Last week’s article explained best practices for handling foams for rheology testing. Below, foam analysis techniques are discussed for selecting test methods and interpreting data specific to foams.
Prior to conducting rheometry measurements, knowledge of the average foam cell size and ratio of gas to liquid helps the user understand the general structure of the foam. To support rheological analyses, optical microscopy should be used to help concur or re-evaluate the predicted foam microstructure. After rheology data is collected, changes in shear stresses may be attributed to changes in foam cell size and cell strength in response to the applied shear forces.
For an initial rheometry test, a constant shear rate over time is a great way to establish if the foam structure is stable under a low shear. Shear stress values should remain at constant slope, while viscosity should be a flat line if the foam maintains a constant structure (Figure 1). Likewise, oscillatory time tests at low applied constant strain should display flat lines for shear moduli (G’, G”), with any significant deviation serving as a sign of foam draining or coarsening.
Once the stability of a foam has been confirmed, flow curves can be collected to observe how the wet foam responds to increased shear rotation. Similar to a mixing simulation, flow curves demonstrate how foams may shear thin (1) as they are stirred (Figure 2). Data collected at a low shear rate (< 10 1/s) gives the foam properties near rest, while high shear rates (> 10 1/s) are useful for understanding the flowability in a pipe. Gardiner, Dlugogorski, and Jameson calculated effective shear rates ranging from 700 to 3,500 1/s for sulfate surfactant foams traveling in a pipe under 300 to 500 kPa of applied pressure, (2) while Zhou, Pillai, and Pilon found that shear rates of 3,000 to 8,000 1/s were representative of sodium dodecyl sulfate foams under pipe flow. (3) However, most shear rheometers are unable to test wet foams beyond 100 1/s or even only 15 1/s (4) if the foams slip out of the gap at those high speeds.
To accommodate the limitation of foam slippage from plates, frequency sweeps have
been conducted at an estimated 100 1/s frequency as an analog for high shear rates. (4) Such oscillatory testing with constant shear strain tends to keep the sample within the gap while measuring shear stress, the same variable output from flow curves. Although it should be cautioned that the shear stress values obtained under applied oscillatory frequency cannot be directly or quantitatively compared to the shear stress values from a flow curve, the data can be used in a qualitative manner to predict flowability and identify any potential issues that may occur under the high shear conditions.
In addition to wet foam stability and flow properties, foam strength can be measured in a shear rheometer. An amplitude sweep (aka “strain sweep”) displays the foam response to increased strains and stresses, including the point of foam breakage. The deviation from elastic (G’) or viscous modulus (G”) signifies the strain or stress needed to begin to disrupt the foam structure, while the shear moduli crossover point is the degree of stress or strain required for full deformation. (5) Elastic modulus magnitude represents the degree of solid-like behavior and the viscous modulus serves as a measure of liquid-like characteristics. A greater G’ means that the foam is stronger, while a greater G” is a sign of a weak foam. These variables depend on foam bubble size, foam age, and liquid volume fraction. (5)
With appropriate planning, foams can be analyzed for their physical characteristics for use in processing, product development, and research using these techniques.
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1. 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.
2. Gardiner, B. S., Dlugogorski, B. Z., and Jameson, G. J., “Rheology of fire-fighting foams”, Fire Safety Journal, 1998, 31, 61.
3. Zhao, J., Pillai, S., and Pilon, L., “Rheology of colloidal gas aphrons (MICROFOAMS) made from different surfactants”, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2009, 348, 1.
4. Marze, S., Langevin, D., and Saint-Jalmes, A., “Aqueous foam slip and shear regimes determined by rheometry and multiple light scattering”, Journal of Rheology, 2008, 52, 5, 1091.
5. 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.