The Chemistry of Rheology

http://www.chem.com.au/science/rheology/

Rheology and its terminologyThe chemistry of particlesUsing chemistry to modify rheologySome applications of rheologyRheology: rheology and its terminology

Rheology is concerned with the flow and deformation of materials experiencing an applied force. Two extremes of rheological behaviour are:

ELASTIC behaviour - e.g. perfectly rigid solids - where any deformation reverses spontaneously when an applied force is removed. Energy is stored by the system, then released.

VISCOUS (or PLASTIC) behaviour - e.g. ideal Newtonian liquids - where any deformation ceases when the applied force is removed. Energy performs work on the material.

In between elastic and viscous behaviour lies the real world of most substances, which are viscoelastic materials. The rheology described on these net-pages is for slurries and emphasises the underlying chemical effects. We continue with some definitions and common terminology used in rheology.

Newtonian (or viscous) behaviour.

For ideal viscous materials, the rate of deformation is in proportion to the force applied. Deformation ceases when the applied force is removed. The apparent viscosity is constant with changing shear rates. This behaviour is typical of simple liquids such as water.

Simple Newtonian behaviour

Thixotropy and RheopexyA thixotropic material becomes more fluid with increasing time of applied force. The applied force could be stirring, pumping or shaking. This effect is sometimes called work softening. It is often reversible, so that if left undisturbed for some time a thixotropic slurry regains its viscosity. Quicksand is an example of a thixotropic material.

A rheopectic material becomes more viscous with increasing time of applied force. This effect is the opposite of thixotropy, and is sometimes called work hardening.

Dilatancy (shear thickening)

A dilatant material resists deformation more than in proportion to the applied force. For example, the more effort you put into stirring a dilatant material, the more resistant it becomes to stirring. This is usually an indication that the applied force is causing the material to adopt a more ordered structure. A thick slurry of wet beach sand is often dilatant.

Plastic and Pseudoplastic (shear thinning)

PLASTIC materials initially resist deformation, until a yield stress is reached. When that stress is exceeded, the shear rate becomes measurable. Further stress leads finally to linear (Newtonian) behaviour.

PSEUDOPLASTIC materials exhibit shear thinning without the initial resistance to deformation. Like plastic materials, they also show linear (Newtonian) behaviour at the highest levels of stress and shear rate.

Rheology: the chemistry of particles

Size and Shape

Particle size & surface chemistry are critical to the rheological behaviour of a slurry. As particle size decreases, surface effects such as dispersion and flocculation become increasingly of practical concern. Similarly with decreasing particle size, those factors which affect surface behaviour, such as surface charge & adsorbed species, become increasingly significant. Some typical particle sizes & the physical model for their behaviour are given in Table 1.Table 1: Particle Size and Surface ChemistrySize(diam)Area/MassExampleBehaviour

1000 um0.1coarse sandNewtonian

1001fine sand

1010coarse clayStokesian

1100clay

0.11000milkColloid

Surface effects dominate in particles of 1 micron or less, and surface forces are often significant for particles up to 20 microns in diameter.

Particle shape is also important in determining the rheology of a suspension or slurry. In the absence of other differences, spherical particles will interact less than plate-like or needle-like particles, and produce slurries of lower viscosity.

Surface Charge

The effective surface charge of particles primarily determines their dispersion and aggregation. This effective charge is measured as the zeta potential, which is derived from the actual surface charge modified by the molecules and ions which are dragged along with a particle as it moves in a solution.

The zeta potential is a measure of the charge at the moving boundary, or shear plane (see diagram below) which exists in the solution close to the moving particle. The zeta potential depends upon the concentration of ions in the solution, the pH of the solution, and the presence or otherwise of ions, especially multivalent ions.

The ionic concentration affects the zeta potential by causing more of the surface charge to be neutralised in the solution close to the surface of the particle. The effective surface charge decreases rapidly as ionic concentration increases, up to about 0.1 M depending on the ions involved.

Effective surface charge can also be modified by pH, since almost all particles contain surface species or functional groups which cause them to act as acids or bases. Therefore there is some pH at which the overall charge of the particle, including its passenger ions is zero. This pH is the particles isoelectric point. Most inorganic and organic particles have isoelectric points. This includes minerals, polymers, proteins and even bacteria.

Multivalent ions preferentially adsorb at oppositely charged surfaces and greatly reduce a particles effective surface charge. Particles tend to coagulate when their effective surface charge is close to zero. This is because under these conditions the electrostatic forces of repulsion are very small.

Modifying Rheology

Dispersion and flocculation

The effect of aqueous surface chemistry is most important for small particles, such as those with a diameter less than 10 microns. Typically this occurs in slurries. Weak interparticle bonds in a slurry of flocculated particles make the slurry more viscous than a slurry of dispersed particles.

Therefore, a proportionately smaller number of particles are required to solidify a slurry containing flocculated particles. Conversely, if a slurry contains highly dispersed particles it will have a low viscosity, since the internal structure of a dispersed slurry approaches that of a liquid.

Particles in flocculated slurries settle faster than particles in well dispersed slurries, and this allows faster dewatering or solid-liquid separation. However, flocculated particles pack more loosely than dispersed particles, so a larger final volume of sediment is obtained from a flocculated slurry than from a dispersed slurry.

Flocculated systems exhibit shear thinning (plastic behaviour) as the floc structure breaks up. Sometimes this is irreversible. Dispersed systems containing uniformly sized particles and pseudo-spherical particles can show both dilatant and thixotropic behaviour.

Significance of pH

A slurry will be most viscous at a pH when its particles are at their point of zero charge. Mineral surfaces are generally acidic, and the point of zero charge occurs at acidic pH. The point of zero charge for common clays is ~pH 5. For clay mineral slurries, increasing the pH into alkaline regions will increase the effective charge on each particle, and create conditions for particle dispersion. The apparent viscosity of such a clay slurry will markedly reduce. Not all minerals are acidic, however.

The point of zero charge for some alumina minerals is near pH 8. In all cases, a slurry will be least viscous at a pH most distant from its particles point of zero charge. This can occur at pH regions both higher and lower than the particles isoelectric point.

Effect of polyelectrolytes and multivalent ions

In addition to changing the pH, slurries can be dispersed or flocculated by adding suitable ions and polymers. If the slurry contains dispersed charged particles, these can be neutralised by adding multivalent ions of the opposite charge. An example is flocculation of negatively charged organic and inorganic particles using Al3+, or Fe3+.

If particles in a slurry are not sufficiently dispersed, the particle charge can be increased by adding a polyelectrolyte. The most common polyelectrolytes used as dispersants are polyphosphates and polymers of organic acids. The amounts of ions and polymers required to effect a change are typically very small (20 - 1,000 ppm).

Typical Organic Additives:Sodium polyacrylate, which forms a low molecular weight, negatively charged polyelectrolyte, used as a dispersant for neutral to alkaline conditions. Formula: [-CH2-CH(COONa)-]nPolyacrylamide, which forms a high molecular weight, variable charge polyectrolyte. This is common as a flocculant, which acts to bridge particles. Formula: [-CH2-CH(CONH2)-]nTypical Inorganic Additives:Sodium tripolyphosphate Na5P3O10 and sodium hexametaphosphate (Calgon)

these are negatively charged dispersants for alkaline conditions. The active species are phosphorus polyanions, which are unstable in acid.

sodium tripolyphosphate is sometimes known as STPP, and is used in laundry detergent.

Alum, AlCl3, FeCl3, lime.

in these additives, the multivalent cations are the active species, so these are positively charged coagulants and flocculants.

Applications of rheology and areas of use

Pumping slurries - materials transport.

Thickening and dewatering of mineral slurries.

Filtration - more viscous, less speed.

Forming materials e.g. brick & ceramic products.

Paint manufacture e.g. non-drip paints.

Reactions involving mineral slurries e.g. gold extraction.

Food chemistry and manufacture - texture of ice-cream , pasta , desserts, processed meats.

Cosmetics chemistry.

Drilling muds for the petroleum industry.

Polymer chemistry - solutions and melts.

Soil chemistry e.g. effect of clay rheology on soil friability and structure.