Chemistry Reference
In-Depth Information
6.5.3 Deformable Particles
Deformable particles covers a wide range of materials from human erythrocytes
to emulsions and microgels. The rheological measurements on these systems
show a high sensitivity to the chemical environment. Energy-dissipation
mechanisms will include the hydrodynamic drag of the particles through the
medium and interparticle forces. In addition, they will include mechanisms
associated with deforming the shape of the particles and internal circulation
within them where possible. Thus, the effect of the applied field is a subtle
balance between the internal and external structural rearrangements that are
possible. The most widely studied deformable systems are emulsions. These can
come in many forms, oil in water (O/W) and water in oil (W/O) the most
commonly encountered. However, there are multiple emulsions where oil or
water droplets become trapped inside another drop such that they are W/O/W
or O/W/O. Silicone oils can become incompatible at certain molecular weights
and with different chemical substitutions and this can lead to oil in oil emulsions
O/O. At high concentrations, typical of some pharmaceutical creams, cosmetics
and food stuffs the droplets are in contact and deform. Volume fractions in
excess of 0.90 can be achieved. The drops are separated by thin surfactant films.
Self-bodied systems are multicomponent systems where the dispersion is a
mixture of droplets and precipitated organic species such as long-chain alcohols.
The solids can form part of the stabilising layer and these are termed Pickering
emulsions. The surface characteristics of these species are determined by the
particulates and stress transfer across the membrane will tend to be low,
reducing internal circulation within the drop. The structure of the interface
surrounding the drop plays a significant role in determining the characteristics of
the droplet behaviour. We can begin our consideration of emulsion systems by
looking at the role of this layer in determining linear viscoelastic properties. This
was undertaken by Princen and Kiss
36
and also by Mason et al.
37
They
suggested a simple semiempirical linearisation of G(0) the static modulus with
volume fraction.
Princen G
ð
0
Þ¼
Ag
12
j
1
=
3
a
32
ð
j
j
m
Þ
ð
6
:
127
Þ
Mason G
ð
0
Þ¼
Ag
12
j
a
32
ð
j
j
m
Þ
ð
6
:
128
Þ
where A is a constant typically between 1 and 2. The radius a
32
is the third
moment to second moment average and g
12
is the interfacial tension between the
droplet and the continuous phase. The term j
m
is the maximum packing
fraction before the drops distort. The basic idea underlying this model is that
at a critical concentration the droplets pack together and thin films begin to
form between them. As the concentration is increased further the droplets
deform. This type of emulsion is termed a high internal phase emulsion or HIPE
or sometimes a ''gel emulsion''. If a strain is applied this results in droplet
distortion and an increase in the film area that is opposed by the interfacial
