Newtonian Systems

• Newtonian Systems show constant viscosity, independent of applied shear rate.
• Newtonian Systems is a fluid whose flow behavior follows Newton’s law of viscosity, which means:
• The viscosity remains constant regardless of the shear rate or the force applied.
• The relationship between shear stress (F/A) and shear rate (du/dx) is linear.

Definition:

• A Newtonian fluid is one for which the shear stress is directly proportional to the shear rate.

Mathematical Form (Newton’s Law of Flow):

                                             $\tau = \eta \cdot \gamma$

Where:

• τ = shear stress (force per unit area, N/m² or dynes/cm²)
• η = viscosity (resistance to flow)
• γ = shear rate (rate of change of velocity across the fluid)

Graphical Representation:

If you plot:

• X-axis: Shear rate γ ​
• Y-axis: Shear stress τ

Examples in Pharmaceutics:

• Water
• Ethanol
• Glycerin
• Some dilute solutions (e.g., dilute sugar or salt solutions)

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