Introduction to Rheology and Deformation of Solids
Rheology is the study of the flow of liquids and the deformation of solids. This critically vital unit explains how pharmaceutical liquids behave when poured, pumped, or spread on the skin under stress (shear rate). It distinguishes simple liquids (Newtonian) like water from complex formulations (Non-Newtonian) like creams and ointments. The second half covers powder mechanics—how solid powder particles bend, break, and permanently deform (plastic vs elastic deformation) when extreme pressure is applied during tablet punching, governed by the Heckel framework.
Syllabus & Topics
- 1Newtonian systems: Law of flow, kinematic viscosity, effect of temperature.
- 2Non-Newtonian systems: Plastic, pseudoplastic, and dilatant flow.
- 3Thixotropy: Concept, rheograms, and application in formulation.
- 4Determination of viscosity: Capillary, falling sphere, and rotational viscometers.
- 5Deformation of solids: Plastic and elastic deformation.
- 6Heckel equation and Yield value.
- 7Solid state physics and Modulus of elasticity.
Learning Objectives
Frequently Asked Questions (FAQs)
Q1. What is Thixotropy and Why is it Desirable in Pharmacy?
Thixotropy is a reversible, time-dependent decrease in viscosity of a non-Newtonian system when shear stress is applied (such as shaking), followed by gradual recovery of viscosity when the system is left undisturbed. It is desirable in pharmaceutical suspensions because the formulation becomes less viscous and easy to pour upon shaking, but regains higher viscosity at rest, thereby preventing particle settling.
Q2. What is the Difference Between Plastic and Pseudoplastic Flow?
Pseudoplastic flow is shear-thinning behavior in which viscosity decreases immediately as shear rate increases, without a yield value, and the flow curve starts at the origin (e.g., polymer solutions, ketchup). Plastic flow (Bingham flow) behaves like a solid until a minimum force, known as the yield value, is applied; beyond this point, it flows like a liquid. This behavior is typical of flocculated suspensions and toothpaste.
Q3. How Does One Determine the Viscosity of a Non-Newtonian System?
Because non-Newtonian fluids exhibit viscosity that varies with shear rate, single-point viscometers such as an Ostwald viscometer are unsuitable. Instead, multi-point rotational viscometers, such as Cup-and-Bob (Couette) or Cone-and-Plate viscometers, are used to obtain measurements at different shear rates and construct a complete rheogram.
Q4. What is the Heckel Equation Used For?
The Heckel equation is applied in powder compaction studies and relates the relative density of a powder bed to the applied compression pressure. It is important for analyzing the plastic and elastic deformation behavior of drugs and excipients during tablet manufacturing.