Phase Change Material Particles and Their Application in Heat Transfer Fluids - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 465

Fig. 13 Pictures of ethanol-
in-PAO nanoemulsion fluids
(Bottle A) and pure PAO
(Bottle B). Liquids in both
bottles are transparent. The
Tyndall effect (i.e., a light
beam can be seen when
viewed from the side) can be
observed only in Bottle
A when a laser beam is passed
through Bottles A and B [ 11 ,
44 ]

Bottle B Bottle A

Table 2 Comparison of nanoemulsion fluids and emulsions [ 44 ]

Sample Property Nanoemulsion Emulsion

1 Appearance Transparent Turbid

2 Interfacial tension Ultra low (usually \\1 mN/m) low

3 Droplet size \50 nm [500 nm

4 Stability Thermodynamically stable,

long shelf-life

Thermodynamically unstable

5 Preparation Self-assembly Need of external shear

6 Viscosity Newtonian Non-newtonian

5.1.1 Microstructure of Ethanol-in-PAO Nanoemulsion Fluids

The microstructure of nanoemulsion fluids is affected by many factors, including
surfactant type and concentration, dispersed liquid type and concentration,
molar ratio of dispersed liquid to surfactant, temperature, pH value, and salinity
[ 56 , 69 - 75 ]. The characterization of the microstructure of nanoemulsion fluids is a
challenging task. Small angle neutron scattering (SANS) and Small angle X-ray
scattering (SAXS) are often used because they can be applied to the ''concen-
trated'' colloidal suspensions (e.g., [1 vol%) [ 76 - 82 ]. Figure 14 shows the SANS
data for ethanol-in-PAO nanoemulsion fluid. The wave vector is given by:

q ¼ 4p sin h =ð = k ð 6 Þ

where k is the wavelength of the incident neutrons and h is the scattering angle.
The analysis of the SANS data suggests that the inner cores of the swollen
micelles, i.e., the ethanol droplets are spherical and have a radius of about 0.8 nm
for 9 vol%.