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Structural and mechanical properties of the dispersed systems. Plan Rheology of dispersion system . Sol’s viscosity . Liquid dispersed systems : suspensions , emulsions , foams . Gas dispersed systems : aerosols , powders . Assistant Kozachok S.S. prepared.
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Assistant Kozachok S.S. prepared
It is a subject of tremendous andincreasing technological importance - in many industries, such asrubber, plastics, food, paint and textiles, the suitability of theproducts involved is to a large extent judged in terms of theirmechanical properties. In biology and medicine (particularly
haematology) rheological behaviour is also of major importance.
The most straightforward rheological behaviour is exhibited on theone hand by Newtonian viscous fluids and on the other by Hookeanelastic solids. However, most materials, particularly those of acolloidal nature, exhibit mechanical behaviour which is intermediatebetween these two extremes, with both viscous and elastic characteristicsin evidence. Such materials are termed viscoelastic.
The rheological behaviour ofcolloidal 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.
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, ηis awell-defined quantity for a given temperature and pressure which isindependent of σand D, provided that the flow is streamlined (i.e.laminar). For many other solutions and dispersions, especially ifconcentrated and if the particles are asymmetric and/or aggregateddeviations from Newtonian flow are observed. The main causes ofnon-Newtonian flow are the formation of a structure throughout thesystem and orientation of asymmetric particles caused by the velocitygradient.
Capillary flow methods
The most frequently employed methods for measuring viscosities arebased on flow through a capillary tube. The pressure under which theliquid flows furnishes the shearing stress.The relative viscosities of two liquids can be determined by using asimple Ostwald viscometer (Figure). Sufficient liquid is introducedinto the viscometer for the levels to be at B and C. Liquid is thendrawn up into the left-hand limb until the liquid levels are aboveA and at the bottom of the right-hand bulb. The liquid is thenreleased and the time for the left-hand meniscus to pass between themarks A and B is measured.
Since the pressure at any instant driving the liquid through thecapillary 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,
whereη – viscosity
ρ – the density of the liquid
К – the viscometer constant,
t – the flow time
[η] = Pa·s
wherd – density of the globule’s material,
ρо – liquid density,
- translational velocity of the uniform motion of the globule
g - acceleration of gravity
Functions of viscosity
When colloidal particles are dispersed in a liquid, the flow of theliquid is disturbed and the viscosity is higher than that of the pureliquid. The problem of relating the viscosities of colloidal dispersions(especially when dilute) with the nature of the dispersed particles hasbeen the subject of much experimental investigation and theoreticalconsideration. In this respect, viscosity increments are of greatersignificance than absolute viscosities, and the following functions of
viscosity are defined:
Einstein (1906) made a hydrodynamic calculation (under assumptionssimilar to those of Stokes;) relating to the disturbance ofthe flow lines when identical, non-interacting, rigid, sphericalparticles are dispersed in a liquid medium, and arrived at theexpression:
The effect of such particles on the viscosity of a dispersion depends,therefore, only on the total volume which they occupy and isindependent of their size.
For dispersions of non-rigid spheres (e.g. emulsions) the flow linesmay be partially transmitted through the suspended particles, makingk in Einstein's equation less than 2.5.
Relative viscosity(it’s true when the spherical particles don’t react with each other):
Reduced viscosity (or viscosity number):
1/η - fluidity
Aggregates which closeafixed fluid inside temselves
Shear-thinning, as the term suggests, is characterised by a gradual(time-independent) decrease in apparent viscosity with increasingrate of shear, and can arise from a number of causes.
If particle aggregation occurs in a colloidal system, then an increasein the shear rate will tend to break down the aggregates, which willresult, among other things, in a reduction of the amount of solventimmobilised by the particles, thus lowering the apparent viscosity of
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.
asymmetric particles. Asymmetric particles disturb the flow lines to agreater extent when they are randomly orientated at low-velocitygradients than when they have been aligned at high-velocitygradients. In addition, particle interaction and solvent immobilisationare favoured when conditions of random orientation prevail.
The apparent viscosity of a system which thins on shearing is mostsusceptible to changes in the shear rate in the intermediate rangewhere there is a balance between randomness and alignment, andbetween aggregation and dispersion.
Plasticity is similar to shear-thinning, except that the system does notflow noticeably until the shearing stress exceeds a certain minimumvalue. The applied stress corresponding to a small but arbitrarilychosen rate of deformation is termed the yield value.
Plasticity is due to a continuous structural network which impartsrigidity to the sample and which must be broken before flow canoccur. It is often difficult to distinguish between plastic and ordinaryshear-thinning behaviour. Modelling clay, drilling muds and certain
pigment dispersions are examples of plastic dispersions.
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 is time-dependent shear-thickening, and is sometimesobserved as an acceleration of thixotropic recovery - for example,bentonite clay suspensions often set only slowly on standing but quiterapidly when gently disturbed,
Shear-thickening is characterised by an increase in apparent viscositywith increasing rate of deformation.
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
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
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 filmand/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
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 volumeThe 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
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
diluted concentrated jelly emulsion
< 0,1 %< 74 %>74 %
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
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