Viscosity is the measure of a fluid's resistance to flow. The Brookfield viscometer determines viscosity by measuring the torque required to rotate a spindle at a constant speed in a fluid. The resistance to the spindle’s movement caused by the fluid is converted into viscosity units (centipoise, cP).

Torque :
In a Brookfield viscometer, viscosity is measured based on the torque required to rotate a spindle in the sample.
When the spindle rotates, the fluid resists the motion due to its internal friction. This resistance generates torque, which is directly proportional to the viscosity of the sample.
Higher viscosity → Higher resistance → Higher torque

Spindle Speed:
The spindle rotates at a fixed speed (rpm), which determines the rate of deformation (shear rate) applied to the sample.
• Low speed → low shear rate
• High speed → high shear rate
The measured viscosity may vary depending on spindle speed, especially for non-Newtonian fluids.

Shear Rate Dependence (Non-Newtonian Behavior) :
Unlike Newtonian liquids (e.g., water), many pharmaceutical semisolids (such as creams, gels, and ointments) exhibit non-Newtonian flow behavior, meaning their viscosity changes with shear rate.

Types of behavior include: :
• Pseudoplastic (shear-thinning): viscosity decreases with increasing shear rate
• Dilatant (shear-thickening): viscosity increases with shear rate
The Brookfield viscometer helps study this behavior by measuring viscosity at different spindle speeds.