Questions
2 questions per theory paper
Difficulty
Medium
Importance
Core for B.Pharm formulation exams
Overview
Micromeritics and Rheology are fundamental studies of the science of small particles and the flow of matter, respectively. Understanding these principles is critical for formulation science in pharmacy and material engineering, as they dictate the stability, processing, and delivery efficiency of dosage forms. Mastery of these concepts ensures you can correlate physical particle dimensions with bulk flow behavior.
Fundamental Particle Properties
Micromeritics focuses on the characteristics of individual and bulk particles, including size, shape, and surface area. These properties govern the dissolution rate and bioavailability of medicinal powders.
- Equivalent spherical diameter concept
- Methods: Optical microscopy, Sieving, Sedimentation (Andreasen pipette)
- Surface area determination via Adsorption and Air permeability (Fisher sub-sieve sizer)
- Derived properties: Porosity, Bulk density, Tapped density
Particle Size Distribution and Statistics
Since particles are rarely uniform, they are characterized by their distribution curves. Understanding the log-normal distribution is vital for predicting product homogeneity.
- Normal vs Log-normal distribution curves
- Number distribution vs Weight distribution
- Mean diameters: Arithmetic mean, Geometric mean, Harmonic mean
- Coefficient of Variation (CV) as a measure of polydispersity
Rheological Foundations
Rheology is the study of the flow and deformation of matter under applied stress. It helps in distinguishing between Newtonian and non-Newtonian systems, which is essential for manufacturing liquid dosage forms.
- Newton's Law of Flow: F/A = η (dv/dr)
- Newtonian systems: Viscosity (η) remains constant
- Non-Newtonian systems: Pseudoplastic, Dilatant, Plastic flow
- Thixotropy: Time-dependent decrease in viscosity under shear
- Bingham bodies: Require yield stress before flow initiates
Flow Properties of Powders
The flowability of powders determines the efficiency of hopper discharge and tablet compression. Key metrics like the Angle of Repose and Carr's index are used to evaluate powder flow potential.
- Angle of Repose (tan θ = h/r)
- Carr's Compressibility Index = (Tapped density - Bulk density) / Tapped density * 100
- Hausner Ratio = Tapped density / Bulk density
- Interparticulate friction and cohesive forces
Formula Sheet
Newton's Law: F = ηA(dv/dr)
Angle of Repose: tan θ = h/r
Carr's Index: ((ρt - ρb)/ρt) * 100
Stokes' Law: v = (d^2(ρs - ρl)g) / 18η
Exam Tip
Always draw and label the Rheogram (Shear stress vs Shear rate) for Newtonian, Pseudoplastic, and Dilatant systems; it is the most frequently asked diagram in this unit.
Common Mistakes
- Confusing Angle of Repose with Hausner ratio in terms of flowability correlation (lower angle means better flow, higher Hausner ratio means worse flow).
- Failing to specify the type of flow (e.g., Dilatant vs Pseudoplastic) when sketching rheograms.
- Neglecting units for viscosity (Poise, Centipoise, Pascal-seconds) in numerical problems.
More Revision Notes
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