Rheology of aqueous foams under pressure

Aqueous foams are found in a wide range of industrial applications such as cosmetic, pharmaceutical, and food industries. These foam-based products are typically packaged in aerosol cans that are pressurized up to 3 bar. With pressure fluctuations over time, issues like coarsening and liquid drainage could affect the formulation. However, the rheological studies of foams under pressure are limited by the poor sensitivity and complex designs of traditional pressure cells that impose a lower torque limit of approximately 100 μN ⋅m. In this study, we detail the design and validation of a high-sensitivity pressurized Couette cell that utilizes a novel design to provide sample pressurization (up to 5 bar) with significantly improved torque sensitivity (1 μN ⋅m in oscillatory and 10 μN ⋅m in shear flows). Foam samples present experimental challenges due to a reduction in sample volume when pressurized. This has serious implications if a standard cylindrical cup and bob geometry is used with foam samples as it will be underfilled under pressurized conditions leading to erroneous results. To overcome this difficulty, we have designed a new rotor appropriately dimensioned that ensures a sufficient sample level over the entirety of the rotor under pressurized conditions. We demonstrate the application of this device by characterizing a commercial aqueous foam using traditional rheological methods such as flow ramps, oscillatory strain amplitude sweep, frequency sweep, time sweep, and stress growth under pressurized conditions.