Two-dimensional velocity profiles and laminar boundary layers in flowing soap films

DOI PDF 5 Citations
  • M. A. Rutgers
    Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
  • X-l. Wu
    Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
  • R. Bhagavatula
    Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
  • A. A. Petersen
    Department of Physics, Morehouse College, Atlanta, Georgia 30314
  • W. I. Goldburg
    Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260

Abstract

<jats:p>In this study we examine laminar velocity profiles of freely suspended flowing soap films. We introduce a new device which supports large uniform films for indefinite periods of time. The geometry of the flow is two-dimensional (2D), yet the measured velocity profiles depart from ideal 2D behavior. The main reason for this departure is that the soap film experiences an air drag force across its entire surface. Describing the air with Prandtl boundary layer theory, we predict the observed flow patterns with good accuracy. The downstream development of the profiles is self similar. Our models set an apparent upper limit on the film 2D viscosity of 5⋅10−6 surface poise for dilute soap concentrations. This measurement implies that the surfactant layers on the film may not contribute measurably to the 2D viscosity. For higher soap and glycerol concentrations the opposite appears to be true.</jats:p>

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