RHEOLOGY

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RHEOLOGY

• Roselyn Aperocho-Naranjo
• Faculty, College of Pharmacy
• USPF

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Rheology

• rheo to flow
• logos science
• ology the study of
• is the study of the flow of materials that behave
  in an interesting or unusual manner.
• Unusual materials such as mayonnaise, peanut
  butter, chocolate, bread dough, paints, inks,
  road building aterials, cosmetics, dairy
  products, etc.

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Rheology

• The study of viscosity is of true liquids,
  solutions, dilute and concentrated colloidal
  systems is of much importance in this study
• 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.

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Rheology

• Can affect the patients acceptability of the
  product, physical stability, biologic
  availability, absorption rate of drugs in the
  gastrointestinal tract
• Influence the choice of processing equipments in
  the pharmaceutical system

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Two Categories of Flow Deformation

• 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 as 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 stress
• Measured in (SI unit) pascal
• Commonly used symbols t
• Expressed in other quantities t F / A

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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.
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Two Categories of Flow Deformation

• Newtonian (Newtonian Law of Flow)
• 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.
• A simple equation to describe Newtonian fluid
  behavior is where
• t is the shear stress exerted by the fluid
  ("drag") Pa
• µ is the fluid viscosity - a constant of
  proportionality Pas
• du is the velocity gradient perpendicular to
  the direction
• dy of shear s-1

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Two Categories of Flow Deformation

• Newtonian (Newtonian Law of Flow)
• 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.

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Two Categories of Flow Deformation

• Newtonian (Newtonian Law of Flow)
• 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.
• If the fluid is incompressible and viscosity is
  constant across the fluid, the equation governing
  the shear stress is expressed in the Cartesian
  coordinate system

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Two Categories of Flow Deformation

• Newtonian (Newtonian Law of Flow)
• Cartesian coordinate system
• where, by the convention of tensor notation,
• tij is the shear stress on the ith face of a
  fluid element in the jth direction
• ui is the velocity in the ith direction
• xj is the jth direction coordinate

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Two Categories of Flow Deformation

• Newtonian (Newtonian Law of Flow)
• Cartesian coordinate system
• Tensor - are geometrical entities introduced into
  mathematics and physics to extend the notion of
  scalars, (geometric) vectors, and matrices
• - Components of stress, a second-order tensor,
  in
• three
  dimensions. The tensor in the image is the
• row vector, of
  the forces acting on the X, Y, and
• Z faces of the
  cube. Those forces are represented
• by column
  vectors. The row and column vectors
• that make up
  the tensor can be represented
• together by a
  matrix.

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Two Categories of Flow Deformation

• Non-Newtonian
• A non-Newtonian fluid is a fluid whose flow
  properties are not described by a single 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 Newtonian fluid, the relation between the
  shear stress and the strain rate is linear (and
  if one were to plot this relationship, it would
  pass through the origin), the constant of
  proportionality being the coefficient of
  viscosity.

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Two Categories of Flow Deformation

• Non-Newtonian
• A In a non-Newtonian fluid, the relation between
  the shear stress and the strain rate is
  nonlinear, and can even be time-dependent.
  Therefore a constant coefficient of viscosity
  cannot be defined.
• A ratio between shear stress and rate of strain
  (or shear-dependent viscosity) can be defined,
  this concept being more useful for fluids without
  time-dependent behavior.

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Two Categories of Flow Deformation

• Non-Newtonian Examples
• An inexpensive, non-toxic example of a
  non-Newtonian fluid is a suspension of starch
  (e.g. cornflour) in water, sometimes called
  oobleck (uncooked imitation custard, being a
  suspension of primarily cornflour, has the same
  properties).
• The sudden application of force for example
• by stabbing the surface with a finger, or rapidly
  inverting the container holding it leads to the
  fluid behaving like a solid rather than a liquid.
  
• This is the "shear thickening" property of this
  non-Newtonian fluid. More gentle treatment, such
  as slowly inserting a spoon, will leave it in its
  liquid state.
• Trying to jerk the spoon back out again, however,
  will trigger the return of the temporary solid
  state.
• A person moving quickly and applying sufficient
  force with their feet can literally walk across
  such a liquid.

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Two Categories of Flow Deformation

• Non-Newtonian Examples
• There are fluids which have a linear shear
  stress/shear strain relationship which require a
  finite yield stress before they begin to flow.
  That is the shear stress, shear strain curve
  doesn't pass through the origin.
• These fluids are called
• 1. Bingham plastics.
• 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.

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Two Categories of Flow Deformation

• Non-Newtonian Examples
• These fluids are called
• 1. Pseudoplastic Flow
• Polymers in solutions such as tragacant, sodium
  alginate, methylcellulose
• Viscosity decreases with an increase in shear
  thinning
• Caused by the re-alignment of polymer and/or the
  release of solvents associated with the polymers
• 2. Dilatant Flow
• Volume increases when sheared
• Shear thickening
• Suspension containing high-concentration of small
  deflocculated particles

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Two Categories of Flow Deformation

• Non-Newtonian Examples
• 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).

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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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THIXOTROPY

• pseudoplastic fluids
• Shear thinning is an effect where viscosity
  decreases with increasing rate of shear stress.
  Materials that exhibit shear thinning are called
  pseudoplastic.
• This property is found in certain complex
  solutions, such as lava, ketchup, whipped cream,
  blood, paint, and nail polish.
• Pseudoplasticity can be demonstrated by the
  manner in which shaking a bottle of ketchup
  causes the contents to undergo an unpredictable
  change in viscosity. The force causes it to go
  from being thick like honey to flowing like
  water.
• thixotropic fluid viscosity decreases over time
  at a constant shear rate.

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THIXOTROPY

• The distinction between a thixotropic fluid and a
  shear thinning fluid
• A thixotropic fluid displays a decrease in
  viscosity over time at a constant shear rate.
• A shear thinning fluid displays decreasing
  viscosity with increasing shear rate.
• Some fluids are anti-thixotropic constant shear
  stress for a time causes an increase in viscosity
  or even solidification. Constant shear stress can
  be applied by shaking or mixing. Fluids which
  exhibit this property are usually called
  rheopectic. They are much less common.