Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session M19: Non-Newtonian Flows III |
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Chair: Gareth McKinley, Massachusetts Institute of Technology Room: 28E |
Tuesday, November 20, 2012 8:00AM - 8:13AM |
M19.00001: Flow-induced structures in polymer-colloid mixtures with attractive interactions Patrick Underhill, Rangarajan Radhakrishnan In some polymeric or surfactant systems, structures can form in flow that would not normally form at equilibrium. This can occur for hydrophobic poylelectrolytes, associative polymers, wormlike micelles, and mixtures of polymers and nanoparticles. These systems are particularly challenging to model because of the separation of time and length scales between the polymer radius of gyration and conformational rearrangements and the scale of the attractive and repulsive interactions. We have developed a new coarse grained model of polymer solutions that show flow-induced structures and have performed Brownian Dynamics simulations. These simulations provide insight into the mechanisms for forming flow-induced structures and the similarities between different systems. We will discuss how the effective solvent quality and balance of attractive and repulsive interactions alter the flow transitions and whether the structures are reversible or irreversible. We have also analyzed the dependence on the flow type. In particular, we have looked at oscillatory shear and elongational flows. [Preview Abstract] |
Tuesday, November 20, 2012 8:13AM - 8:26AM |
M19.00002: On singularity of the UCM and Oldroyd-B models in viscoelastic fluids: resolving the high-Weissenberg number problem Rho Shin Myong Most of methods based on the UCM and Oldroyd-B models in viscoelastic fluids are found to break down at a frustratingly low value of the Weissenberg number around We=1. The rigorous explanation for this mysterious break-down has remained elusive until recently. In this work, the nature of mathematical singularity of these classical models is first elucidated by considering shear, compression, and extension flows. Then a regularization method based on the Rayleigh-Onsager quadratic dissipation function is proposed in order to resolve the high-Weissenberg number problem. In particular, the exact reason why the extensional flow suffers the break-down in high-Weissenberg number cases is explained. In addition, the relationship of the regularized model to other constitutive models such as the Giesekus and the Phan-Thien-Tanner equations is illustrated. [Preview Abstract] |
Tuesday, November 20, 2012 8:26AM - 8:39AM |
M19.00003: Flow of a viscous nematic fluid around a microsphere Manuel Gomez-Gonzalez, Juan C. del Alamo We analyze the creeping flow elicited by a rigid spherical particle in a viscous nematic fluid. The drag force acting on the particle and its far flow velocity field are obtained in closed analytical form. We identify two anisotropy mechanisms arising from the constitutive equations, namely, the anisotropic momentum diffusivity and the resistance to bending of the fluid. Their influence on the flow is studied separately by defining two ``pure'' pseudo-isotropic conditions in which only one of these mechanisms is present. The accuracy of existing Particle Tracking Microrheology methods is analyzed in these fluids, with especial attention to an approach that provides effective directional viscosity coefficients by applying Stokes's drag separately along different directions. Finally, we analyze the effect of anisotropy on the flow structure, finding that directional momentum diffusivity produces asymmetric streamline patterns with enhanced streamline deflection in the directions of lower diffusivity and viceversa. This asymmetry can either be increased or decreased by the bending resistance of the fluid, depending on the sign of the bending stresses. In some cases, their combined effect is found to produce streamline patterns that converge towards the sphere. [Preview Abstract] |
Tuesday, November 20, 2012 8:39AM - 8:52AM |
M19.00004: Experiments on Rearrangements and Forces in 2D Emulsion Hopper Flow Xia Hong, Kenneth Desmond, Dandan Chen, Eric Weeks We conduct experiments with a quasi-two-dimensional binary emulsion flowing through a hopper. Our samples are oil-in-water emulsions confined between two close-spaced parallel plates, so that the droplets are deformed into pancake shapes. In this system, there is only viscous friction and no static friction between droplets. The hopper flow induces a high rate of rearrangement events allowing us to understand how stresses and forces change during the process. By imaging the droplets during flow, we observed T1 events, which are topological changes when droplets exchange neighbors. Simultaneously, we measure forces between the droplets using a technique we have developed. We study the evolution of forces between droplets during rearrangements, finding that rearrangements decrease the inter-droplet forces of nearby droplets. [Preview Abstract] |
Tuesday, November 20, 2012 8:52AM - 9:05AM |
M19.00005: Response of a two-dimensional liquid foam to air injection: swelling rate, fingering and fracture Benjamin Dollet, Imen Ben Salem, Isabelle Cantat The response of a two-dimensional liquid foam to a localised air injection is investigated experimentally and theoretically. The experiments show a rich phenomenology, with two essentially distinct behaviours, depending on the injection conditions. At low flux, the injected air forms a central bubble that grows inside the foam and induces plastic rearrangements, without film rupture. This ``pure swelling'' regime is reminiscent of ductile fracture. In this regime, the central bubble shows fingering patterns beyond a certain velocity. The dependence between the swelling rate, the injection overpressure and the other control parameters: cell gap, bubble size and foam area, is captured by a simple balance between the pressure drop and bubble/wall friction within a radial assumption. Fingering is successfully modelled by the linear stability analysis of an azimuthal perturbation of the radial model; yield stress becomes an important parameter to determine the finger width. At high injection rate, films are broken and narrow cracks form rapidly through the foam, which reminds brittle fracture. Criteria on the transition between ductile and brittle behaviours are investigated, both at the local and global scales. [Preview Abstract] |
Tuesday, November 20, 2012 9:05AM - 9:18AM |
M19.00006: Sedimentation of solid spheres and Newtonian drops in a viscoplastic medium Olga Lavrenteva, Yulia Holenberg, Uri Shavit, Avinoam Nir The slow sedimentation of smooth and rough solid spheres and Newtonian drops in viscoplastic fluid (low concentrated, of 0.07{\%} w/w, aqueous gel of Carbopol 940) is studied experimentally. It was found that when the drops settle in proximity to a vertical solid wall their settling speed is augmented and drops drift slowly toward the wall. This is contrary to what is known with Newtonian or viscoelastic domains. The shape of yielded regions around the particles and the flow field within these regions were determined making use of PTV and PIV methods. It is observed that yielded region around slowly moving rough sphere is almost fore-and-aft symmetric and resembles that obtained via numerical modeling. In contrast to this, yielded regions around drops and smooth particles are not symmetric. A striking similarity between flow fields generated by the motion of the later two types of inclusions was observed. This phenomenon as well as the increase of settling speed in the vicinity of walls can be attributed to the dynamic formation of a thin clear layer providing effective slip at the interface of smooth particles or at the wall. The existence of the wall slip was demonstrated also in our studies of Carbopol gel rheology. [Preview Abstract] |
Tuesday, November 20, 2012 9:18AM - 9:31AM |
M19.00007: Dynamics of Non-Newtonian Liquid Droplet Collision Xiaodong Chen, Vigor Yang Collision of Newtonian liquid droplets has been extensively investigated both experimentally and numerically for decades. Limited information, however, is available about non-Newtonian droplet collision dynamics. In the present work, high-fidelity numerical simulations were performed to study the situation associated with shear-thinning non-Newtonian liquids. The formulation is based on a complete set of conservation equations for the liquid and the surrounding gas phases. An improved volume-of-fluid (VOF) method, combined with an innovative topology-oriented adaptive mesh refinement (TOAMR) technique, was developed and implemented to track the interfacial dynamics. The complex evolution of the droplet surface over a broad range of length scales was treated accurately and efficiently. In particular, the thin gas film between two approaching droplets and subsequent breakup of liquid threads were well-resolved. Various types of droplet collision were obtained, including coalescence, bouncing, and reflexive and stretching separations. A regime diagram was developed and compared with that for Newtonian liquids. Fundamental mechanisms and key parameters that dictate droplet behaviors were identified. In addition, collision-induced atomization was addressed. [Preview Abstract] |
Tuesday, November 20, 2012 9:31AM - 9:44AM |
M19.00008: Lateral migration of a viscoelastic drop in a shear flow near a wall Swarnajay Mukherjee, Kausik Sarkar Deformation, orientation and lateral migration of a viscoelastic drop suspended in a wall-bounded shear flow of Newtonian fluid is numerically investigated using a front-tracking finite-difference method. The viscoelasticity is modeled using the modified FENE-CR constitutive equation. The initial position is found to not affect the quasi-steady migration dynamics of a viscoelastic drop just like that of a Newtonian drop. As viscoelasticity is increased, lateral migration is enhanced initially, then a saturation is reached and finally for very high viscoelasticity lateral migration decreases. This non-monotonicity is due to the presence of two opposite factors: the interfacial stresslet term and the non-Newtonian normal stresses inside the drop phase. Viscoelasticity increases the orientation angle which hinders migration by decreasing the interfacial stresslet term. Deformation is non-monotonic affecting the stresslet term and adding to the non-monotonicity of migration. Finally, the viscoelastic normal stresses inside promote migration in contrast to the case of a Newtonian drop migrating in a viscoelastic matrix, where the viscoelastic stresses outside hinders migration. The viscoelastic effect is enhanced at lower \textit{Ca} and higher viscosity ratios. [Preview Abstract] |
Tuesday, November 20, 2012 9:44AM - 9:57AM |
M19.00009: The effects of low Reynolds number viscoelasticity on linked sphere swimmers Mark Curtis, Eamonn Gaffney A simple model for a swimmer immersed in a zero Reynolds number environment consisting of three linked spheres attached by extensible rods contracting out of phase to break reciprocal motion is analysed. By prescribing the forces acting on the three spheres due to the rods, asymptotic methods are used to derive analytic expressions for the net displacement of the swimmer in both a Newtonian Stokes fluid and a zero Reynolds number viscoelastic fluid. The model indicates that the swimmer, during one beat cycle, can actually move a greater distance when immersed in the viscoelastic fluid compared to the Newtonian fluid. [Preview Abstract] |
Tuesday, November 20, 2012 9:57AM - 10:10AM |
M19.00010: Droplet Size Distributions in Atomization of Dilute Viscoelastic Solutions Bavand Keshavarz, Gareth McKinley, Eric Houze, John Moore, Michael Pottiger, Patricia Cotts The droplet size probability distribution functions (PDF) for atomization/fragmentation processes in Newtonian fluids are now generally accepted to be close to Gamma distributions. Despite the great practical importance, little is known about the nature of corresponding distributions for viscoelastic liquids, e.g. polymeric solutions such as pesticide sprays and paints. We present data from air-assisted atomization experiments for model viscoelastic solutions composed of very dilute solutions of polyethylene oxide. Although the addition of small amounts of high molecular weight polymer keeps the fluid shear viscosity and surface tension close to the solvent values, the size distributions are skewed towards higher values of the Sauter mean diameter. We show that the PDF curves for these weakly-elastic fluids are well described by Gamma distributions, but the exponent \emph{n} is systematically decreased by fluid elasticity. Flow visualization images show that this behavior arises from the non-linear dynamics close to the break-up point which are dominated by an elasto-capillary force balance within the thinning ligaments and the magnitude of the extensional viscosity in the viscoelastic fluid. [Preview Abstract] |
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