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.

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We propose a novel mechanism for the origin of non-Gaussian tails in the probability distribution functions (PDFs) of local variables in nonlinear, diffusive, dynamical systems including passive scalars advected by chaotic velocity fields. Intermittent fluctuations on appropriate time scales in the amplitude of the (chaotic) noise can lead to exponential tails. We provide numerical evidence for such behavior in deterministic, discrete-time passive scalar models. Different possibilities for PDFs are also outlined.

It is shown that bulk thermodynamic effects, in addition to surface tension effects, can play a crucial role in thermocapillary phenomena ͑e.g., bubble or droplet thermomigration and phase separation in multi-phase systems subjected to an imposed temperature gradient͒ due to asymmetric thermodynamic properties of the underlying coexisting phases. The resultant new phenomena are elucidated in a representative model of diffusive systems with a conserved order parameter.

Interface equations are derived for both binary diffusive and binary fluid systems subjected to nonequilibrium conditions, starting from the coarse-grained (mesoscopic) models. The equations are used to describe thermo-capillary motion of a droplet in both purely diffusive and fluid cases, and the results are compared with numerical simulations. A mesoscopic chemical potential shift, owing to the temperature gradient, and associated mesoscopic corrections involved in droplet motion are elucidated.