Colloids and Surfaces A: Physicochem. Eng. Aspects 338 (2009) 80–86 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa Time dependent scattering properties of slow decaying liquid foams J.N. Swamy a , C. Crofcheck a,∗ , M.P. Mengüc ¸ b a Department of Biosystems and Agricultural Engineering, 128 C E Barnhart Building, University of Kentucky, Lexington, KY 40546, USA b Department of Mechanical Engineering, 269 Ralph G. Anderson Building, University of Kentucky, Lexington, KY 40506, USA article info Article history: Received 15 July 2008 Received in revised form 19 December 2008 Accepted 31 December 2008 Available online 8 January 2009 Keywords: Foam Polarized light scattering Mueller matrix Monte Carlo method abstract An experimental procedure is described to explore the physical characteristics of foam structures using a diagnostics tool based on elliptically polarized light scattering (EPLS) concept. The experimental system was constrained in cylindrical geometry and used to test a slow decaying, stable foam. Measurements were taken over a period of 30h to utilize the scaling behavior of foam over long time scales. The normalized Mueller matrix elements were obtained for backscattering angles in the range of 120–150 ◦ . Additionally, Monte Carlo simulations were conducted and compared to the experiments. Experimental results suggest that backscattering signals, as quantified with the matrix elements (S 11 , S 12 , and S 33 ), are sensitive to foam age and show a dynamic range especially at angles between 120 ◦ and 135 ◦ . The Monte Carlo simulations display a qualitative agreement with the angular profiles obtained from experiments. The age dependence of the properties of shaving foam in terms of the liquid fraction, bubble size, and polydispersity is used to understand the correlation with the measured profiles of S 11 , S 12 , and S 33 . © 2009 Elsevier B.V. All rights reserved. 1. Introduction Foams and foam-like structures can be seen in a broad range of materials, ranging from familiar soap froths and food foams (whipped cream, ice-cream, aerated desserts, beer head, and bread) to foamed/reticulated metals and polymers, and biological tissues. The efficiency of the processes involving foams may have a strong dependence on the relative gas–liquid fractions and bubble size distribution which subsequently affect the area available for mass transfer [1]. During the process of formation/production, the foam structure evolves dynamically in an attempt to minimize energy, changing the bubble size distribution and liquid fraction. Such continuously evolving systems require measurement using non-intrusive diag- nostic techniques. Being cellular structures, foams exhibit a highly multiple scattering behavior resulting in a familiar white appear- ance. This limits the utility of conventional measurement tech- niques like photography or video imaging. AC electrical conductiv- ity has been widely employed to understand the drainage behavior of foams based on the measurement of the liquid fraction [2]. Some of these techniques have a disadvantage of providing information only about the average bubble size/liquid fraction [3], while others are often intrusive [4] or require an elaborate set up. More sophis- ticated methods such as optical tomography [5] and magnetic resonance imaging [6] have been used with success to study the ∗ Corresponding author. Tel.: +1 859 257 3000; fax: +1 859 257 5671. E-mail addresses: crofcheck@bae.uky.edu (C. Crofcheck), menguc@engr.uky.edu (M.P. Mengüc ¸ ). topology of three-dimensional foams. Although such techniques provide important insights on the dynamics of foam, they usu- ally require complex image reconstruction algorithms, which are computationally expensive limiting their utility as a research tool. In the past decade or so, multiple light scattering measurements have evolved as a useful tool for probing the structural evolution of liquid foams. Several studies have been conducted to investigate multiple scattering of light in foams using a diffusion approxima- tion [3,7–9]. Using such techniques, Durian and co-workers studied the transient behavior of foam due to coarsening and drainage [3,7]. They modeled foam as air bubbles separated by liquid films and a model for photon transport based on random walk was applied. The resulting photon transport mean free path was correlated with the foam microstructure. Studies utilizing the scattering optics of foam have contributed significantly to the understanding of foam dynamics including drainage, coarsening, and rheological proper- ties. Yet, the potential of elliptically polarized light scattering (EPLS) has never been considered for possible diagnostics of transient behavior of foams. The premise of the current approach is that cel- lular structures, like foam, are likely to alter the polarization of the incident radiation due to successive scattering events. Thus, moni- toring polarization changes in addition to the attenuated intensity can lead to additional information about foams. If the properties of the foam layer can be related to the changes in polarization of the incident light, an intelligent diagnostic scheme can be developed for practical applications to monitor foam properties. Polarized light scattering techniques have been used with success to characterize colloidal particle suspensions [10], soot agglomerates [11], milk [12], cotton fiber [10], bubble-laden media [13], and nanoparticle suspensions [14,15]. Depolarization and 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.12.038