Lecture - 20 Applied Rheology 4

8/10/2019 Lecture - 20 Applied Rheology 4

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Applied Rheology in Polymer

Processing



•

The rheological behaviour of dilatent fluids is exactly opposite to that of pseudoplastic fluids, i.e., their viscosity increases with rate of shear or shear

stress and the plot of τ and γ◦ gives concave upward curve.

•These fluids show a constant viscosity at low shear rate, increasing viscosity

with increasing rate of shear in the intermediate region but at high shear

region the behavior is not well established due to experimental difficulty incollecting the reliable rheological data.

•Examples of such fluids are not very common and only some highly filled

polymer propellant systems like hydroxyl terminated polybutadiene

propellants or the suspensions of beach sand or the PVC plastisols display this

nature.

Dilatent fluids



•The shearing of suspensions in which the amount of solids

present is so high that the liquid is just enough to fill the voids

will tend to release the solids out of fluids contact due to

expansion during shearing there by increasing the viscosity.

•At very low shear rate the liquid fills and voids causing

lubrication effect and giving constant viscosity.

Dilatent fluids...

Dilatent fluids under shear deformation



• Bingham plastic fluids require a certain

force to develop what is known as yield

stress, τy before they start flowing andthen flow like Newtonian liquids, i.e.,

the plot of τ and γ◦ is a straight line andit intercepts the stress axis at some finite

value equal to the yield stress.

•The viscosity of these fluids is constant

during the flow at shear stresses higher

than yield stress, τ > τy and increases

sharply as the stress approaches yield

stress.

Bingham plastic fluids

•These fluids, when at rest, are believed to

develop a three-dimensional structure due

to the presence of intermolecular or inter

particle forces. This structure resists the

deformation to such an extent that it does

not allow the fluid to deform till the

applied force gives enough energy to break

it down and once the fluid begins to

deform it flows like a Newtonian liquid.

After the deforming force is removed the

structure forms again when at rest.



• These liquids also show yield stress

but during flow do not follow the

Newton’s

law of viscosity but show ashear rate dependent viscosity

beyond the shear stress τ > τy .

Viscoplastic fluids



• The time dependent fluids are those

liquids, which show either a decrease or

increase in the viscosity with time at aparticular rate of shear, i.e., if these

fluids are subjected to a constant shear

rate then the shear stress will either

decrease or increase. These fluids are

respectively thixotropic fluids and

rheopectic fluids.

Time dependent fluids

ɳ and τ versus time plots for time dependent fluids



• Thixotropic fluids are essentially

pseudoplastic type where the orientation

of molecules at a particular shear rate is

a time dependent process as comparedto being instantaneous for the

pseudoplastic liquids.

•The rheopectic fluids are dilatent type

where the dilatency is time dependent.

Time dependent fluids...

•Both these fluids, when deformed first

with increasing shear stress and thenwith decreasing, do not follow the same

path back but show hysteresis.

•The area under the hysteresis curves

represents the loss of energy during the

cycle of deformation.

•This energy is consumed in the changes

in the molecular configuration and other

structural changes, these liquids might

have undergone as a result of shearing.Hysteresis curves for i) thixotropic and ii) rheopectic fluids



• As the name implies these fluids posses

both purely elastic and purely viscouscharacteristics, i.e., they store a part of

the deformational energy and dissipate

the remaining as heat due to viscous

drag.

Viscoelastic fluids.

•Weissenberg demonstrated these effects

by shearing such fluids in a cone andplate rheogoniometer, which is fitted

with a set of piezometric tubes.

•The stored elastic energy manifests

itself in the form of normal stresses

resulting in the climbing of fluid in thetubes.

•The intensity of normal stresses is

maximum at the center and minimum at

the periphery. Normal stresses as observed in a rheogoniometer



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Lecture - 20 Applied Rheology 4