Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session L26: Suspensions: Rheology |
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Chair: Roseanna Zia, Assistant Professor, Chemical and Biomolecular Eng. Cornell Univ. Room: E146 |
Monday, November 21, 2016 4:30PM - 4:43PM |
L26.00001: Gravitational collapse of colloidal gels: Origins of the tipping point Roseanna Zia, Poornima Padmanabhan Reversible colloidal gels are soft viscoelastic solids in which durable but reversible bonds permit on-demand transition from solidlike to liquidlike behavior; these O(kT) bonds also lead to ongoing coarsening and age stiffening, making their rheology inherently time dependent. To wit, such gels may remain stable for an extended time, but then suddenly collapse, sedimenting to the bottom of the container (or creaming to the top) and eliminating any intended functionality of the material. Although this phenomenon has been studied extensively in the experimental literature, the microscopic mechanism underlying the collapse is not well understood. Effects of gel age, interparticle attraction strength, and wall effects all have been shown to affect collapse behavior, but the microstructural transformations underlying the ‘tipping point’ remain murky. To study this behavior, we conduct large-scale dynamic simulation to model the structural and rheological evolution of colloidal gels subjected to various gravitational stresses, examining the detailed micromechanics in three temporal regimes: slow sedimentation prior to collapse; the tipping point leading to the onset of rapid collapse; and the subsequent compaction of the material as it approaches its final bed height. [Preview Abstract] |
Monday, November 21, 2016 4:43PM - 4:56PM |
L26.00002: Microfluidic rheology of active particle suspensions: Kinetic theory Roberto Alonso-Matilla, Barath Ezhilan, David Saintillan We analyze the effective rheology of a dilute suspension of self-propelled slender particles between two infinite parallel plates in a pressure-driven flow. We use a continuum kinetic model to study the dynamics and transport of particles, where hydrodynamic interactions induced by the swimmers are taken into account. Using finite volume simulations we study how the activity of the swimmer and the external flow modify the rheological properties of the system. Results indicate that at low flow rates, activity decreases the value of the viscosity for pushers and increases its value for pullers. Both effects become weaker with increasing the flow strength due to the alignment of the particles with the flow. In the case of puller particles, shear thinning is observed over the entire range of flow rates. Pusher particles exhibit shear thickening at intermediate flow rates, where passive stresses start dominating over active stresses, reaching a viscosity greater than that of the Newtonian fluid. Finally shear thinning is observed at high flow rates. Both pushers and pullers exhibit a Newtonian plateau at very high flow rates. We demonstrate a good agreement between numerical results and experiments. [Preview Abstract] |
Monday, November 21, 2016 4:56PM - 5:09PM |
L26.00003: 3D Numerical Study of the Shear Rheology of a Semi-dilute Viscoelastic Suspension Mengfei Yang, Sreenath Krishnan, Eric Shaqfeh The stress in suspensions of rigid particles in polymer solutions is of considerable interest in applications such as manufacturing processes and fracturing technologies. Deriving an analytic expression for the material functions of a viscoelastic suspension under shear is difficult due to the nonlinear particle-fluid and particle-particle interactions, and theoretical studies have been limited to dilute suspensions at low shear Weissenberg number (Wi) or low polymer concentrations. Previously, we performed 3D single-particle simulations and showed that the results agreed well with the existing theories in the appropriate parameter regimes. We found that suspensions in constant-viscosity elastic fluids shear-thicken over a range of Wi and their material properties plateau at higher Wi. However, discrepancies between simulation and existing experimental measurements for volume fractions as low as 2.5{\%} suggested that interparticle hydrodynamic interactions could not be neglected. We now present 3D high fidelity numerical simulations of multiple spheres freely suspended in a sheared viscoelastic fluid using an immersed boundary framework to study the relationship between hydrodynamic interactions, particle structure formation, and the bulk rheology of viscoelastic suspensions. We observe that in a non-shear thinning elastic fluid, particles do not ``chain'', but their interactions induce additional polymer stresses in the fluid which contribute to a stronger particle effect than predicted in the dilute limit. [Preview Abstract] |
Monday, November 21, 2016 5:09PM - 5:22PM |
L26.00004: Tunable shear thickening: from understanding suspension thickening to controlling viscosity on the fly Itai Cohen, Neil Lin, Chris Ness, Jin Sun, Mike Cates, Ben Guy, Michiel Hermes, Wilson Poon Whether contact forces play a role in shear thickening of colloidal systems where hydrodynamic contributions are thought to dominate remains highly controversial. By performing shear reversal experiments on silica and latex colloidal particles, we directly measure the hydrodynamic and contact force contributions to the suspension viscosity. We find that contact forces are not only present, but dominate the shear thickening response. More importantly, this finding directly suggests a strategy for active controlling the thickening viscosities of dense suspensions. We demonstrate that by strategic imposition of a high-frequency and low-amplitude shear perturbation orthogonal to the primary shearing flow, we can largely eradicate thickening. The orthogonal shear effectively becomes a regulator for controlling thickening in the suspension, allowing the viscosity to be reduced by up to two decades on demand. [Preview Abstract] |
Monday, November 21, 2016 5:22PM - 5:35PM |
L26.00005: Yield and flow-induced phase transition in colloidal gels under startup shear Lilian Johnson, Benjamin Landrum, Roseanna Zia We study the micro-mechanical origins of the transition from solid-like to liquid-like behavior during flow startup of colloidal gels via large-scale dynamic simulation, with a view toward understanding connections to energy storage and phase transition. Such materials often exhibit an overshoot in stress, and prior studies of strong, dilute colloidal gels with a stringy microstructure connect this ``yield'' event to loss of network connectivity. Owing to the importance of Brownian transport in phase separation processes in colloids, here we study a reversible colloidal gel of hard spheres that interact via a short-range attraction of several \textit{kT}, for which Brownian motion can lead to rapid quiescent coarsening. In the present study, we interrogate the shear stress for a range of imposed flow strengths, monitoring particle-level structure and dynamics, to determine the microscopic picture of gel yield. Our detailed studies of the microstructural evolution and macroscopic response during startup provide insight into the phase behavior during yield. We present a new model of stress development, phase transition, and structural evolution during transient yield in colloidal gels for which ongoing phase separation informs gel phenomenology. [Preview Abstract] |
Monday, November 21, 2016 5:35PM - 5:48PM |
L26.00006: Rheological properties of Cubic colloidal suspensions. Arman Boromand, Joao Maia Colloidal and non-colloidal suspensions are ubiquitous in many industrial application. There are numerous studies on these systems to understand and relate their complex rheological properties to their microstructural evolution under deformation. Although most of the experimental and simulation studies are centered on spherical particles, in most of the industrial applications the geometry of the colloidal particles deviate from the simple hard sphere and more complex geometries exist. Recent advances in microfabrication paved the way to fabricate colloidal particles with complex geometries for applications in different areas such as drug delivery where the fundamental understanding of their dynamics has remained unexplored. In this study, using dissipative particle dynamics, we investigate the rheological properties of cubic (superball) particles which are modeled as the cluster of core-modified DPD particles. Explicit representation of solvent particles in the DPD scheme will conserve the full hydrodynamic interactions between colloidal particles. Rheological properties of these cubic suspensions are investigated in the dilute and semi-dilute regimes. The Einstein and Huggins coefficients for these particles with different superball exponent will be calculate which represent the effect of single particle's geometry and multibody interactions on viscosity, respectively. The response of these suspensions is investigated under simple shear and oscillatory shear where it is shown that under oscillation these particles tend to form crystalline structure giving rise to stronger shear-thinning behavior recently measured experimentally. [Preview Abstract] |
Monday, November 21, 2016 5:48PM - 6:01PM |
L26.00007: Enhancing Shear Thickening Fatemeh Madraki, Sarah Hormozi, Guillaume Ovarlez, Elisabeth Guazzelli, Olivier Pouliquen A cornstarch suspension is the quintessential particulate system that exhibits shear thickening. By adding large non-Brownian spheres to a cornstarch suspension, we show that shear thickening can be significantly enhanced. More precisely, the shear thickening transition is found to be increasingly shifted to lower critical shear rates. This enhancement is found to be mainly controlled by the concentration of the large particles. [Preview Abstract] |
Monday, November 21, 2016 6:01PM - 6:14PM |
L26.00008: Mathematical inference in one point microrheology Christel Hohenegger, Scott McKinley Pioneered by the work of Mason and Weitz, one point passive microrheology has been successfully applied to obtaining estimates of the loss and storage modulus of viscoelastic fluids when the mean-square displacement obeys a local power law. Using numerical simulations of a fluctuating viscoelastic fluid model, we study the problem of recovering the mechanical parameters of the fluid's memory kernel using statistical inference like mean-square displacements and increment auto-correlation functions. Seeking a better understanding of the influence of the assumptions made in the inversion process, we mathematically quantify the uncertainty in traditional one point microrheology for simulated data and demonstrate that a large family of memory kernels yields the same statistical signature. We consider both simulated data obtained from a full viscoelastic fluid simulation of the unsteady Stokes equations with fluctuations and from a Generalized Langevin Equation of the particle's motion described by the same memory kernel. From the theory of inverse problems, we propose an alternative method that can be used to recover information about the loss and storage modulus and discuss its limitations and uncertainties. [Preview Abstract] |
Monday, November 21, 2016 6:14PM - 6:27PM |
L26.00009: Motion of a Spherical Particle Near Porous Boundaries Phani Kanth Sanagavarapu, Prabhu Nott The flow of suspensions near porous boundaries occur in many industrial processes and living systems. The questions that arise are, how do the permeability of the porous walls and the concomitant velocity slip affect the dynamics of individual particles, and the rheology of the suspension? Here we consider the motion of a single sphere near a plane permeable slab. We solve the Stokes equations using an eigenfunction expansion in the bi-spherical coordinate system. The boundary conditions at the porous wall are Darcy’s flux condition normal to the surface and Saffman’s slip condition in the tangential direction. In addition, we impose the condition of zero net flux across the porous slab and obtain the non-zero pressure on the other side of the porous slab. The drag and torque on the particle moving near a porous wall are computed and compared with those obtained for an impermeable wall. We comment on the implications of our results on recent measurements of the suspension stress near porous boundaries. [Preview Abstract] |
Monday, November 21, 2016 6:27PM - 6:40PM |
L26.00010: Normal stress differences in a sheared gas-solid suspension Saikat Saha, Meheboob Alam The stress tensor and normal stress differences are analyzed for a homogeneously sheared gas-solid suspension using Enskog-Boltzmann equation. Inelastic particles are suspended in a viscous fluid of viscosity $\mu_f$ and experience a Stokes drag force. Viscous heating due to shear is compensated by (i) the inelastic collisions between particles and (ii) the drag force experienced by the particles due to the interstitial fluid. Rheology of the particle phase is analyzed with anisotropic-Gaussian as the single particle distribution function. The first (${\mathcal N}_1$) and second (${\mathcal N}_2$) normal stress differences are computed as functions of the density ($\nu$), Stokes number ($St$) and restitution coefficient ($e$). A comparison with the existing simulation data shows an excellent agreement for both ${\mathcal N}_1$ and ${\mathcal N}_2$ over the predictions from other Grad-level theories. Finally, in the limit of $St\rightarrow\infty$ ($\mu_f\rightarrow 0$), the related results from the conventional theory of dry granular flows are recovered. [Preview Abstract] |
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