INTRODUCTION

The semisolid pharmaceuitical preparations such as creams, gels and suspensions comprises of the largest part in the pharmaceutical industry. Hence, Viscosity and rheology are monitored for quality control, consistency and pharmacopoeial regulations during formulation of ointments and gels. Rheological properties such as viscosity and thixotropy of semi-solid dosage forms can influence their drug delivery by directly influencing the diffusion rate of the drug.

PRINCIPLE

Classical Brookfield viscometers employ the principle of rotational viscometry - the torque required to turn an object, such as a spindle, in a fluid indicates the viscosity of the fluid. Torque is applied through a calibrated spring to a disk or bob spindle immersed in test fluid and the spring deflection measures the viscous drag of the fluid against the spindle. The amount of viscous drag is proportional to the amount of torque required to rotate the spindle, and thus to the viscosity of a Newtonian fluid. In the case of non-Newtonian fluids, Brookfield viscosities measured under the same conditions (model, spindle, speed, temperature, time of test, container, and any other sample preparation procedures that may affect the behavior of the fluid) can be compared. When developing a new test method, trial and error is often necessary in order to determine the proper spindle and speeds. Successful test methods will deliver percentage torque reading between 10 and 100. The rheological behavior of the test fluid can be observed using the same spindle at different speeds, but because the geometry of the fluid around a rotating bob or disk spindle in a large container does not allow a single shear rate to be assigned, proper rheometry is not feasible using this setup.