PROCESS SYSTEMS ENGINEERING A Multi-Point Electrical Resistance Measurement System for Characterization of Foam Drainage Regime and Stability Chanhyuk Park Dept. of Civil and Environmental Engineering, University of California at Berkeley, Berkeley, CA 94720 Center for Water Resource Cycle Research, Korea Institute of Science and Technology, Seoul 136-791, South Korea Slawomir W. Hermanowicz Dept. of Civil and Environmental Engineering, University of California at Berkeley, Berkeley, CA 94720 DOI 10.1002/aic.14489 Published online May 20, 2014 in Wiley Online Library (wileyonlinelibrary.com) Foam drainage regimes are significantly associated with the nature of the hydrodynamic resistance in foam structure. A multi-point electrical resistance measurement technique has been applied for characterization of the drainage regimes and quantifying stability within standing foams. The capacity of the technique was confirmed by the estimation of mac- roscopic drainage rates for aqueous foams stabilized with sodium dodecyl sulfate. The drainage of sodium dodecylben- zenesulfonate, a commercial form of linear alkylbenzene sulfonate that is the most frequently used in household detergents was studied in detail by two complementary methods (forced and free drainage). The experimental data could be fitted using a power-law with an exponent of 1/3 for forced drainage and of 1.0 for free drainage. These data indi- cate the following drainage behavior: mobile bubble surfaces, causing plug-like flow within Plateau borders, thus dissi- pation mainly occurs inside the nodes. This research introduced an accurate method for quantifying foam stability that can be assessed by variations of real-time measured foam heights that incorporate the evolution of the liquid content. VC 2014 American Institute of Chemical Engineers AIChE J, 60: 3143–3150, 2014 Keywords: drainage regime, electrical resistance, forced drainage, stability, sodium dodecylbenzenesulfonate Introduction Aqueous foam is basically a large volume of a gas, envel- oped by a much smaller amount of a liquid in the presence of surfactant molecules stabilizing the gas-liquid interfaces. In various applications, foams could be produced by physical methods such as shaking, pouring, or sparging gas, or could be generated by chemical methods including fermentation, (electro-) chemical reaction, or ultrasound. 1–3 When freshly formed, foams are thermodynamically unstable and some- times dissipate within a few seconds. 4,5 In other cases, some foam systems persist for a long time, for instance, the pres- ence of a substantial stable layer in a bioreactor could lead to serious deleterious effects. 6–9 One of the main characteris- tics of foams is that they irreversibly evolve in time because they are not in equilibrium under normal gravity condi- tions. 10,11 Liquid drains out of the foam until such an equi- librium is attained, and some liquid is kept inside the foam due to capillary forces. 12 Liquid drainage is a complex phys- icochemical hydrodynamic process at the gas-liquid interfa- ces. 13,14 The main hydrodynamic energy-dissipation through the foam is due to the liquid flowing in the network of the channels known as Plateau borders (PBs) interconnected at the nodes. 15,16 These elementary structures (PBs and nodes) within the foam strongly depend on the liquid content, which is directly coupled to foam permeability. 13,17–19 When the liquid drains, the foam structure becomes thinner and more fragile, leading foam collapse. 20,21 Fast drainage rates are expected to indicate less stable foams, leading to a quick breakdown of foam structure. Such changes in the liquid content and the drainage rates are important for predicting foam stability. The main objective of drainage studies was to find out how long it takes for the foam to achieve a steady state. Sev- eral methods have been extensively used to characterize liq- uid distribution within the foam. 15,22–24 Measuring the drainage of a standing foam (free drainage) is conceptually the simplest. The drainage rates and regimes could be deter- mined as a decrease of the local liquid fraction at a given location. 25–27 Recent studies focused on a method with gravity-driven liquid flow with continuous supply of liquid called forced drainage. Numerous researchers carried out forced drainage experiments to investigate the drainage prop- erties of the foam. 28–30 In such experiments, the surfactant solution is added to the top of initially dried and drained foam at a controlled flow rate, causing propagation of a con- tinuous wet traveling wave throughout the foam height. A front of the wave is detected, separating the initially dried foam below from the wetter foam above. The wave front moves down with a constant drainage velocity, thus forming Correspondence concerning this article should be addressed to C. Park at chpark@kist.re.kr. VC 2014 American Institute of Chemical Engineers AIChE Journal 3143 September 2014 Vol. 60, No. 9