Rheology (Greek, Rheo = To Flow & Logos = Science)
¨ Rheology means the study of the flow of matter under the effect of an applied stress which might be applied at any angle to the surface of the matter.
¨ The term ‘rheololgy’ was coined by Eugene Bingham,working as a professor at Lehigh University, in 1920, from an idea by a colleague, Markus Reiner. The term was inspired by Heraclitus's famous expression panta rei, "everything flows". (Ref- Internet)
¨ Scott-Blair recognized the importance of rheology in pharmacy and suggested its application in formulation as well as analysis of some pharmaceutical products eg. emulsions, pastes, suppositories and tablet coatings.
Viscosity
¨ Definition: Viscosity is an expression of resistance of a fluid to flow. Viscosity of a fluid is the internal resistance or friction involved in the relative motion of one layer of molecules with respect to the next. It is symbolized by η.
¨ Example: Water, which is easier to stir than syrup, is said to have the lower viscosity & the viscosity of syrup is more because it resists more to flow than water do.
¨ Explanation: Viscosity occurs due to several attraction forces (van der Waals force, dipole attraction etc.) between molecules of a liquid. For strong attractions, the viscosity will be high and develop greater resistance and for weak attraction, the viscosity will be weak and low resistance will develop.
¨ Unit: The unit of viscosity is poise (p).
Based on rheologic properties materials can be categorized into 2 systems --
- Newtonian System
- Non-Newtonian System
Newtonian System
Definition: Newtonian system is the system which follows the Newton’s equation of flow.
Example: Water, simple organic liquid, dilute suspensions, true solutions and emulsions.
Newtonian’s Law of Flow
¨ In this system, a ‘block’ of liquid comprising of parallel plates of molecules, similar to a deck of cards may be considered.
¨ The bottom layer is supposed to be fixed in place.
¨ As the top level of liquid is stirred at a constant velocity, each lower layer will travel with a velocity directly proportional to its distance from the stationary bottom layer.
Shearing rate: The difference of velocity between planes of liquid separated by an infinitesimal distance is termed as rate of shear or velocity gradient. Rate of shear is given the symbol G. So,
Shearing rate, G =dv/dr
Shearing Stress: Force per unit area essential to bring about flow is called shearing stress. And it is given the symbol F. So,
