RHEOLOGY

Rheology

Study of deformation and flow of matter

A fluid is a substance that deforms continuously under the action of a shearing force.

Intuitively, a fluid flows!

Inquiry into the flow behavior of complex fluids

Complex fluids do not follows Newton's Law or Hooke's Law (of elasticity)

 
Newton and Simple Fluids

Reflected upon the resistance of liquids to a cylinder rotating in a vessel.

Newton (-Stokes) Law

Deformation rate is expected to be proportional to stress and the constant coefficient of proportionality is called viscosity.

The study of simpler fluids have their own well-defined field, called fluid mechanics.

Purely viscous fluid.

"Rheology is the study of the flow of materials that behave in an interesting or unusual manner. Oil and water flow in familiar, normal ways, whereas mayonnaise, peanut butter, chocolate, bread dough, and silly putty flow in complex and unusual ways. In rheology, we study the flows of unusual materials."

"… all normal or Newtonian fluids (air, water, oil, honey) follow the same scientific laws. On the other hand, there are also fluids that do not follow the Newtonian flow laws. These non-Newtonian fluids, for example mayo, paint, molten plastics, foams, clays, and many other fluids, behave in a wide variety of ways. The science of studying these types of unusual materials is called rheology"

Examples of Complex Fluids
Foods
Emulsions (mayonaisse, ice cream)
Foams (ice cream, whipped cream)
Suspensions (mustard, chocolate)
Gels (cheese)
Biofluids
Suspension (blood)
Gel (mucin)
Solutions (spittle)
Personal Care Products
Suspensions (nail polish, face scrubs)
Solutions/Gels (shampoos, conditioners)
Foams (shaving cream)
Electronic and Optical Materials
Liquid Crystals (Monitor displays)
Melts (soldering paste)
Pharmaceuticals
Gels (creams, particle precursors)
Emulsions (creams)
Aerosols (nasal sprays)
Polymers

 
Rheology's Goals

• Establishing the relationship between applied forces and geometrical effects induced by these forces at a point (in a fluid).
  • The mathematical form of this relationship is called the rheological equation of state, or the constitutive equation.
  • The constitutive equations are used to solve macroscopic problems related to continuum mechanics of these materials.
  • Any equation is just a model of physical reality.
    
     

2. Establishing the relationship between rheological properties of material and its molecular structure (composition).

• Related to:
  • Estimating quality of materials
  • Understanding laws of molecular movements
  • Intermolecular interactions
• Interested in what happens inside a point during deformation of the medium.

 
(Material) Structure

More or less well-organized and regularly spaced shapes

Arrangements, organization or intermolecular interactions

Structured Materials – properties change due to the influence of applied of applied forces on the structure of matter

Rheology sometimes is referred to as mechanical spectroscopy.

"Structure Mechanisms" are usually proposed, analogous to reaction mechanisms in reaction kinetics

Structural probes are used to support rheological studies and proposed mechanisms.

Rheological analysis is based on the use of continuum theories
meaning that:

• There is no discontinuity in transition from one geometrical point to another, and the mathematical analysis of infinitesimal quantities can be used; discontinuities appear only at boundaries
• Properties of materials may change in space (due to gradients) but such changes occur gradually

 

• changes are reflected in space dependencies of material properties entering equations of continuum theories

 
Rheological Properties

• Stress
  • Shear stress
  • Normal stress
  • Normal Stress differences
• Viscosity
  • Steady-state (i.e. shear)
  • Extensional
  • Complex
• Viscoelastic Modulus
  • G' – storage modulus
  • G" – loss modulus
• Creep, Compliance, Decay
• Relaxation times
• and many more …

Common Non-Newtonian Behavior

• shear thinning
• shear thickening
• yield stress
• viscoelastic effects
  • Weissenberg effect
  • Fluid memory
  • Die Swell

Shear Thinning and Shear Thickening

shear thinning – tendency of some materials to decrease in viscosity when driven to flow at high shear rates, such as by higher pressure drops.

Shear Thickening

Shear Thickening– tendency of some materials to increase in viscosity when driven to flow at high shear rates