfactant) to the liquid. There are many possibilities to stabilize a colloid.
The most important are steric and electrostatic stabilization. Figure 6.1 displays
these two possibilities in a simplified way.
In Figure 6.1a, steric stabilization is sketched. Simply stated, steric stabilization
is obtained by adding long organic molecules perpendicular at the surface of the
particles. Now, these molecules act as distance holders, they avoid direct contact
in-between the particles and, therefore, agglomeration. In Figure 6.1a the distanceholder molecules are perpendicular to the surface. When they are not well ordered,
one obtains a transition to the gel network stabilization. When the molecules carry
electric dipole moments, they can form an electrostatic double layer. As the surfaces of the particles now carry electrostatic charges of equal sign, they repel each
other. (It is important to mention that the net charge of each particle remains nil.)
In both cases, the particles are kept at a certain distance, this thwarts the formation
of van der Waals bond clusters. Therefore, the Brownian molecular movements of
the particles, kept at a distance, prevents sedimentation of the particles.
As a general rule, one can say that a nanofluid contains up to 10 vol% particles
and approximately the same amount of surfactants.
6.2
Nanofluids for Improved Heat Transfer
In general, the heat capacity of nanoparticles is larger than that of liquids; additionally, the same is valid for the thermal conductivity. Therefore, it seems straightforward to add nanoparticles to coolants to improve their performance. In fact,
these expected phenomena were found. The remaining open questions are the
compatibility of the particles in the fluids with the technical environment to be
Nanoparticles – Nanocomposites – Nanomaterials: An Introduction for Beginners, First Edition. Dieter Vollath.
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.