1. Motivations

Colloidal Gas Aphrons (or microfoams) can be used in numerous applications including oil recovery, remediation of soils and waters contaminated with hydrocarbons, and firefighting where foam is currently used. Unresolved questions remain regarding the structure and rheology of CGA. The most widely accepted structure was suggested by Sebba [1]. It speculates that bubbles have a multilayered shell but no direct observation have demonstrated such a structure. Sebba [1] did not elaborate on the thickness of the speculative soapy shell either.

2. Rheology of Colloidal Gas Aphrons

As part of an undergraduate research project, the effect of surfactant concentration and pipe shape and size on the rheological properties of colloidal gas aphrons (CGA) or microfoams have been investigated. The effort resulted in a publication [2]. In brief, pipe flow experiments were performed in cylindrical pipes with various diameters. The porosity, bubble size distribution, surface tension, and pH were systematically measured. It was first established that there was no slip velocity at the wall and CGA did not change morphology and porosity. Compressibility effects were accounted for through the volume equalization approach [3]. To make the experimental results more general, the volume equalized apparent shear rate  and shear stress are non-dimensionalized in terms of Capillary number Ca^* and dimensionless stress.

First, Experiments where performed with pipe diameters 1., 1.5, and 2 mm and surfactant Tween 20 mass fraction of 0.22 wt.%. It establishes that (1) CGA can be considered as a shear thinning fluid, (2) the pipe shape and diameter have no effect on the CGA rheology, (3) the dimensionless volume equalized shear stress is proportional to (Ca^*)^m where Ca^* is the Capillary number and m = 0.65±0.06.

Furthermore, the effect of concentration of surfactant on CGA rheology was assessed with various aqueous solutions of Tween 20. The results are plotted in Fig. 2 in terms of dimensionless shear stress as a function of volume equalized Capillary number (Ca*)^2/3 and show a linear relationship. Increase in shear stress with surfactant concentration at a given value of Ca^* could be attributed to the reduction in the maximum packing of spherical bubbles as their size distribution narrows. As the maximum packing decreases so does the viscosity.

3. CGA Morphology

It is proposed to determine the morphology of CGA bubbles through scattering technique based on depolarization of a polarized radiation as it travels through the CGA. The experimental setup used to measure the scattered light from CGA is shown in Figure 3 and described elsewhere [4]. A HeNe laser (632 nm) is employed as the light source. A flow cells is used to flow the freshly generated CGA through the beam. The experimental apparatus and procedure were successfully validated using polystyrene microspheres. Then, experiments with CGA were performed with surfactant concentration of 2 ml Tween-20 per liter of deionized water. The flow cell thickness was 0.2-0.5 mm. The intensity vector is defined as (I,Q,U,V)^T where I is the total intensity, Q represents the tendency for horizontal polarization, U the tendency for ± 45° polarization, and V for circular polarization. The scattered intensity vector is related to the incident intensity vector through the Mueller matrix as [4]. The scattering Mueller matrix element S₁₁ relates to total intensity, S₁₂ to the linear polarization, S₂₂ represent the deviation of the scatterers' shape from sphericity (for perfect spheres S₁₁=S₂₂), and S₃₄ corresponds to the transformation from circular to linear polarization. The collected data was normalized with their value at scattering angle of 5 degree. The experimental data collected during the reporting period are of very good quality given the strongly scattering nature of CGA and the sensitivity of the measurements. However, the physical phenomena responsible such as dependent scattering and theoretical analysis are quite complex. Development of a theoretical model accounting for dependent scattering due to the close proximity of bubbles in CGA is currently under way.

4. References

[1]     Sebba, F., 1987. Wiley & Sons, New York.

[2]     S. Larmignat, D. Vanderpool, H. Lai, and L. Pilon, 2008. Colloids and Surfaces A: http://dx.doi.org/10.1016/j.colsurfa.2008.03.010 .

[3]     P. Valko and M.J. Economides, 1992. Journal of Rheology, (36), 111-127.

[4]     M.M. Aslan, C. Crofcheck, D. Tao, and M. Pinar Menguc, 2006. Journal of Quantitative Spectroscopy and Radiation Transfer, (101), 527-539.