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DrX Whiz Niraj
Unit 2: Rheology
Flow of Liquids & Deformation of Solids
1. What is Rheology?
The word Rheology comes from the Greek words 'Rheo' (to flow) and 'Logos' (science). It is the branch of physics that deals with the flow of liquids and the deformation of solid materials.
Why do Pharmacists study Rheology?
When you squeeze toothpaste out of a tube, shake a suspension bottle, or spread an ointment on your skin—you are applying a force (Shear Stress) to make the product flow. Rheology helps us design products that pour easily but don't drip randomly!2. Newtonian & Non-Newtonian Flow
Fluids are divided into two main categories based on whether they obey Newton's Law of Flow.
Newtonian Fluids: Liquids that obey Newton's Law. Their viscosity remains constant regardless of how hard or fast you stir them (e.g., Water, Glycerin, True solutions).
Non-Newtonian Fluids: Liquids whose viscosity changes when you apply force. They are further divided into 3 types:
| Flow Type | Behavior | Example in Pharmacy |
|---|---|---|
| 1. Plastic Flow | Does not flow until a minimum force (Yield Value) is applied. Behaves like a solid initially. | Ointments, Pastes, Flocculated Suspensions. |
| 2. Pseudoplastic | Shear-Thinning: Viscosity decreases on shaking/stirring. Becomes thinner. | Syrups, Polymer solutions (Tragacanth). |
| 3. Dilatant Flow | Shear-Thickening: Viscosity increases on stirring. Becomes thicker/harder. | Concentrated starch paste (Cornstarch in water). |
💡 Exam Tip (How to Read the Graph):
Look at the Plastic Curve (Pink). It does not start from zero! It starts from a specific point on the X-axis called the Yield Value. This proves that plastic fluids (like toothpaste) won't flow until you apply a certain amount of squeezing force!3. Thixotropy (Gel-Sol-Gel Transformation)
Thixotropy is an isothermal, slow, and reversible process where a material converts from a Gel (thick solid-like) to a Sol (flowable liquid) upon shaking, and returns back to a Gel when left undisturbed.
The area trapped between the Up-curve (Red) and Down-curve (Green) is called the Hysteresis Loop. A larger loop means the material takes a longer time to rebuild its Gel structure.
Ideal for Suspensions: A thixotropic suspension remains as a thick gel in the bottle (no settling of particles). When the patient shakes it, it becomes a fluid sol, making it easy to pour an exact dose!
4. Rotational Viscometers (Cup & Bob)
While Capillary Viscometers (like Ostwald) are only used for Newtonian liquids, Rotational Viscometers are used for both Newtonian and Non-Newtonian systems.