Characterizing any material for flow behavior involves testing it over a range of shear rates relevant to how it is processed in manufacturing or how it is used by the consumer. Ointments, for example, are used to coat human body parts like skin, lips, finger/toe nails, hair, eyeballs, etc. The coating action is such that the medicinal mixture, once applied to the surface of the object, spreads as it is rubbed into place. Therefore, a relevant viscosity test would employ shear rates that mimic this type of spreading action.
Squeezing the ointment out of the tube is another type of flow behavior that should be tested for acceptable performance. The situation that we are all familiar with is not being able to get the ointment out of the tube, no matter how hard we squeeze. This type of flow behavior measurement is known as “yield stress” and it quantifies the force needed to initiate flow of material out of the tube.
It’s worth noting that pharmaceutical marketing departments have increased their focus on product packaging to make sure that the customer experience is acceptable in all respects. This includes providing user friendly tubes which open easily and expel controlled amounts of the ointment according to the recommendation of the physician.