Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session D2: Suspensions: Rheology |
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Chair: Itai Cohen, Cornell University Room: 3002 |
Sunday, November 23, 2014 2:15PM - 2:28PM |
D2.00001: Effect of friction on the rheology of dense suspensions Stany Gallier, Elisabeth Lemaire, Fran\c{c}ois Peters, Laurent Lobry This work reports three-dimensional numerical simulations of sheared non-Brownian concentrated suspensions using a fictitious domain method. Contacts between particles are modeled using a DEM-like approach (Discrete Element Method), which allows for a more physical description, including roughness and friction. This study emphasizes the effect of friction between particles and its role on rheological properties, especially on normal stress differences. Friction is shown to notably increase viscosity and second normal stress difference $|N_2|$ and decrease $|N_1|$, in better agreement with experiments. The hydrodynamic and contact contributions to the overall particle stress are particularly investigated and this shows that the effect of friction is mostly due to the additional contact stress since the hydrodynamic stress remains unaffected by friction. Simulation results are also compared with experiments and the agreement is improved when friction is accounted for: this suggests that friction is operative in actual suspensions. [Preview Abstract] |
Sunday, November 23, 2014 2:28PM - 2:41PM |
D2.00002: Force-induced diffusion in hydrodynamically interacting colloidal dispersions: non-monotonic connections between fluctuation and dissipation Nicholas Hoh, Roseanna Zia Effects of hydrodynamic interactions on equilibrium self-diffusivity are well known; here we explore their influence on the force-induced diffusion of a microrheological probe by tuning the strength of such interactions via an excluded-annulus model. As the probe is driven through the suspension, its force-induced diffusion (microdiffusivity) is determined analytically in the limits of strong and weak forcing, and numerically for all forcing. The total diffusivity comprises that of an isolated probe, the entropic hindrance of the equilibrium microstructure, and non-equilibrium interactions between probe and bath particles. When hydrodynamic interactions are important, three factors contribute to the microdiffusivity: a reduction in probe mobility; Brownian flux due to microstructural deformation; and entropic exclusion (collisions). Long-range hydrodynamic interactions diminish all components of the microdiffusivity; however, lubrication interactions enhance longitudinal encounters. This manifests as a monotonic increase with flow strength, but with a surprising non-monotonic dependence on the strength of hydrodynamic interactions. That is, the role of hydrodynamics in the connection between diffusive fluctuation and viscous dissipation is non-monotonic. [Preview Abstract] |
Sunday, November 23, 2014 2:41PM - 2:54PM |
D2.00003: Transient, nonlinear rheology of reversible colloidal gels by dynamic simulation Benjamin Landrum, William Russel, Roseanna Zia We study the nonlinear rheology of reversible colloidal gels via dynamic simulation as they undergo age- and flow-induced structural evolution, with a view toward understanding and predicting transient behaviors such as multi-step and delayed yield. The gel is formed from 750,000 Brownian spheres interacting via hard-sphere repulsion and O(kT) short-range attraction, where thermal fluctuations are strong enough to allow continued structural rearrangement in the absence of flow. During startup of imposed strain rate, the transition to steady state is characterized by one or more ``overshoots'' in the stress which suggest initial yield, formation of a stronger gel, and subsequent yield of the new gel. When subjected to step-shear stress, the microstructure undergoes limited creep, followed by viscous flow. This macroscopic ``delayed flow'' is consistent with previously proposed models of competition between breakage and formation of particle bonds among static load-bearing structures. Our findings suggest, however, that the load-bearing structures evolve, and that the gel's resistance to delayed failure depends upon this structural evolution and reinforcement. We put forth a micro-mechanical model of stress gradient-driven particle transport that captures this macroscopic behavior. [Preview Abstract] |
Sunday, November 23, 2014 2:54PM - 3:07PM |
D2.00004: Normal stress differences in suspensions of rigid fibers Braden Snook, Levi Davidson, Jason Butler, Olivier Pouliquen, Elisabeth Guazzelli Numerical and experimental studies of normal stress differences in suspensions of rigid, non-Brownian fibers were carried out for length (L) to diameter (d) ratios of 11 to 30 at concentrations nL$^{\mathrm{2}}$d$=$1.5 to 3, where n is the number density of fibers. The numerical results are in quantitative agreement with the experimental results and allow calculation of the hydrodynamic and contact contributions to the stress in the suspension. The simulations show that the contact contribution to the rheology is dominant in determining the normal stress differences, where the first normal stress difference is positive and approximately twice the magnitude of the second normal stress difference, which is negative. [Preview Abstract] |
Sunday, November 23, 2014 3:07PM - 3:20PM |
D2.00005: The effect of geometry on the particle stress in suspensions of rigid particles in simple shear Mohsen Daghooghi, Iman Borazjani The contribution of particles on the total stress of a suspension is known as particle stress, which consists of three sources: moment of stress on the particle surface, inertial term and Reynolds stress term. The symmetric part of the first term, i.e. stresslet, is considered as the most important term in rheological calculation and contribution of other terms is mainly ignored in low Reynolds regimes. For suspensions of rigid spheres at steady state these terms are negligible comparing to stresslet of the suspension, however this might not be the case for complex particle shapes. Using immersed boundary method, we simulate suspensions of complex shaped particles in simple shear flow to investigate the role of other two terms on the total particle stress and effective viscosity. We validated our results against classical analytical results for the low Reynolds-Stokes problem of suspension of ellipsoidal particles by Jeffery. We studied the effect of volume fraction of suspension and particle shape (aspect ratio) on the rheology of suspensions at Reynolds number range of $0.01 |
Sunday, November 23, 2014 3:20PM - 3:33PM |
D2.00006: Constant stress and pressure rheology: a unified perspective on the arrest of colloidal suspensions Mu Wang, John Brady We study the constant pressure and stress rheology of dense hard-sphere colloidal suspensions using a novel Brownian dynamics simulation algorithm. The simulations show an arrested region exhibiting viscosity divergence between the glass and the jamming transitions. Both the suspension shear and normal viscosities near the arrested region display universal power law divergences that depend solely on the volume fraction distance from the corresponding arrest point. We further found that the microscopic particle diffusion correlates with the suspension pressure through a Stokes-Einstein-Sutherland-like relation. With an estimation of the effect of hydrodynamic interactions and a careful analysis of the accessible volume in the experiments, the simulations are in quantitative agreement with the experiments of Boyer et al. [PRL 107, 188301 (2011)] in the non-Brownian limit. The simulations clearly show the fundamental role of the jamming transition in the dense suspension rheology, and illustrate the great care needed when performing and analyzing experiments and simulations near the maximum allowable volume fractions. [Preview Abstract] |
Sunday, November 23, 2014 3:33PM - 3:46PM |
D2.00007: When hard spheres overlap - generalization of the Rotne-Prager-Yamakawa hydrodynamic tensors Eligiusz Wajnryb, Pawel Zuk, Krzysztof Mizerski, Piotr Szymczak The Rotne-Prager-Yamakawa (RPY) approximation is commonly used to model the hydrodynamic interactions between small spherical particles suspended in a viscous fluid at a low Reynolds number. It takes into account long-range contribution to hydrodynamic interactions and yields positive definite diffusion matrix, which is essential for Brownian dynamics modeling. However, when the particles overlap, the RPY tensors lose their positive definiteness, which leads to numerical problems in the Brownian dynamics simulations as well as errors in calculations of the hydrodynamic properties of rigid macromolecules using bead modeling. We extend the RPY approach to the case of overlapping spherical particles of different radii in a consistent way that preserves positive definiteness of diffusion tensors for translational, rotational and dipolar degrees of freedom. Moreover we show how the Rotne--Prager--Yamakawa approximation can be generalized for other geometries and boundary conditions. [Preview Abstract] |
Sunday, November 23, 2014 3:46PM - 3:59PM |
D2.00008: Measuring Mechanical Properties by Staring: Using Stress Assessment from Local Structural Anisotropy (SALSA) to Probe Viscosity and Visualize Stress Networks in Colloidal Suspensions Itai Cohen, Matthew Bierbaum, James Sethna, Neil Lin Measurement of stress induced thermal fluctuations in materials can be used to determine macroscopic mechanical properties including viscosity in fluids, as well as bulk and shear moduli in solids. When extended to the single particle scale, such measurements also reveal underlying spatially inhomogeneous response mechanisms in systems such as glasses, gels, and polycrystals. Unfortunately, it is not possible to experimentally measure these temporal and spatial stress fluctuations in a colloidal suspension using conventional rheometers. Here however, we show that using fast confocal microscopy it is possible conduct a Stress Assessment from Local Structural Anisotropy (SALSA) to measure such spatio-temporal stress fluctuations. We directly image the microstructure of a nearly hard-sphere suspension using a high-speed confocal microscope and determine particle positions. We compute the structure anisotropy of the suspension and building on the Brady formalism, calculate particle-level stresses. In conjunction with the fluctuation-dissipation theorem, we then determine the bulk viscosity of a colloidal liquid. Furthermore, we show our local measurements allow direct visualization of the complex stress networks in a 3D supercooled liquid under compression. Our method provides an experimental approach that applies to a broad range of processes arising in sheared glasses, compressed gels, and even indented crystals. [Preview Abstract] |
Sunday, November 23, 2014 3:59PM - 4:12PM |
D2.00009: Paradoxical ratcheting in cornstarch suspensions Troy Shinbrot, Theo Siu, Matthew Rutala Cornstarch suspensions are well known to exhibit strong shear thickening, and we show as a result that they must -- and do -- climb vertically vibrating rods and plates. This occurs because when the rod moves upward, it shears the suspension \textit{against} gravity, and so the fluid stiffens, but when the rod moves downward, the suspension moves \textit{with} gravity, and so the fluid is more compliant. This causes the fluid to be dragged up by the upstroke more than it is dragged down by the downstroke, effectively ratcheting the fluid up the rod every cycle. We show experimentally and computationally that this effect is paradoxically caused by gravity -- and so goes away when gravity is removed -- and we show that the suspension can be made to balance on the uphill side of an inclined rod in an analog of the inverted ``Kapitza pendulum,'' closely related to the recent report by Ramachandran {\&} Nosonovsky, Soft Matter \textbf{10} (2014) 4633. [Preview Abstract] |
Sunday, November 23, 2014 4:12PM - 4:25PM |
D2.00010: The role of contact forces in rheology of hard-sphere colloidal suspensions Safa Jamali, Arman Boromand, Joao Maia Dense colloidal suspensions show a wide range of non-Newtonian behavior in response to the flow. While at low and intermediate shear rates the fluid shear-thins, increasing the shear rate above a critical rate gives rise to microstructural changes in the fluid and consequently shear-thickening. It is widely accepted that shear-thickening of a suspension is due to the short-range hydrodynamics (so-called lubrication) forces between colloidal particles which consequently gives rise to formation of hydro clusters that resist against the flow. However, computational efforts based on lubrication theory have not been able to explain discontinuous shear-thickening in suspensions. Recently, some reports have incorporated contact potentials and their dissipative role in colloidal interactions and have successfully reproduced higher viscosity ratios at high shear rates in the shear-thickening regime. We study the effect of contact forces and lubrication potential in rheological behavior of the colloidal suspensions. To do so, we have modified Dissipative Particle Dynamics (DPD) method in order to include the lubrication potentials. Furthermore, different types of contact potentials have been included in our DPD potentials in order to understand the physical nature of contact forces and their effect on the rheology of suspensions. Finally, efforts will be made in order to correlate the microstructural changes and different types of interactions to macroscopic flow behavior of suspensions. [Preview Abstract] |
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