pharmacetics

• Rheology


  The study of the flow and deformation of materials under the influence of stress
• Rheology is a branch of science in which we study about the flow
• Flow


  Permanent shape change which occurs in fluids
• Types of materials


  • Solids (cannot flow)
  • Semisolids (ability to flow)
  • Liquids (easily flow)
  • Gases (always flow)
• Strain


  Relative deformation or change in shape and size of elastic, plastic, and fluid materials under applied forces

  
• Viscosity


  A measure of resistance of a fluid to deformation under stress
• The unit of viscosity is the poise. A more convenient unit is the centipoise
• Viscosity


  Affects flow, higher viscosity decreases flow
• Kinematic viscosity


  The ratio of viscosity of fluids to its density at a specific temperature
• Temperature increases

  Viscosity of liquids decreases, flow increases
• Temperature increases


  Viscosity of gases increases
• Characteristics of Newtonian flow

  • Rate of deformation is proportional to force applied
  • Deformation ceases when force is removed
  • Apparent viscosity is constant with changing shear rates
• Non-Newtonian fluids


  Fluids where shear stress and rate of shear are not constant

  
• Types of non-Newtonian fluids

  • Plastic (viscoelastic)
  • Pseudoplastic
  • Dilatant
• Plastic (viscoelastic) flow


  Requires an initial stress (yield value) before flow occurs, behaves as an elastic solid below yield value
• Pseudoplastic flow


  Viscosity decreases with increasing shear rate (shear thinning)
• Dilatant flow


  Viscosity increases with increasing shear rate (shear thickening)
• Thixotropy


  Time-dependent reversible process where a fluid's structure is broken down under shear and rebuilt at rest
• Thixotropic materials exhibit a thixotropic hysteresis loop on a rheogram
• Thixotropy


  The change or transition of a substance due to mechanical load
• Thixotropy is a phenomenon by which the structure of a fluid is broken down under shear and rebuilt at rest
• Thixotropy is a time-dependent reversible process
• Thixotropy in liquid pharmaceutical systems

  Pouring from the container and spreading to the skin will be easy
• A well-formulated thixotropic suspension does not immediately collapse in the container, it becomes liquefied by shaking and remains as dispersed for a sufficient period of time for dosing
• Thixotropy is desirable for emulsions, lotions, creams, ointments and some i.m. parenterals
• As the thixotropy grade increases

  The sedimentation rate decreases
• Negative thixotropy (Antithixotropy)


  Viscosity of system is increased on applying shear stress and when we remove shear stress, it regains its viscosity
• In Pseudoplastic & Plastic fluids, on reducing the shearing stress gradually, a down curve is obtained but the down-curve is shifted to the left side meaning that the viscosities of the down-curve are lower than that of the up-curve
• Bulges


  Substances which can swell in presence of water give a bulge
• Spurs


  In some highly structured thixotrophic materials, the bulged curve actually develop into spur
• Rheopexy


  A phenomenon in which a solid forms a gel more readily when gently shaken or otherwise sheared than when allowed to form the gel while the material is kept at rest
• In a rheopectic system, the gel is the equilibrium form, whereas in anti-thixotropy, the equilibrium state is the sol
• The greater the thixotropy, the higher is the stability
• Capillary viscometer


  Instrument that operate at a single rate of shear such as capillary viscometer can be used for Newtonian system
• Non-Newtonian system


  Multi-point instruments that can operate at different rates of shear are required for characterizing the rheological properties
• Types of viscometers


  • Capillary instrument
  • Falling Sphere viscometer
  • Rotational viscometer
• Ostwald viscometer


  Mostly used in capillary viscometers, also known as ''U-tube'' viscometer, mostly used for Newtonian fluids
• Ubbelohde viscometer


  Modified form of capillary viscometer, has hanging capillary, suitable for viscous liquids
• Falling Sphere Viscometer


  Also called Falling Ball viscometer, can be used over the range 0.5 to 200,000 poise
• Hoeppler Viscometer


  A modified version of Falling ball viscometer, using bigger balls, almost touching the cylinder wall, suitable for both clear and dark liquid