Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session AJ: Minisymposium: Foams: Linking the Mechanics of Fluids and Solids |
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Chair: Sascha Hilgenfeldt, Northwestern University Room: Hilton Chicago Williford C |
Sunday, November 20, 2005 8:00AM - 8:26AM |
AJ.00001: Effective Temperatures in Sheared Glassy Systems Andrea Liu, Ajay Gopinathan, Tal Danino One of the outstanding challenges in condensed matter physics is to describe the collective properties of many-body systems far out of equilibrium. Jammed systems belong in this category; even when they are steadily sheared and can explore different packing configurations, they are still out of equilibrium. Statistical mechanics is a powerful tool for understanding many-body systems at or near thermal equilibrium. I will discuss recent evidence that fluctuations in steadily-sheared systems near jamming can be described by “effective temperatures” that can be many orders of magnitude higher than the ambient temperature. [Preview Abstract] |
Sunday, November 20, 2005 8:26AM - 8:52AM |
AJ.00002: Forces exerted by a flowing foam : viscous, elastic and plastic behaviours Francois Graner, Benjamin Dollet We have built a 1 m long, 10 cm wide foam channel, in which we produce 2D foam flows in the range 0.05 - 50 mm/s. In the middle of the channel, we place an obstacle: circle, square, cogwheel, ellipse or airfoil. We perform measurements of the drag, lift and torque exerted by the flowing foam on the obstacle. We observe both a dissipative contribution characteristic of a liquid, and a yielding behaviour typical of a solid. We simultaneously image the foam. In each region of the flow, we measure locally the pressure field, as well as the velocity field, as for a liquid, but also elastic deformation and plastic rearrangements. We discuss how to link the local and global descriptions, and how beyond a few bubble diameters the foam behaves as a continuous material. However, its triple viscous, elastic, plastic behaviour is complex, and most features we observe are not yet explained by current models. [Preview Abstract] |
Sunday, November 20, 2005 8:52AM - 9:18AM |
AJ.00003: Flowing Foam: T1 events and solid-liquid transitions. Michael Dennin Flowing aqueous foam is found in many applications ranging from oil recovery, to fire fighting, to spreading shaving cream. Aqueous foam consists of gas bubbles with liquid walls. One of the striking features of foam is that despite being composed entirely of fluids, its mechanical properties are either those of a solid (elastic response) or fluid (viscous flow), depending on the nature of the applied stress and strains. We study the transition between these two regimes using a model foam system: bubble rafts. Bubble rafts are a single layer of bubbles floating on the air-water surface. This allows us to track the motion of all the bubbles during flow. In this talk, we will present two main results. First, we will discuss the observation of the coexistence between a solid-like and fluid-like state during flow. Second, we will discuss the role played by nonlinear, topological rearrangements, known as T1 events, in determining the mechanical response of the system. [Preview Abstract] |
Sunday, November 20, 2005 9:18AM - 9:44AM |
AJ.00004: Transition from solid-like to liquid-like rheological behavior in foam Reinhard H\"{o}hler, S\'{e}bastien Vincent-Bonnieu, Sylvie Cohen-Addad Aqueous foams can pass from solid-like to liquid-like behaviour not only as a function of applied stress but also as a function of experimental time scale. This is demonstrated by our experiments with stable 3D foams that age only due to coarsening: When subjected for a long time to stresses well below the yield stress, these foams slowly creep like Maxwell fluids. We present 2D numerical simulations that provide detailed insight about the local bubble rearrangements at the origin of macroscopic flows. In the case of creep flow, rearrangements are induced by coarsening, whereas for plastic flow they are triggered by the applied shear. We show that elementary rearrangements involving 4 bubbles can be represented in a continuum model as force dipoles whose moment is determined by the length and orientation of the created bubble edge. Based on our simulations, we have constructed an analytical model that explains quantitatively the macroscopic creep flow. Concerning strain induced rearrangements, we discuss how a mesoscopic yield stress may be defined, in the aim of constructing a physical model of plastic flow. [Preview Abstract] |
Sunday, November 20, 2005 9:44AM - 10:10AM |
AJ.00005: Rheology of Coarsening Foam Andrew Kraynik, Sascha Hilgenfeldt, Douglas Reinelt Gas diffusion between bubbles causes soap froth to coarsen and soften because the modulus scales with inverse bubble diameter and also decreases as foam polydispersity increases. Computer simulations with the Surface Evolver are used to elucidate the rheology and evolution of 3D foam structure during diffusive coarsening. The instantaneous cell growth rates are evaluated directly from the foam microstructure because the counterpart of von Neumann's law, which provides an exact relationship between growth rate and topology in 2D, is unavailable in 3D. Two mechanisms are responsible for foam topology changes: cell-neighbor switching triggered by cell edges shrinking to zero length (T1) and small cells disappearing (T2). We will discuss the topological, geometric and growth-rate statistics of the foam and individual cells as the system evolves toward what is presumed to be a scaling state. [Preview Abstract] |
Sunday, November 20, 2005 10:10AM - 10:36AM |
AJ.00006: Beyond deformation: Foam fracture Sascha Hilgenfeldt, Adrian Staicu When a flow of air is driven into a quasi-2D layer of ordered foam in a rectangular Hele-Shaw cell, the foam reacts in a fashion reminiscent of processes in both liquids and solids. On the macroscopic scale of the entire cell, an air finger analogous to that in non-Newtonian fluids develops. On the microscopic scale of single bubbles, elementary structural transitions (T1s) parallel the rearrangement of atoms around a crack tip in a solid. Above a critical air flow rate, the finger propagation abruptly changes to a fast cleavage process based on the rupture of successive films in the foam. We show that the interplay of surface tension and drag forces accounts for the transition and that this foam experiment allows for a detailed observation of both quasistatic and dynamic fracture processes. [Preview Abstract] |
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