1.12 Stability Mechanisms
of Nano fluids
Particles in dispersion
may adhere together and form aggregates of increasing size which may settle out
due to gravity. Stability means that the particles do not aggregate at a
significant rate. The rate of aggregation is in general determined by the frequency
of collisions and the probability of cohesion during collision. Derjaguin,
Verway, Landau, and Overbeek (DVLO) developed a theory which dealt with
colloidal stability. DLVO theory suggests that the stability of a particle in
solution is determined by the sum of van der Waals attractive and electrical
double layer repulsive forces that exist between particles as they approach
each other due to the Brownian motion they are undergoing. If the attractive
force is larger than the repulsive force, the two particles will collide, and
the suspension is not stable. If the particles have a sufficient high
repulsion, the suspensions will exist in stable state. For stable nano fluids
or colloids, the repulsive forces between particles must be dominant. According
to the types of repulsion, the fundamental mechanisms that affect colloidal
stability are divided into two kinds, one is steric repulsion, and another is
electrostatic (charge) repulsion, shown in Figure . For steric stabilization, polymers
are always involved into the suspension system, and they will adsorb onto the
particles surface, producing an additional steric repulsive force. For example,
Zinc oxide nano particles modified by PMAA have good compatibility with polar
solvents. Silver nano fluids are very stable due to the protective role of PVP,
as it retards the growth and agglomeration of nano particles by steric effect.
PVP is an efficient agent to improve the stability of graphite suspension . The
steric effect of polymer dispersant is determined by the concentration of the
dispersant. If the PVP concentration is low, the surface of the graphite
particles is gradually coated by PVP molecules with the increase of PVP. Kamiya
et al. studied the effect
of polymer dispersant structure on electrosteric interaction and dense alumina suspension
behaviour . An optimum hydrophilic to hydrophobic group ratio was obtained from
the maximum repulsive force and minimum viscosity. For electrostatic
stabilization, surface charge will be developed through one or more of the
following mechanisms: (1) preferential adsorption of ions, (2) dissociation of
surface charged species, (3) isomorphic substitution of ions’, (4) accumulation
or depletion of electrons at the surface, and (5) physical adsorption of
charged species onto the surface.