2-DIMENSIONAL VELOCITY PROFILES AND LAMINAR BOUNDARY-LAYERS IN FLOWING SOAP FILMS

In this study we examine laminar velocity profiles of freely suspended flowing soap films. We introduce a new device which supports large un iform films for indefinite periods of time. The geometry of the flow i s two-dimensional (2D), yet the measured velocity profiles depart from ideal 2D behavior. The main reason for this departure is that the soa p film experiences an air drag force across its entire surface. Descri bing the air with Prandtl boundary layer theory, we predict the observ ed flow patterns with good accuracy. The downstream development of the profiles is self similar. Our models set an apparent upper limit on t he film 2D viscosity of 5 . 10(-6) surface poise for dilute soap conce ntrations. This measurement implies that the surfactant layers on the film may not contribute measurably to the 2D viscosity. For higher soa p and glycerol concentrations the opposite appears to be true. (C) 199 6 American Institute of Physics.