Wet foams made from solutions and surfactants are prevalent in consumer products, food items, and industrial processing. However, despite the ubiquitous presence of foams in daily life, there is a lack of available information on foam rheology for product development, quality control, and manufacturing design. Compiled below are useful methods for handling and testing foams in a shear rheometer.
Select a Measuring System
Parallel Plates are the traditional favorite for wet foams due to their adjustable gap size. Most wet foam data has difficulties with reproducibility on account of wall slip, which distorts shear stress and viscosity readings due to either the foam sliding out of the gap or the foam not rotating at the applied velocity of the plate. (1) To avoid useless data output, roughened plate surfaces significantly reduce the amount of wall slip. Options include using sandblasted metal surfaces provided by rheometer manufactures or simply applying sandpaper with adhesive backing to a smooth plate to solve the problem. Gap size is recommended to be at least five if not ten times larger than the largest bubble/cell size, (2) just as with other samples with particulates or additives, to prevent data inaccuracies due to bubble or particulate jamming. Although the adjustable gap is ideal for testing foams, plate systems need to be lowered slowly onto the foam to avoid crushing the structure. (1)
Other researchers have conducted rheology experiments with foams placed in a free-
standing cup with a vane measuring system. (1) The same cup that held the solution during foaming with a high-speed mixer was then placed under a vane stirrer connected to a rheometer for a relative measurement of viscosity and shear stress. (1) Although this technique was shown not to disturb the foam structure during switching between the mixer and the rheometer, the data output is extrapolated rather than absolute on account of the cup not being a standard system for the rheometer and the lack of temperature control during the measurement.
To gain a better understanding of the influence of gas on the rheology of a foam, it is recommended to test the non-foamed solution with a double-gap measuring system. Most solutions have a very low viscosity that requires high torque sensitivity that can only be obtained from using a high-surface area measuring system. The double-gap is the best system to use for the solution if there are no particulates or emulsified droplets larger than nanoscale. In contrast, the foamed sample should be tested in a measuring system with a larger gap such as a plate or in special cases a cup and vane. Most studies found that foam viscosity and shear stresses were drastically different from the Newtonian solutions. (3 – 5)
Foam Testing Concerns: Time
Foams have experimental limitations due to the transient changes in cell size, stability, and physical properties over the course of the measurement. Structural instabilities often arise due to draining and coarsening: the gradual flow of liquid within the foam to the bottom due to gravity, and bubbles expanding over time on account of gas diffusion. (6) Contrary to the measurement of other delicate soft materials, wet foams should be tested under the shortest time span possible to deter the influence of draining and coarsening on the data. Higher viscosity solutions (η > 100 mPa-s) used to make foams tend to be more prone to draining unless polymer stabilizers are used. (3) Foam stability may need to be adjusted by altering the chemical formulation prior to taking measurements in a rheometer.
Like many liquidus samples, wet foams are vulnerable to evaporation. Rapid evaporation reduces the foam volume such that it does not completely fill the gap in a plate system. In turn, this affect results in distorted data that cannot be properly assessed. Researchers have overcome this concern through careful use of perfluoroethane (C2F6) gas for use during both foam creation and during rheometry tests because of its slower diffusion rate. (7) Special set-ups are necessary, but the results achieved a high level of sensitivity and repeatability.
Rheometers are capable of characterizing wet foams for their physical properties with use for product formulation, process control, and research. Next week we will discuss foam rheology analyses for the most efficient rheometry methods for handling and interpreting foam data.
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Gardiner, B. S., Dlugogorski, B. Z., and Jameson, G. J., “Rheology of fire-fighting foams”, Fire Safety Journal, 1998, 31, 61.
Bureiko, A., et al., “Bulk and surface rheology of AculynTM 22 and AculynTM 33 polymeric solutions and kinetics of foam drainage”, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2013, 434, 268.
Marze, S., Guillermic, R. M., and Saint-Jalmes, A., “Oscillatory rheology of aqueous foams: surfactant, liquid fraction, experimental protocol and aging effects”, Soft Matter, 2009, 5, 1937.
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