A constant stress, so you can measure yield stress easily

cone/plate rheometer.

the type of instrument known as the “cone/plate rheometer,” is typically used by R&D to characterize flow behavior of ointments. Test material is placed on the flat metal plate and cone spindle is brought down into contact with the ointment, causing a spreading action. Sample material flows out to the circumference of the cone spindle and perhaps beyond if there is too much sample volume. Excess material is removed and the instrument is ready to run tests.

pharmaceutics

Viscoelastic and Deformation Characteristics of Structurally Different Commercial Topical Systems Maryam Dabbaghi 1,†, Sarika Namjoshi 1,†, Bhavesh Panchal 1 , Jeffrey E. Grice 1, Sangeeta Prakash 2 , Michael Stephen Roberts 1,3,4 and Yousuf Mohammed 1,*

ll rheological experiments were conducted using an AR-G2 rheometer (TA instrument®, New Castle, DE, USA) equipped with a Peltier stage for temperature control and a 40 mm diameter parallel plate geometry.

Rheological characteristics and shear response have potential implication in defining the pharmaceutical equivalence, therapeutic equivalence, and perceptive equivalence of commercial topical products. Three creams (C1 and C3 as oil-in-water and C2 as water-in-oil emulsions), and two gels (G1 and G2 carbomer-based) were characterized using the dynamic range of controlled shear in steady-state flow and oscillatory modes. All products, other than C3, met the Critical Quality Attribute criteria for high zero-shear viscosity (η0) of 2.6 × 104 to 1.5 × 105 Pa·s and yield stress (τ0) of 55 to 277 Pa. C3 exhibited a smaller linear viscoelastic region and lower η0 (2547 Pa·s) and τ0 (2 Pa) , consistent with lotion-like behavior. All dose forms showed viscoelastic solid behavior having a storage modulus (G′) higher than the loss modulus (G′′) in the linear viscoelastic region. However, the transition of G′ > G′′ to G′′ > G′ during the continual strain increment was more rapid for the creams, elucidating a relatively brittle deformation, whereas these transitions in gels were more prolonged, consistent with a gradual disentanglement of the polymer network. In conclusion, these analyses not only ensure quality and stability, but also enable the microstructure to be characterized as being flexible (gels) or inelastic (creams)