RHEOLOGY

RHEOLOGY Rheology is the science concerned with the deformation

of matter under the influence of stress

Stress applied perpendicular to the surface of a body is known as Tensile Stress

If tangentially to the surface of a body is known as Shearing Stress

Tensile Stress Shearing Stress

RHEOLOGY

Deformation that result from the application of stress divided into two types:-

• Reversible Deformation (Elastic Deformation) When the body returns to its original shape after the removal of applied stress

• Irreversible Deformation (Permanent Deformation) When the body does not returns to its original shape after the removal of applied stress

RHEOLOGY

Viscosity is a property of fluids that indicates resistance to flow. Newton's Law states that the shear stress (the force divided by area parallel to the force, F/A) is proportional to the shear rate (V/H). The proportionality constant is known as the Coefficient of Viscosity (η).

The ratio of applied shear stress to the Rate of Shear is known as Coefficient of Viscosity (η).

η = Shear Stress/ Rate of Shear

On the basis of Newton’s Law fluids can be divided into two types:• Newtonian Fluids• Non-Newtonian Fluids

NEWTONIAN FLUIDS Fluid which obey Newton’s Law is called Newtonian Fluids

(rate of flow of liquid is directly proportional to applies stress).

Flow of this type of fluid can be illustrated by hypothetical cube of fluid containing infinite layer of liquid (laminae).

When tangentially stress is applied the rate of movement of laminae very strong a maximum value in layer adjacent to the upper plan to a value that is close to zero in the layer adjacent to the lower plane.

NEWTONIAN FLUIDS• Thickness of Cube = x• Stress = S• Velocity difference = µ• Applied Stress = F/A• Rate of Shear = µ/x• Viscosity = Applied Stress/ Rate of Shear

η = F/A/dµ/dx• Unit of η is Ns/m2 other unit of η poise (P) & centipoise

(cP) 1cP = 10-3 ns/m2Rheogram

Sh

ear

Rat

e

Shear Stress

NEWTONIAN FLUIDS

• Straight line of graph shows direct relation of shear stress with shear rate.

• The slope of which is equal to reciprocal of viscosity of the fluid a value to as fluidity.

ϕ = 1/η

Examples of Newtonian Fluids:-• Water• Simple Organic Liquids• True Solutions • Dilutes Suspensions • Emulsions

NON-NEWTONIAN FLUIDS

The fluid which do not obey Newton Law’s they are termed as Non-Newtonian Fluids.

Types of Non-Newtonian Fluids:-

Time Dependent

Thixotropy Rheopexy

Time Independent

Plastic Flow

Pseudo Plastic Flow

Dilatant Flow

Non-Newtonian Fluids

Time Dependent Effects These are properties which depend on duration of shear.• Thixotropy means change by touch.

Any reversible time dependent decrease in Viscosity that result form the application of shearing stress.

The decrease in Viscosity arise from a breakdown of structure within a system when it is sheared after the shearing force are remove a time lag occurs before structural reformation is complete.

The rheogram for this system is:-

NON-NEWTONIAN FLUIDSS

hear

rat

e

Shear Stress

Example of Thixotropy

• Bentonite gel

• Hydrated Bentonite Stress Particular elongated aligning themselves with respect to flow (parallel to flow liquid) this orderly arrangement breaks the interparticle links and therefore apparent viscosity decreases

• On removal of shearing force the arrangement of dispersed particle gradually become less orderly and the gel network reform after a time lag.

Irreversible Thixotropy

There is structural deformation to that extend they cannot go to original state on removal of shear stress. Bentonite Solution .

NON-NEWTONIAN FLUIDS

Thixotropy

NON-NEWTONIAN FLUIDS

Negative Thixotropy

• Samyn and Wan 1967 suggest that Negative Thixotropy observed on clay suspension was caused breakdown of relatively large compact produced, which therefore an increase in apparent viscosity.

• When the system subjective to stress time depend increase in viscosity.

• Reason Large compact flocules by the application of stress change into smaller flocules and its orientation change and increase in the friction between them so viscosity increases.

NON-NEWTONIAN FLUIDS

Rheopexy

• The time lag occurs when stress is removed and system reformed. This time lag is reduced by application of mild rolling or drumming motion. This motion provide a mild turbulence that aids particle of the system to a random orientation when reformation can occurs. This whole effect is known as Rheopexy.

• Negative Rheopexy Reversible deformation by the application of stress. When stress is removed time lag for reformation increased.

NON-NEWTONIAN FLUIDS

Time Independent Fluids

• Plastic Flow. The rheogram for plastic flow show that the line does not pass through the origin of the graph but rises at some point on the shear stress axis. This indicate that certain shearing stress must be exerted before flow began. These stress is termed the Yield Value. If the stress is applied is lower than yield value the system exhibit elastic deformation that are reversible when these small stress removed.

NON-NEWTONIAN FLUIDS

Shear Stress

Sh

ear

Rat

e

Bingham Bodies.

• Material that shows plastic behaviour are often known as Bingham Bodies. The quantitative behaviour of these system usually expressed in terms of Bingham Equations.

U or ηp = S — fB

du/dx

fB = Bingham Yield Value

U = Plastic Velocity,

du/dx = Shear Rate,

S = Shear Stress

NON-NEWTONIAN FLUIDS

• This equation employs that flow diagram is straight line that arises on the shear stress axis at yield value. In practice usually occurs at the lower stress. (Yield Value fH = Higher Yield Value to which the flow curve become linear) When System extremely plastic fL is used in place of fB

NON-NEWTONIAN FLUIDS

Pseudoplastic Flow• It can be seen that the curve arises at the origin of the graph

so no yield value exist.• As soon as stress applied system will began to flow, the slop

of curve gradually increases until it reach maximum value since the apparent of viscosity at any shear rate is given by the reciprocal of the slope, that apparent viscosity decreases as the shear rate increases until a constant value is reached.

• An empirical equation for this system is Sn = k du/dx (K & n are constant) if n=1 the it is similar to Newtonian Equation.

NON-NEWTONIAN FLUIDS

• At the high value of stress the graph of plastic and pseuds plastic are superimposable. So to differentiate between them we applied less stress.

Why increasing stress viscosity decreases?

• Dependent on the nature of the liquid.

• When stress increases in concentrated suspension aggregates of particles are break/ disperse and the friction between particles decreases, so viscosity decreases.

NON-NEWTONIAN FLUIDS

Dilatant Flow• Dilatnacy is usually exhibited by concentrated

dispersion of deflocculated particle. .• At lower shear rate in these systems the particle are

arrange in a state of close packing and the small amount of liquid present is sufficient to fill the narrow spaces between adjacent particles.

NON-NEWTONIAN FLUIDS

Lower Shear Rate High Shear Rate

• These thin liquids films allow the system to flow like a liquid.

• At higher shear rate the particle will become displace from their close pack arrangement which result in the formation of large white spaces in the system.

• The liquid continuous medium is now insufficient to fill all the spaces between particles, hence the movement of particle relative to each other involves a greater amount of friction and the apparent viscosity therefore increases.

• This effect may be trouble some in high speed milling process because the viscosity of dilatant suspension may increase so much that led to overloading of the motors.

NON-NEWTONIAN FLUIDS

• The Rehogram for Dilatant Flow

• The slope of this curve gradually decreases to a constant value which indicate that the apparent viscosity must increase with increase in shear rate up to a maximum value.

NON-NEWTONIAN FLUIDS

Sh

ear

Rat

e

Shear Stress