Pharmaceutics · Short notes

Rheology and Micromeritics: Short Notes & Cheat Sheet

Newtonian and non-Newtonian flow, thixotropy, viscometers, particle size methods, powder flow and porosity.

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Short notes

Read these before you attempt the topic's MCQs.

Types of flow at a glance

Only Newtonian liquids have a single viscosity. Every other flow type needs a rotational viscometer to describe it.

Flow types

Newtonian liquids have a constant viscosity regardless of shear rate (water, simple syrups, glycerin).
Plastic (Bingham) flow needs a yield value before it flows (e.g. ointments, concentrated suspensions).
Pseudoplastic flow is shear-thinning: viscosity falls as shear rate rises (polymer solutions such as CMC or tragacanth).
Dilatant flow is shear-thickening: viscosity rises with shear rate (concentrated suspensions of deflocculated particles, above about 50% solids).
Thixotropy is a reversible, time-dependent fall in viscosity with shaking, followed by recovery at rest. It is ideal for suspensions.

Measuring viscosity

The capillary (Ostwald) viscometer and the falling-sphere (Hoeppler) viscometer suit Newtonian liquids only, because they measure at a single shear rate.
Rotational viscometers (cup-and-bob, cone-and-plate, Brookfield) measure at many shear rates, so they suit non-Newtonian systems.
Viscosity usually falls as temperature rises in liquids (Arrhenius-type relationship).

Micromeritics

Micromeritics is the science of small particles: size, shape, surface area, density and flow.
Particle size methods: sieving (above about 50 µm), microscopy (0.2–100 µm; the reference method), sedimentation (Andreasen pipette) and laser diffraction or Coulter counter.
Powder flow is assessed by angle of repose, Carr's compressibility index and Hausner ratio.
Surface area is measured by gas adsorption (BET method) or air permeability.
Bulk density = mass / bulk volume (including voids); true density excludes all pores. Porosity is the fraction of voids.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Flow type → example

Flow	Example
Newtonian	Water, glycerin, simple syrup
Plastic (Bingham)	Ointments, pastes, concentrated flocculated suspensions
Pseudoplastic	Polymer solutions: CMC, tragacanth, alginate
Dilatant	≥ 50% deflocculated suspensions, starch in water
Thixotropic	Suspensions that gel at rest and thin on shaking

Powder flow

Measure	Good flow
Angle of repose	< 30° excellent; > 40° poor
Carr's index	≤ 15% good
Hausner ratio	< 1.25 good
Improve flow	Granulate, add glidant, reduce fines

Memory aids

Pseudoplastic = Polymer; Dilatant = Dense: Shear-thinning is typical of polymer solutions; shear-thickening of dense, deflocculated suspensions.

Exam traps

An Ostwald viscometer cannot characterise non-Newtonian liquids, because it works at a single shear rate.
Thixotropy is time-dependent; pseudoplastic flow alone is not.
A lower angle of repose means better flow.
Carr's index and Hausner ratio are both calculated from bulk and tapped density.

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