Structural and mechanical properties
of the dispersed systems
Plan
1. Rheology of dispersion system. Sol’s
viscosity.
2. Liquid
dispersed
systems:
suspensions, emulsions, foams.
3. Gas dispersed systems: aerosols,
powders.
Assistant Kozachok S.S. prepared
Rheology is the science of the deformation and flow of matter
It is a subject of tremendous and increasing technological
importance - in many industries, such as rubber, plastics, food,
paint and textiles, the suitability of the products involved is to
a large extent judged in terms of their mechanical properties.
In biology and medicine (particularly
haematology) rheological behaviour is also of major
importance.
The most straightforward rheological behaviour is exhibited
on the one hand by Newtonian viscous fluids and on the other
by Hookean elastic solids. However, most materials,
particularly those of a colloidal nature, exhibit mechanical
behaviour which is intermediate between these two extremes,
with both viscous and elastic characteristics in evidence. Such
materials are termed viscoelastic.
The rheological behaviour of colloidal dispersions depends
mainly on the following factors:
1. Viscosity of the dispersion medium.
2. Particle concentration.
3. Particle size and shape.
4. Particle-particle and particle-dispersion medium nteractions.
Newtonian viscosity
The viscosity of a liquid is a measure of the internal resistance offered
to the relative motion of different parts of the liquid. Viscosity is
described as Newtonian when the shearing force per unit area σ
between two parallel planes of liquid in relative motion is proportional
to the velocity gradient D between the planes - i.e.
where η is the coefficient of viscosity. The dimension of η is,
therefore, (mass) (length)-1 (time)-1. For most pure liquids and for many
solutions and dispersions, η is a well-defined quantity for a given
temperature and pressure which is independent of σ and D, provided that
the flow is streamlined (i.e. laminar). For many other solutions and
dispersions, especially if concentrated and if the particles are asymmetric
and/or aggregated deviations from Newtonian flow are observed. The
main causes of non-Newtonian flow are the formation of a structure
throughout the system and orientation of asymmetric particles caused by
the velocity gradient.
Measurement of viscosity
Capillary flow methods
The most frequently employed methods for measuring
viscosities are based on flow through a capillary tube. The
pressure under which the liquid flows furnishes the shearing
stress. The relative viscosities of two liquids can be
determined by using a simple Ostwald viscometer (Figure).
Sufficient liquid is introduced into the viscometer for the
levels to be at B and C. Liquid is then drawn up into the lefthand limb until the liquid levels are above A and at the bottom
of the right-hand bulb. The liquid is then released and the time
for the left-hand meniscus to pass between the marks A and B
is measured.
Since the pressure at any instant driving the liquid through the
capillary is proportional to its density,
where k is the viscometer constant, p is the density of the
liquid and t is the flow time. Therefore, for two different
liquids,
Ostwald’s capillary flow
methods
  Kt
where η – viscosity
ρ – the density of the liquid
К – the viscometer constant
t – the flow time
[η] = Pa·s
Stock’s viscometer
2r ( d   ) g

r
r
9 (1  2.4 )(1  3.3 )
R
L
2
wher d – density of the globule’s
material,
ρо – liquid density,
 - translational velocity of the
uniform motion of the globule
g - acceleration of gravity
Viscosities of dilute colloidal solutions and dispersions
Functions of viscosity
When colloidal particles are dispersed in a liquid, the flow of
the liquid is disturbed and the viscosity is higher than that of
the pure liquid. The problem of relating the viscosities of
colloidal dispersions (especially when dilute) with the nature
of the dispersed particles has been the subject of much
experimental investigation and theoretical consideration. In
this respect, viscosity increments are of greater significance
than absolute viscosities, and the following functions of
viscosity are defined:
Spherical particles
Einstein (1906) made a hydrodynamic calculation (under
assumptions similar to those of Stokes;) relating to the
disturbance of the flow lines when identical, non-interacting,
rigid, spherical particles are dispersed in a liquid medium, and
arrived at the expression:
The effect of such particles on the viscosity of a dispersion
depends, therefore, only on the total volume which they
occupy and is independent of their size.
For dispersions of non-rigid spheres (e.g. emulsions) the flow
lines may be partially transmitted through the suspended
particles, making k in Einstein's equation less than 2.5.
Structural and mechanical
behavior of the dispersion systems
Elasticity
Plasticity
Strength
Viscosity
Relative viscosity (it’s
true when the spherical
particles don’t react with
each other):
  0
i 
 2.5
0
Reduced viscosity (or
viscosity number):
1/η - fluidity
  i / С
Scheme formation of the
dimensional structure
Aggregates which close
a fixed fluid inside temselves
Non-Newtonian flow
Shear-thinning
Shear-thinning, as the term suggests, is characterised by a
gradual (time-independent) decrease in apparent viscosity with
increasing rate of shear, and can arise from a number of
causes.
If particle aggregation occurs in a colloidal system, then an
increase in the shear rate will tend to break down the
aggregates, which will result, among other things, in a
reduction of the amount of solvent immobilised by the
particles, thus lowering the apparent viscosity of
the system.
Shear thinning is an effect where viscosity decreases with
increasing rate of shear stress. Materials that exhibit shear
thinning are called pseudoplastic. This property is found in
certain complex solutions, such as lava, ketchup, whipped
cream, blood, paint, and nail polish. It is also a common
property of polymer solutions and molten polymers.
A shear stress, denoted (tau), is defined as a stress which is
applied parallel or tangential to a face of a material, as
opposed to a normal stress which is applied perpendicularly.
Shear-thinning is particularly common to systems containing
asymmetric particles. Asymmetric particles disturb the flow
lines to a greater extent when they are randomly orientated at
low-velocity gradients than when they have been aligned at
high-velocity gradients. In addition, particle interaction and
solvent immobilisation are favoured when conditions of
random orientation prevail.
The apparent viscosity of a system which thins on shearing is
most susceptible to changes in the shear rate in the
intermediate range where there is a balance between
randomness and alignment, and between aggregation and
dispersion.
Plasticity and yield value
Plasticity is similar to shear-thinning, except that the system
does not flow noticeably until the shearing stress exceeds a
certain minimum value. The applied stress corresponding to a
small but arbitrarily chosen rate of deformation is termed the
yield value.
Plasticity is due to a continuous structural network which
imparts rigidity to the sample and which must be broken
before flow can occur. It is often difficult to distinguish
between plastic and ordinary shear-thinning behaviour.
Modelling clay, drilling muds and certain
pigment dispersions are examples of plastic dispersions.
Time-dependent phenomena
Thixotropy
Thixotropy is the time-dependent analogue of shear-thinning and
plastic behaviour, and arises from somewhat similar causes. If a
thixotropic system is allowed to stand and is then sheared at
a constant rate, the apparent viscosity decreases with time until a
balance between structural breakdown and structure re-formation is
reached. If the sheared system is then allowed to stand, it eventually
regains its original structure.
Solutions of high polymers are, in general, thixotropic to a certain
extent; intermolecular attractions and entanglements are overcome
and the extent of solvent immobilisation is reduced on shearing,
while Brownian motion restores the system to its original condition
when left to stand. The classical examples of thixotropic behaviour
are given by the weak gel systems, such as flocculated sols of iron(III)
oxide, alumina and many clays (particularly bentonite clays), which
can be liquefied' on shaking and 'solidify' on standing. Thixotropy is
particularly important in the paint industry, as it is desirable that the
paint should flow only when being brushed on to the appropriate
surface (high rate of shear) and immediately after brushing.
Rheopexy
Rheopexy is time-dependent shear-thickening, and is
sometimes observed as an acceleration of thixotropic
recovery - for example, bentonite clay suspensions often set
only slowly on standing but quite rapidly when gently
disturbed,
Shear-thickening
Shear-thickening is characterised by an increase in apparent
viscosity with increasing rate of deformation.
Emulsion (liquid/liquid)
Emulsifying agents and emulsion stability
Probably the most important physical property of an emulsion is its
stability. The term 'emulsion stability' can be used with reference to
three essentially different phenomena - creaming (or sedimentation),
coagulation and a breaking of the emulsion due to droplet coalescence.
If an emulsion is prepared by homogenising two pure liquid
components, phase separation will usually be rapid, especially if the
concentration of the dispersed phase is at all high. To prepare
reasonably stable emulsions, a third component - an emulsifying
agent (or emulsifier) - must be present. The materials which are most
effective as emulsifying (and foaming) agents can be broadly
classified as:
1. Surface-active materials.
2. Naturally occurring materials.
3. Finely divided solids.
The functions of the emulsifying agent are to facilitate emulsification
and promote emulsion stability. The emulsifying agent forms an
adsorbed film around the dispersed droplets which helps to prevent
coagulation and coalescence. The stabilising mechanism is usually
complex and may vary from system to system. In general, however,
the factors which control droplet coagulation are the same as those
which control the stability of sols.
The following factors (which depend on the nature of the
emulsifying agent and/or on a suitable choice of formulation and
manufacturing conditions) favour emulsion stability:
1, Low interfacial tension The adsorption of surfactant at oil-water
interfaces causes a lowering of interfacial energy, thus facilitating
the development and enhancing the stability of the large interfacial
areas associated with emulsions.
2. A mechanically strong and elastic interfacial film This is particularly
important when the volume fraction of the dispersed phase is
high
3. Electrical double layer repulsions (see page 212) Interparticle
repulsion due to the overlap of similarly charged electric double
layers is an important stabilising mechanism in O/W emulsions.
When ionic emulsifying agents are used, lateral electric double
layer repulsion may prevent the formation of a close-packed film.
This film-expanding effect can be minimised by using a mixed
ionic plus non-ionic film and/or by increasing the
electrolyte concentration in the aqueous phase.
4. Relatively small volume of dispersed phase (see below).
5. Narrow droplet size distribution Larger droplets are less unstable
than smaller droplets on account of their smaller area-to-volume
ratio, and so will tend to grow at the expense of the smaller droplets.
6. High viscosity A high Newtonian viscosity simply retards the
rates of creaming, coalescence, etc. If a weak gel network is
formed by, for example, dissolving sodium carboxymethyl cellulose
in an O/W emulsion, genuine stability might ensue. However, the
overall Theological properties of such an emulsion may not be
acceptable.
Emulsifying agents and emulsion type
The type of emulsion which is formed when a given pair of immiscible
liquids is homogenised depends on (1) the relative volumes of the two
phases, and (2) the nature of the emulsifying agent.
1. Phase volume The higher its phase volume, the more likely a*
liquid is to become the dispersion medium. However, the liquid
with the greater phase volume need not necessarily be the
dispersion medium.
If the emulsion consisted of an assembly of closely packed
uniform spherical droplets, the dispersed phase would occupy 0.74
of the total volume. Stable emulsions can, however, be prepared
in which the volume fraction of the dispersed phase exceeds 0.74,
because (a) the droplets are not of uniform size and can,
therefore, be packed more densely, and (b) the droplets may
be deformed into polyhedra, the interfacial film preventing
coalescence.
Types of emulsion:
diluted
< 0,1 %
concentrated
< 74 %
jelly emulsion
>74 %
2. Nature of the emulsifying agent Alkali-metal soaps favour the
formation of O/W emulsions, whereas heavy-metal soaps favour
the formation of W/O emulsions. O/W emulsions in the middle
concentration region stabilised by alkali-metal soaps can often be
broken, and even inverted into W/O emulsions, by the addition of
heavy-metal ions.
The most satisfactory general theory of emulsion type is that
originally proposed for emulsions stabilised by finely divided solids
(see Figure). If the solid is preferentially wetted by one of the
phases, then more particles can be accommodated at the interface if
the interface is convex towards that phase
The amphiphilic nature of many emulsifying agents (particularly
non-ionic surfactants) can be expressed in terms of an empirical scale
of so-called HLB (hydrophile-lipophile balance) numbers222 (see
Table 10.1). The least hydrophilic surfactants are assigned the lowest
HLB values.
Suspension (solid/liquid)
Foams (gas/liquid)
Aerosol (liquid/gas, solid /gas)
Powder (solid with high concentration / gas)
Characteristics of
powder:
Bulk density(It is defined as the mass of
many particles of the material divided by
the total volume they occupy).
 The clumping behavior (of a powder arises
because of the molecular Van der Waals
force that causes individual grains to cling
to one another).
