RHEOLOGY OF FLUIDS

BY: VIDHI DOSHIVIDHI DOSHI

RHEOLOGY

rheo – to flowlogos – scienceology – the study of

Thus, rheology is the science that concerns with the flow of

liquids and deformation of solids.

The study of viscosity is of true liquids, solutions, dilute and concentrated colloidal systems.

It is involved in the mixing and flow of materials, their packaging into containers, the pouring from the bottle, extrusion from a tube or a passage of the liquid to a syringe needle.

It can affect the patient’s acceptability of the product, physical stability, biologic availability, absorption rate of drugs in the gastrointestinal tract.

It influence the choice of processing equipments in the pharmaceutical system

TWO CATEGORIES OF FLOW & DEFORMATION

Newtonian (Newtonian Law of Flow)Newtonian (Newtonian Law of Flow)“the higher the viscosity of a liquid, the greater is the force per unit area (shearing stress) required to produce a certain rate of shear”Shear – is a stress which is applied parallel or tangential to a face of a material, as opposed to a normal stress which is applied perpendicularly. Shear stressMeasured in (SI unit): pascal Commonly used symbols: τ Expressed in other quantities: τ = F / A

A shear stress, is applied to the top of the square while the bottom is held in place. This stress results in a strain, or deformation, changing the square into a parallelogram.

A Newtonian fluid (named for Isaac Newton) is a fluid whose stress versus rate of strain curve is linear and

passes through the origin. The constant of proportionality is known as the viscosity.

In common terms, this means the fluid continues to flow, regardless of the forces acting on it. For example, water is Newtonian, because it continues to exemplify fluid properties no matter how fast it is

stirred or mixed.

For a Newtonian fluid, the viscosity, by definition, depends only on temperature and pressure (and also the chemical composition of the fluid if the fluid is not a pure substance), not on the

forces acting upon it.

Non-Newtonian Non-Newtonian

A A non-Newtonian fluidnon-Newtonian fluid is a fluid whose flow is a fluid whose flow properties are not described by a single properties are not described by a single constant value of viscosity.constant value of viscosity.

Many polymer solutions and molten polymers are non-Newtonian fluids, as are many commonly found substances such as ketchup, starch suspensions, paint, blood and shampoo.

In a non-Newtonian fluid, the relation between In a non-Newtonian fluid, the relation between the shear stress and the strain rate is nonlinear, the shear stress and the strain rate is nonlinear, and can even be time-dependent. Therefore a and can even be time-dependent. Therefore a constant coefficient of viscosity cannot be constant coefficient of viscosity cannot be defined.defined.

1. Bingham plastics. They have linear shear stress and strain relationship which require a finite yield stress before they begin to flow, i.e., the shear stress-strain curve doesn’t pass through origin.Eg- clay suspensions, drilling mud, toothpaste, mayonnaise, chocolate, and mustard. The classic case is ketchup which will not come out of the bottle until you stress it by shaking.

TYPES OF NON NEWTONIAN FLUIDS

2. Pseudoplastic FlowPolymers in solutions such as tragacant, sodium alginate, methylcelluloseViscosity decreases with an increase in shear thinningCaused by the re-alignment of polymer and/or the release of solvents associated with the polymers

3. Dilatant FlowVolume increases when shearedShear thickeningSuspension containing high-concentration of small deflocculated particles

THE RELATIONSHIP OF SHEAR STRESS-STAIN FOR

ALL FLUIDS:

There are also fluids whose strain rate is a function of time. Fluids that require a gradually increasing shear stress to maintain a constant strain rate are

referred to as rheopectic.

An opposite case of this, is a fluid that thins out with time and requires a decreasing stress to

maintain a constant strain rate (thixotropic).

THIXOTROPY is the property of some non-Newtonian

pseudoplastic fluids to show a time-dependent change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity.

A thixotropic fluid is a fluid which takes a

finite time to attain equilibrium viscosity when introduced to a step change in shear rate.

the term is sometimes applied to pseudoplastic fluids without a viscosity/time component. Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated.

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