19.1 Rheological properties and terminology (Chemistry)
Description
This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
19.1 Rheological properties and terminology (Chemistry)
Contributed by Jim Oliver
RHEOLOGY
What is RHEOLOGY ?
RHEOLOGY describes the deformation of a material under the influence of
stresses. Materials in this context can be solids, liquids or gases. In this
FAQ we will be concerned only with the rheological properties of liquids.[1]
Perry discusses the some aspects of the behaviour of gases, and Ullmann
discusses elastic solids.
When liquids are subjected to stress they will deform irreversibly and flow.
The measurement of this flow is the measurement of VISCOSITY. IDEAL liquids
are very few, whereas non-ideal examples abound. Ideal liquids are : water
and pure paraffin oil. Non-ideal examples would be toothpaste or cornflour
mixed with a little water. [2]
What is VISCOSITY ?
VISCOSITY is expressed in Pascal seconds (Pa.s) and to be correct the
conditions used to measure the VISCOSITY must be given. This is due to the
fact that non-ideal liquids have different values of VISCOSITY for different
test conditions of SHEAR RATE, SHEAR STRESS and temperature. [3,4]
A graph describing a liquid subjected to a SHEAR STRESS (y axis) at a
particular SHEAR RATE (x axis) is called a FLOW CURVE. The shape of this
curve reveals the particular type of VISCOSITY for the liquid being studied.
[3]
What is a NEWTONIAN LIQUID ?
NEWTONIAN LIQUIDS are those liquids which show a straight line drawn from the
origin at 45 degrees, when graphed in this way. Examples of NEWTONIAN liquids
are mineral oil, water and molasses. (Isaac NEWTON first described the laws
of viscosity) [1] All the other types are NON NEWTONIAN.
What does NON NEWTONIAN mean ?
a. PSEUDOPLASTIC liquids are very common. These display a curve starting at
the origin again and curving up and along but falling under the straight
line of the NEWTONIAN liquid. In other words increasing SHEAR RATE results
in a gradual decreasing SHEAR STRESS, or a thinning of viscosity with
increasing shear. Examples are toothpaste and whipped cream.
b. DILATANT liquids give a curve which curves under then upward and higher
than the straight line NEWTONIAN curve. (Like a square law curve) Such
liquids display increasing viscosity with increasing shear. Examples are
wet sand, and mixtures of starch powder with small amounts of water. A car
may be driven at speed over wet sand, but don't park on it, as the car may
sink out of sight due to the lower shear forces (compared to driving over)
the wet sand.
There are other terms used which include :
THIXOTROPY - this describes special types of PSEUDOPLASTIC liquids. In this
case the liquid shows a YIELD or PLASTIC POINT before starting to thin out.
What this means is the curve runs straight up the y axis for a short way then
curves over following ( but higher and parallel to ) the PSEUDOPLASTIC curve.
This YIELD POINT is time dependant. Some water based paints left overnight
develop a FALSE BODY which only breaks down to become useable after rapid
stirring. Also: the curve describing a THIXOTROPIC liquid will be different
on the way up (increasing shear rate) to the way down (decreasing shear rate).
The area inside these two lines is a measure of it's degree of THIXOTROPY.
This property is extremely important in industrial products, e.g to prevent
settling of dispersed solids on storage. [3]
A RHEOPECTIC liquid is a special case of a DILATANT liquid showing increasing
viscosity with a constant shear rate over time. Again, time dependant but in
this case _increasing_ viscosity.
Why do some liquids become solid ?
A few special liquids (dispersions usually) display extraordinary DILATANT
properties. A stiff paste slurry of maize or cornflour in water can appear to
be quite liquid when swirled around in a cup. However on pouring some out
onto a hard surface and applying extreme shear forces (hitting with a hammer)
can cause a sudden increase in VISCOSITY due to it's DILATANCY. The
VISCOSITY can become so high as to make it appear solid. The "liquid" then
becomes very stiff for an instant and can shatter just like a solid material.
It should be noted that the study of viscosity and flow behaviour is
extremely complex. Some liquids can display more than one of the above
properties dependant on temperature, time and heat history.
What are Electrorheological Fluids? ( added by Bruce Hamilton )
Electrorheological (ER) fluids change their flow properties when an electric
field is applied, and are usually dispersions of polarizable particles in an
insulating base fluid [5]. Their apparent viscosity can change by orders of
magnitude in milliseconds when a fews watts of electrical power are applied.
The shear stress versus shear rate properties of ER fluids vary as a function
of the applied electric field, When an electric field is applied, the fluid
switches from a liquid to semisolid. The particles are usually irregularly-
shaped 0.5-100um and present at concentrations of 10-40% by mass. ER fluids
are dielectric particles in an insulating medium ( such as silicone oil ),
along with additives ( such as surfactants, dispersants, and possibly a
polar activator ). ER fluid effectively function as leaky capacitors. The
electric field can be either AC, pulsed DC, or DC, with AC producing less
electrophoresis of particles to electrodes.
There are two categories of ER particulate materials, extrinsically
polarizable materials ( which require a polar activator ), and intrinsically
polarizable materials. Extrinsically polarizable materials can be polar
nonionic compounds ( such as silica, alumina, or polysaccharides ), or polar
ionic materials ( such as the lithium salt of polymethacrylic acid ),
Intrinsically polarizable materials provide simpler systems - because a polar
activator is not required, and they have a lower thermal coefficient of
conductance. The most common examples are the ferroelectrics like barium
titanate (BaTiO3 ) and polyvinylidene difluoride, however their performance
has been poor, as has been that of metal powders ( such as iron and
aluminium - even when coated with an insulating layer ), and research is
concentrating on conducting polymers ( such as polyanilines and pyrolysed
hydrocarbons ) [5,6].
The ability to utilise computer-based electrical switching to control ER
fluid properties has resulted in vehicle suspension and industrial vibration
control as major target applications for ER fluids. Demonstration systems
have been built, and they match performance predictions, however cost and
durability issues still have to be solved [7].
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