Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session A35: Suspensions I: Structure and Phase Transitions |
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Room: 406 |
Sunday, November 24, 2013 8:00AM - 8:13AM |
A35.00001: An immersed boundary method for investigating the rheology of heavy crude oil Mohsen Daghooghi, Iman Borazjani Heavy crude oil is a colloidal suspension of aggregates of many compounds (resins and asphaltenes), which give rise to very unusual rheological properties such as non-Newtonian behavior and very high viscosity. To optimize processes to lower viscosity and improve flow properties of heavy crude oil, a better understanding of the relationship between micro-structure and macro rheological behavior is required. To date, this relationship has been investigated mostly using theoretical models with many simplifying assumptions on the shape, size, and concentration of aggregates that limit their validity and use. We extend our immersed boundary method to simulate arbitrarily shaped suspensions of particles and calculate the viscosity of the suspension. We validate our method against the classical analytical and experimental results for the low Reynolds-Stokes problem of particle suspensions based on the work of Einstein, Batchelor, and others. We apply our method to simulate colloidal suspensions of asphaltenes with their experimentally observed micro-structure. We investigate the effects of asphaltene aggregates' concentration, size, shape, and polydispersity (different aggregate sizes) on the viscosity of the heavy crude oil. [Preview Abstract] |
Sunday, November 24, 2013 8:13AM - 8:26AM |
A35.00002: Acoustic Properties of Dilute Microstructured Suspensions: Theory and Experiment Wuhan Yuan, Liping Liu, Jerry Shan It is known that the orientation of ellipsoidal ferromagnetic particles in suspensions can be readily manipulated by external magnetic fields. This variable suspension microstructure can lead to a novel acoustic medium with tunable sound speed and attenuation. In pursuit of a better understanding of the acoustic properties of such microstructured suspensions, we develop a theoretical model for the effective viscosity of suspensions of oriented ellipsoidal particles. We then use this model to predict the acoustic properties of the suspensions, and how they depend upon sound frequency and particle volume fraction, aspect ratio, and orientation. On the other hand, we also conduct a series of experiments measuring sound speed and attenuation in suspensions of nickel microflakes with and without microstructure induced by external magnetic fields. The experimental and theoretical results are presented and compared. [Preview Abstract] |
Sunday, November 24, 2013 8:26AM - 8:39AM |
A35.00003: Derivation of the rheological properties of a dilute suspension of spheres in a dilute polymer solution using the method of ensemble averaged equations Donald Koch, Eric Lee, Ibrahim Mustafa Einstein derived the first effects of spherical particles on the rheology of a Newtonian fluid in the limit of small particle concentration. In the past, the only comparable analysis for a non-Newtonian fluid considered a second order fluid constitutive equation valid for very low Deborah number (defined as the shear rate times the polymer relaxation time). In this paper, the ensemble average stress of a dilute suspension of spheres in a dilute polymer solution governed by the Oldroyd B rheological equation is derived for Deborah numbers up to 4. The extra stress in the suspension arises from three physical processes: the stretching of the polymers due to the disturbance flow of the particles, the increased particle stresslet due to the polymer stress, and the modification of the Newtonian stress due to the flow perturbation caused by the polymers. We make use of an asymptotic analysis for small polymer concentration and the generalized reciprocal theorem to derive the third contribution. While a particle-free Oldroyd B fluid has no shear thinning or thickening, the particle suspension exhibits shear thickening and a nonlinear increase in the second normal stress difference with shear rate. [Preview Abstract] |
Sunday, November 24, 2013 8:39AM - 8:52AM |
A35.00004: Rheology and fluid mechanics of a hyper-concentrated biomass suspension Lorenzo Botto, Xiao Xu The production of bioethanol from biomass material originating from energy crops requires mixing of highly concentrated suspensions, which are composed of millimetre-sized lignocellulosic fibers. In these applications, the solid concentration is typically extremely high. Owing to the large particle porosity, for a solid mass concentration slightly larger than 10{\%}, the dispersed solid phase can fill the available space almost completely. To extract input parameters for simulations, we have carried out rheological measurements of a lignocellulosic suspension of Miscanthus, a fast-growing plant, for particle concentrations close to maximum random packing. We find that in this regime the rheometric curves exhibit features similar to those observed in model ``gravitational suspensions,'' including viscoplastic behaviour, strong shear-banding, non-continuum effects, and a marked influence of the particle weight. In the talk, these aspects will be examined in some detail, and differences between Miscanthus and corn stover, currently the most industrially relevant biomass substrate, briefly discussed. We will also comment on values of the Reynolds and Oldroyd numbers found in biofuel applications, and the flow patterns expected for these parameter values. [Preview Abstract] |
Sunday, November 24, 2013 8:52AM - 9:05AM |
A35.00005: Microstructure and rheology of colloidal suspension in simple shear and dynamic oscillatory flows: theory and simulation Ehssan Nazockdast, Stephanie Marenne, Jeffrey Morris A Smoluchowski theory is developed for analytical prediction of structure and rheology of concentrated colloidal dispersions. Pair distribution function, $g$(\textbf{r}), is computed as a solution to pair Smoluchowski convection-diffusion equation at a given volume fraction, $\phi $, and \textit{Pe} which is the ratio of hydrodynamic to Brownian forces. Pair distribution function is then used to evaluate rheology. Many body interactions are modeled self-consistently through third particle integrals which allows for predictions of structure and rheology at $\phi$ \textless\ 0.50 and the entire range of \textit{Pe}. The predictions of structure and rheology of steady state simple shear flow are in good agreement with Accelerated Stokesian Dynamics simulations (ASD). In this work the theory is extended to study time-dependent pair structure, $g$(\textbf{r} t), and rheology of colloidal dispersions, considering both startup of steady shear and oscillatory shear flows. The predictions of startup flow are compared against ASD simulations over a wide range of \textit{Pe} and $\phi $. The predictions of stress and microstructure in oscillatory shear flow are also presented over a wide range of strain amplitudes and frequencies.~ [Preview Abstract] |
Sunday, November 24, 2013 9:05AM - 9:18AM |
A35.00006: Reversible plastic regime in a 2D jammed material Nathan Keim, Paulo Arratia At the microscopic level, flow of a jammed, disordered material consists of a series of particle rearrangements that cannot be reversed. The same material under infinitesimal deformation is free of rearrangements, perfectly reversible, and dominated by elastic stress. Yet several recent studies have found an intermediate regime with observable plastic activity microscopically, but not globally: there is no net change to the material upon reversing the deformation. We report on the occurrence and structure of these reversible plastic events in experiments with an interfacial material, which do not give rise to global irreversibility. Reversible plasticity couples to the bulk shear stress and so contributes to bulk dissipation and viscoplasticity --- but this is the case for only a limited range of strain amplitudes below and above the yielding transition. [Preview Abstract] |
Sunday, November 24, 2013 9:18AM - 9:31AM |
A35.00007: Analogy between strain-stiffening and jamming in dense flows Gustavo D\"uring, Edan Lerner, Matthieu Wyart Dense granular and suspension flows display peculiar properties near the jamming threshold: the rheology is singular, and a diverging length scale can be identified from the velocity correlation of the particles, or from non-local effects affecting flow. In elastic networks a rigidity transition occurs when the coordination $z$ is increased toward some threshold $z_c$, but can also take place if a large strain is imposed as remarked early on by Maxwell. The latter phenomenon has been proposed to cause the ubiquitous stress-stiffening observed in gels of biopolymers. In my talk I will present the critical properties of a network immersed in a fluid approaching such a strain-induced rigidity transition. Then I will argue that this transition is at play in dense suspension flows, where it corresponds microscopically to the buckling of force chains. Our predictions include the existence of a vanishing strain $\gamma\sim 1/p$ in flow near jamming, where $p$ is the dimensionless particle pressure, and unravels the existence of two length scales affecting flow. [Preview Abstract] |
Sunday, November 24, 2013 9:31AM - 9:44AM |
A35.00008: Age coarsening of colloidal gels: a micro-mechanical study Roseanna Zia, Benjamin Landrum, William Russel We study the evolving structure and time-dependent rheological properties of an aging colloidal gel, with a focus on understanding the non-equilibrium forces that drive late-age coarsening. The gel is formed from a dispersion of Brownian hard spheres that interact via a hard-sphere repulsion and short-range attraction. The O(kT) strength of attractions lead to an arrested phase separation, and the resulting structure is a bi-continuous, space-spanning network that exhibits elastic and viscous behaviors: the gel may sustain its weight under gravity, or flow under shear. With weak attractions the bonds are reversible, giving rise to a continuous breakage / formation process as the gel ages. This balance favors coarsening over time, accompanied by an increase in feature size and elastic strength. We show here that anisotropic surface migration leads to heterogeneous coarsening, and that this migration is driven by gradients in particle-phase stress. [Preview Abstract] |
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