OIA1008 RHEOLOGY

Rheology

The science of flow of liquids and deformation of solids; from Greek "rheo" (flow) + "logia" (study).

Importance in pharmacyx

Crucial in manufacturing and handling of dosage forms like creams, ointments, suspensions, emulsions, and injections.

Practical relevance

Affects patient acceptability, product stability, bioavailability, and drug absorption rate.

Shear

Movement of material relative to parallel layers.

Shear stress (F’)

Force per unit area (F/A) required to induce flow.

Shear rate (S)

Change in velocity (dv) between layers per unit distance (dr), i.e., dv/dr.

Viscosity (η)

Resistance to flow; defined by η = F / G (shear stress / shear rate). Unit: poise (dyne·s/cm²).

Fluidity (Ø)

Reciprocal of viscosity: Ø = 1/η; measures how easily a substance flows.

Kinematic viscosity

A fluid's resistance to flow under gravity; expressed as ratio of dynamic viscosity to density.

Temperature and viscosity (liquids)

Viscosity decreases with increasing temperature (except for methylcellulose).

Temperature and viscosity (gases)

Viscosity increases with increasing temperature.

Viscosity measurement

Performed using viscometers such as Ostwald viscometer.

Newtonian fluids

Follow Newton’s law: shear stress is directly proportional to shear rate; viscosity is constant regardless of shear.

Examples of Newtonian fluids

Water, alcohol, glycerin.

Non-Newtonian fluids

Do not follow Newton’s law; viscosity changes with shear rate. Most pharmaceuticals fall into this category.

Pseudoplastic flow (shear-thinning)

Viscosity decreases as shear rate increases (e.g., polymer solutions like methylcellulose, tragacanth).

Mechanism of pseudoplasticity

Polymer chains align and disentangle with shear, reducing resistance to flow.

Dilatant flow (shear-thickening)

Viscosity increases with shear rate (e.g., starch suspensions, candy slurries).

Mechanism of dilatancy

Particles form open structures under stress, requiring more force to move past each other.

Plastic flow

Material doesn't flow until applied stress exceeds a yield value; e.g., ketchup, toothpaste.

Yield value (f)

Minimum stress needed to initiate flow in plastic systems (e.g., flocculated suspensions).

Above yield value

Plastic fluid flows similarly to a Newtonian system.

Plastic systems example

Slurries, emulsions, foams, gels, paints, blood.

Force of flocculation

Determines the yield value; higher inter-particle contacts = higher resistance.

Time-dependent behavior

Viscosity of certain fluids changes with time at a constant shear rate.

Thixotropy

Viscosity decreases over time under constant shear (e.g., paints, iron oxide gels, egg white).

Thixotropic mechanism

Breakdown of internal structure causes stress to drop until steady-state.

Rheopecty

Viscosity increases over time under constant shear (e.g., printer ink, gypsum paste).

Combined behavior

Time-dependent behaviors may occur with any flow type — seen only at specific shear rates.

Syringibility

Ease of liquid flow through a needle — impacted by viscosity and shear rate.

Pourability

Flow from containers must be controlled — thixotropic suspensions are ideal.

Extrudability

Gels and pastes should flow easily upon squeezing, but not leak during storage.

Stability

Proper rheology prevents sedimentation, creaming, caking in suspensions and emulsions.

Mixing equipment selection

Depends on rheological behavior — pseudoplastics require high shear mixers.

Tube filling

Plastic flow is ideal — holds shape until squeezed.

Topical formulations

Require pseudoplastic or thixotropic gels for easy application and retention.

Brookfield viscometer

Common for semisolid dosage forms; measures torque required to rotate spindle in sample.

Cone and plate viscometer

Used for small volumes and precise rheological data at set shear rates.

Rheogram

Graph of shear stress vs shear rate — helps identify flow type and predict behavior.

Applications of rheology in QA

Ensures batch consistency, performance prediction, packaging suitability, and user compliance.