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    Velocimetry of interstitial flow in freely draining foam

    Year: 2018

    Journal: Colloid Surf. A-Physicochem. Eng. Asp., Volume 540, nov-05

    Authors: Kennedy, Matthew J.; Conroy, Michael W.; Fleming, James W.; Ananth, Ramagopal

    Organizations: Office of Naval Research through Naval Research Laboratory; Office of Naval Research through Strategic Environmental Research and Development Program (SERDP) grant [WP2739]; National Research Council Associateship Award at the Naval Research Laboratory

    Keywords: Aqueous foam; Drainage; Velocimetry; AFFF; Interstitia flow; RF6

    Interstitial flows at the middle height of a freely draining foam column were measured by microparticle image velocimetry (mu PIV), and superficial flow was measured from the bottom of the foam by a digital weighing scale. In addition, Plateau border widths were measured from the mu PIV images. Plateau borders were selected for mu PIV particle-tracking analysis based on their orientation being nearly vertical. Interstitial liquid velocity in the vertically oriented Plateau borders was found to depend linearly on the square of the width consistent with Poiseuille flow. Three foams were examined including a commercial fluorinated Aqueous Film Forming Foam (AFFF), a commercial non-fluorinated firefighting foam (Re-healing Foam Type 6; RF6), and a simple lab-mixed foam composed from sodium dodecyl sulfate (SDS) in water. The interstitial velocity and superficial velocity measurements were compared with theoretical models that assumed either channel-dominated (CD) or node-dominated (ND) viscous dissipation, i.e. rigid or mobile interfaces. The measurements were found to fall in between the two limiting cases for AFFF and RF6, with RF6 showing closer agreement with the CD case than the ND case. Flow measurements in SDS foam agreed approximately with the ND model, as expected, prior to foam breakdown. An approximate liquid volume fraction at the middle height of the foam column was also obtained from the ratio of interstitial flow to superficial flow.
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