Two-dimensional flow of foam around a circular obstacle: local measurements of elasticity, plasticity and flow

Abstract

We investigate the two-dimensional flow of a liquid foam around circular obstacles by measuring all the local fields necessary to describe this flow: velocity, pressure, bubble deformations and rearrangements. We show how our experimental setup, a quasi-2D "liquid pool" system, is adapted to the determination of these fields: the velocity and bubble deformations are easy to measure from 2D movies, and the pressure can be measured by exploiting a specific feature of this system, a 2D effective compressibility. To describe accurately bubble rearrangements, we propose a new, tensorial descriptor. All these quantities are evaluated via an averaging procedure that we justify showing that the fluctuations of the fields are essentially random. The flow is extensively studied in a reference experimental case; the velocity presents an overshoot in the wake of the obstacle, the pressure is maximum at the leading side and minimal at the trailing side. The study of the elastic deformations and of the velocity gradients shows that the transition between plug flow and yielded regions is smooth. Our tensorial description of T1s highlight their correlation both with the bubble deformations and the velocity gradients. A salient feature of the flow, notably on the velocity and T1 repartition, is a marked asymmetry upstream/downstream, signature of the elastic behaviour of the foam. We show that the results do not change qualitatively when various control parameters (flow rate, bubble area, fluid fraction, bulk viscosity, obstacle size and boundary conditions) vary, identifying a robust quasistatic regime. These results are discussed in the frame of the actual foam rheology literature, and we argue that they constitute a severe test for existing rheological models, since they capture both the elastic, plastic and fluid behaviour of the foam.

Damping of liquid sloshing by foams

“…In addition, k n = nπ/L is the wavenumber and ω n is the fluid-free-surface natural frequency. Using the relation (20) we obtain…”

Understanding and predicting viscous, elastic, plastic flows

“…[11,7,43]. Bubble distributions are monodisperse in size and disordered in geometry (shape, number of neighbors).…”

“…It had been suggested that fluctuations remain relevant even at large scale, in which case detailed statistical theories of long-range correlations and avalanches would be required [9,10]. This view is challenged by recent experiments which suggest that even these materials can be treated as continuous materials described by tensorial equations [3,11]; thus in principle partial differential equations could lead to the long-awaited predictions.…”

Cyclic deformation of bidisperse two-dimensional foams

“…the geometrical properties of the different foams studied. Foams were produced by the "liquid-glass" technique [12,13,14], in which bubbles are blown through a nozzle in a detergent solution. Two nozzles with different diameters were used to create small and large bubbles simultaneously, with a size dispersity of about 5% in each.…”